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Antigua and Barduda National Action Plan for Protected Areas.  Submitted by Tropical Ecosystem Consulting to the Caribbean Conservation Association and the Antigua Ministry of Environment. Antigua_and_Barbuda_PA_Action_Plan_-_FINAL_0.pdf

 

 


MINISTRY OF AGRICULTURE, LANDS, HOUSING AND THE ENVIRONMENT

 

PUMPED HYDRO IN ANTIGUA AND BARBUDA

 

report on prefeasibility assessments

 

MAR

14

2014

 


MINISTRY OF AGRICULTURE, LANDS, HOUSING AND THE ENVIRONMENT

 

PUMPED HYDRO IN ANTIGUA AND BARBUDA

 

REPORT ON PREFEASIBILITY ASSESSMENTS

 

MAR

14

2014

Version

Draft Report

Date of issue

14.03.2014

Prepared by

Norplan team

Checked by

 

Approved by

 

Executive summary

Pumped storage hydropower plant is a feasible option for compensating for undulating renewable power such as wind power.

A feasible site has been identified near St. Phillips and is utilizing sea water as the lower reservoir and a new upper reservoir of 70,000 m3 is to be built to yield minimum 15 MWh for the 10 MW pumped storage hydropower project. The plant can be connected to the exiting grid at Lavington substation. The project will have to be looked at as a power compensation for the wind farm and not a standalone project. The cost estimate for the pumped storage hydropower plant is 22.5 MUSD for a 10 MW pumped storage hydropower plant with a 15 MWh storage capacity.

To identify the design criteria for the pumped storage hydropower plant more accurate wind data is necessary together with energy system stability analysis and requirements. It is in general advised to spread the wind mills locations to avoid the same wind gust to hit the mills at the same time but generate a smother wind power from the combined wind mills in the wind park(s) reducing the need for compensation by pumped hydro or other means.

A 1 MW pumped storage hydropower pilot scheme is expensive and has limited capacity for stabilizing the grid being a small installation. An optional pilot scheme is to install a few wind mills (2-5 MW?) without compensating means (without flywheels, pumped hydro) but stabilized by the existing grid and thermal plants. All relevant and high quality data will be recorded from the wind mills themselves and the grid to be used for planning of a bigger wind farm paired with a pumped storage hydropower scheme.
 

Table of contents 

Executive summary. II

Table of contents. III

List of tables. IV

List of figures. IV

List of abbreviations/ackronyms. V

1           Introduction. 6

1.1       Background. 6

1.2       Terms of Reference. 6

2           Background Information. 10

2.1       System Load. 10

2.2       Renewable Energy Mix Challenge and Power System Stability. 12

2.3       Description of Existing Electrical Power System.. 12

2.4       Wind data. 14

2.5       Solar data. 15

3           Proposed Concept Options. 17

3.1       Wind Power at Crabbs Peninsula. 17

3.2       Flywheels. 19

4           Technical Conceptual Layout Optimisation of Pumped Hydropower Scheme. 20

4.1       Technical Approach and Assessment. 20

4.1.1  Regulation Challenges. 20

4.1.2  Potential Locations. 20

4.1.3  The design challenge. 23

4.1.4  Turbine and Pump Setup. 23

4.1.5  Preliminary Waterway Stability Analysis. 27

4.1.6  Pilot scheme. 29

4.2       Conceptual Layout. 30

4.2.1  Civil Works. 30

4.2.2  Electro Mechanical Works. 31

4.2.3  Power Transmission Works. 32

5           Cost Estimate and Energy Production Estimate. 34

5.1       Cost Estimate. 34

5.2       Energy Generation and Consumption. 34

5.2.1  Wind Power Stabilizer. 34

5.2.2  Energy Consumption. 35

6           CO2 Emission and Climate Change. 37

7           Conclusions and Way Forward. 38

7.1       Conclusions. 38

7.2       Further Work. 38

 


 

List of tables

Table 1:         Existing reservoirs potentially for use for pumped hydropower schemes. 9

Table 2:         Potential schemes for pumped hydro by use of existing reservoirs. 9

Table 3:         List of documents provided. 10

Table 4:         Comparison between thermal and hydro power (source: Andritz Hydro) 12

Table 5:         Thermal power plant installations. 13

Table 6:         Transmission lines. 13

Table 7:         Substation transformers. 14

Table 8:         Combinations of Turbine and Pump Setups. 25

Table 9:         Comparison of the two potential sites for pumped storage hydropower scheme. 29

Table 10:      Cost estimate for a 10 MW pumped hydropower project at St. Phillips. 34

Table 11:      Relationship between storage capacity and generating/pumping time. 35

Table 12:      Basic assumptions and estimated annual CO2 emissions reduction due to replacement of thermal power generators (Reference: Engineeringtoolbox.com) 37

 

List of figures

Figure 1:        Weekly load demand in 2011. 11

Figure 2:        Load demand for an average weekday in 2011. 11

Figure 3:        69 kV transmission network in Antigua with 69/11 kV substations. 14

Figure 4:        Mean daily solar intensity 2011. 15

Figure 5:        Monthly variation in Solar Energy 2011. 16

Figure 6:        Windmill plant generation (MW) fluctuations. 17

Figure 7:        Proposed layout of Crabbs wind farm, 10 x Vestas V90-1.8 MW (Source: Pre-Feasibility Study for a grid-parallel Wind Park at Crabbs Peninsula, Aug 2012) 18

Figure 8:        Correlation between windmills in a plant (Ernst, Wan, Kirby, 1999). 18

Figure 9:        The planned localization of the 10 windmills in Crabbs Peninsula. 19

Figure 10:          Conceptual layout of the St. Phillips sea water PSP. 21

Figure 11:          Proposed connection of the St. Phillips sea water PSP to the existing grid. 22

Figure 12:          Conceptual layout of the McNish - Dunnings PSP. 22

Figure 13:          Proposed connection of the McNish - Dunnings PSP to the existing grid. 23

Figure 14:          Power flow for the energy sources. 24

Figure 15:          Power chart for energy sources by separate generating mode and pump mode for pumped storage hydropower project. 25

Figure 16:          Main figures for the simulated 5 MW reversible pump-turbine that has been used for all the simulations in ALAB.  26

Figure 17:          Schematic layout of the waterway for the St. Phillips station in ALAB. 27

Figure 18:          Frequency deviation for the 5 MW RPT for St. Phillips. 28

Figure 19:          Turbine efficiency (red curve) for a conventional Francis turbine (left) and a RPT (right). The graphs have been made in Alab and are based upon the assumptions made for the St. Phillips site. 31

  

List of abbreviations/ackronyms

APUA            -      Antigua Public Utilities Authority

APC                -      Antigua Power Company

CREDP          -      Caribbean Renewable Energy Development Project

EES                 -      Electrical Energy Systems

GEF                -      Global Environment Facility

GWh             -      Giga Watt hours (Derived unit of energy) =1,000 MWh = 1,000,000 kWh

HOMER        -      Hybrid Optimization Model for Electric Renewables

kV                   -      kilo Volt (Derived unit of voltage) = 1,000 V

kVA                -      kilo Volt Ampere (Unit of capacity) = 1,000 VA

kW                 -      kilo Watt (Derived unit of power) = 1,000 W = 0.001 MW

kWh              -      kilo Watt hours (Derived unit of energy) = 0.001 MWh = 1/1,000,000 kWh

OPGW          -      Optical fibre ground wire

MVA              -      Mega Volt Ampere (Unit of capacity) = 1,000 kVA =1,000,000 VA

MW               -      Mega Watt (Derived unit of power) = 1,000 kW

MWh            -      Mega Watt hours (Derived unit of energy) = 1,000 kWh = 0.001 GWh

PSP                -      Pumped Storage Plant

RPT                -      Reversible Pump Turbine

SCADA          -      Supervisory Control and Data Acquisition

SIDS               -      Small Island Developing States

ToR                -      Terms of Reference

UNDESA      -      United Nations Department of Economic and Social Affairs

 

1           Introduction

1.1       Background

The Environment Division of the Government of Antigua and Barbuda and the United Nations Department of Economic and Social Affairs have entered into a contract with Norplan AS to study the pre-feasibility of pumped hydropower as a power stabilizer to mitigate the undulating power supply from future renewable energy projects. In particular an 18 MW wind farm at Crabbs has been identified in earlier studies.

The Consultant and the Clients had a joint site reconnaissance visit to potential pumped hydropower plant sites December 2nd to 6th 2013. 

1.2       Terms of Reference

The Terms of Reference with background, objective, description of required services and outputs and for the study is as follows:

“I.                 Background

Small Island Developing States are under threat from Climate Change in terms of fresh water shortages with sea level rise and increased frequency and severity of weather. They are also faced with high imported diesel costs – providing the energy needed to maintain comfort and desalinate seawater. Making SIDS more energy independent using indigenous renewable energy and energy efficiency will generate more jobs, reduce energy price risks, make the islands more resilient to climate impacts and reduce greenhouse gas emissions. Due to grid stability and the cost of spinning reserve Antigua Public Utilities Authority has identified a threshold of the amount of wind and solar power to 15% of peak demand above which grid management may become more difficult. 15% is 7.5 MW (using the 2012 peak) probably expanding somewhat but in any case the threshold will be surpassed by the proposed wind farm. With a net metering policy in effect for < 50 kW solar PV installations they are already approaching 250 kW of solar PV. Load management will be explored in the water pumping and desalination facilities of APUA as a dump load to remove the effects of fluctuating power generation from wind and solar. Antigua and Barbuda are seeking to shift toward 25% renewable energy and improve resilience to drought.

The Environment Division of the Government of Antigua and Barbuda and the United Nations Department of Economic and Social Affairs have received approval from the Global Environment Facility for the development of a Sustainable Pathways – Protected Areas and Renewable Energy project. The existing domestic water supply reservoirs are in the protected area watershed and will be reforested as part of the project. A wind power plant will be located near the existing diesel plant at Crabbs point remote from the protected area. Solar photovoltaic power will be scattered around the island on rooftops.

The approved Project Identification is available at: http://www.thegef.org/gef/sites/thegef.org/files/gef_prj_docs/GEFProjectDocuments/Multi%20Focal%20Area/Antigua%20And%20Barbuda%20-%20%285390%29%20-%20Sustainable%20Pathways%20-%20Protected%20and%20Renewable%20Ene/04-17-13%20PIF%20and%20PPG%20doc.pdf The project is in a preparatory phase during which the project budget, co-finance and outputs are being established prior to GEF Chief Executive Officer endorsement and receipt of the full project funding.

This consultancy focuses on the pumped hydro energy storage for wind and solar power generation in Antigua. The consultant will work under the supervision Thomas Hamlin of the Division for Sustainable Development UN Department of Economic and Social Affairs, and Fitzmaurice Christian Environment Division, Ministry of Agriculture, Lands, Housing and the Environment. The consultant will work as a team with the above and local consultants hired by the government of Antigua & Barbuda in parallel. The consultancy may be supported by a team of people from the proposing firm in order to cover the electrical as well as hydro aspects.

II.                  Objective

To determine the prefeasibility of pumped hydro and ancillary equipment as required for a spinning reserve energy storage mechanism in Antigua.

III.                Description of the Required Services

The consultant will design and cost scenarios selected from the attached table of pumped hydro facilities, with associated electrolytic capacitors and control equipment to permit shut down of diesel spinning reserve behind wind and solar.

The following steps are envisioned:

1. An Environmental Impact Assessment screening will be executed by Environment Division in parallel with the work of the tendering firms and local engineering firm, and the outcome will be accommodated in the design and location of the facilities.

2. Environment Division will obtain concurrently measured sub-hourly SCADA diesel power output and demand data, wind data and solar data, for a period of at least one year from APUA. UNDESA will assemble the data into HOMER, run scenarios of solar/wind/ pumped-hydro storage and make available the running model and/or validate the nominal installation sizes identified in (3.). HOMER does not consider dynamic transient effects below several minutes.

3. The proposing consultants are requested to prepare a “water to wire” technology package cost estimate, plus reservoir repair (Fishers), penstock, trash grate, sub-station and transmission line, installation and commissioning solutions meeting APUA interconnection and control specifications for two sites. An 11kV line is available on site but would require doubling up to carry the output of the larger installation at Fishers. The main 69kV is not far away, both are at 60 Hz. Climate is steady at 20 to 30 C all year. APUA will provide interconnection specifications or equivalency may be agreed for use of specifications used elsewhere.

  • Pump capacity of nominally 500 kW and a generation capacity of nominally 1 MW hydro capacity operating on Brecknocks 1 and 2 reservoirs. with 1.5 MW wind power The static head is 23 m while the upper reservoir, run empty as is usually the case in dry season is about 3 m lower head. Power house would be at 40 m asl.
  • Nominally 5 MW pump capacity and 10 MW hydro-generation with 15 MW wind power on Walling’s to Fisher reservoirs. Static head is 124 m with empty upper reservoir leaving about 120 m head. Power house at 28 m asl.

The proposing consultants may offer advice on the selection of sites and propose additional designs to those requested.

All components are to be costed including turbine, pumps, generator, governor, inlet valve, and local capacitor or other storage needed, control and switch gear, main setup transformer, reservoir repair (Fishers), penstock, trash grate, sub-station and transmission line, installation commissioning and training.

In order to meet the ramp up, voltage and frequency specifications called for by APUA, capacitor or other storage may be required as well as automatic control of pumps and turbines. The pumps are expected to operate as dump loads to smooth out transients in wind and solar power production and therefore may be variable speed and/or multiple units. The proposing consultants will provide their advice in this regard. Consideration should be given for the controller to also manage reverse osmosis as dump load as well.

4. A report will be written by the pumped hydro consultants as an annex to the GEF Project Document and the consultant will assist in writing the climate change sections of the Project Document as well as the request for GEF CEO endorsement. UNDESA and Environment Division will utilise the costing data and performance characteristics to prepare a project proposal to the Global Environment Facility and other financiers to obtain concessional financing for a pilot scale project of 1.5 MW wind, 1 MW pumped Hydro 1 MWh storage. Further feasibility work can be undertaken during the project execution phase.

5. The cost of energy produced will be estimated using the GIZ feasibility study for wind and the cost of energy storage estimated by the consultants for the proposed pumped hydro and wind as a combined system including any short term storage that may be required. Storage costs are to be compared to costs of batteries, flywheels, diesel only and capacitors only. Fuel, maintenance and operations savings as well as any peak production benefits estimated in the APUA fuel fired plants.

6. It is envisioned that the assets would be held by a national environment fund and operated by APUA. Either a partial Build Operate Transfer or a period of training and assistance from the firm providing the technology will be required to build capacity to operate the system. Review of current BOT, and IPP arrangements, estimation of generation, transmission and distribution cost components and drafting of a wheeling fee or feed in tariff agreement will be prepared by UNDESA in collaboration with APUA and Environment Division. Consultants may comment on this work.

7. A report will be written by the consultant as an annex to the GEF Project Document and the consultant will assist in writing the climate change sections of the Project Document as well as the request for GEF CEO endorsement. 

IV.                Outputs

A report on the technology and costed feasibility for two design scenarios including a pilot scale of 1 to 3 MW, and a larger scale 10 to 20 MW.”

In addition a list of potential reservoirs and projects were attached: 

Table 1:           Existing reservoirs potentially for use for pumped hydropower schemes

Reservoir

Elevation

Storage volume

Siltation factor

Catchment yield

Ft asl

masl

iMgal

m3

ikgal/d

m3/d

Brecknocks 1

240

73.15

5.4

24,549

0.35

15

68

Brecknocks 2

131

39.93

20.1

91,376

 

 

 

Hamiltons

154

46.94

27.4

124,563

 

 

 

Diamond Hole

55

16.76

2

9,092

 

 

 

Bendals Pool (damaged)

49

14.94

1.8

8,183

 

 

 

Body Ponds (broken)

77

23.47

3

13,638

 

40

182

Fishers

89

27.13

14.6

66,373

 

 

 

Fiennes and Swetes

105

32.00

9.1

41,369

 

 

 

Bendals WTP (fouled)

36

10.97

1.3

5,910

 

 

 

Dunnings

100

30.48

35.8

162,750

 

 

 

Wallings

495

150.88

13.6

61,827

 

150

682

Fig Tree

370

112.78

0.6

2,728

 

 

  

 

Table 2:           Potential schemes for pumped hydro by use of existing reservoirs

Antigua reservoirs Data provided by APUA, analyzed by UNDESA

New upper –

Hamiltons

Brecknocks 1 – Brecknocks 2

Brecknocks 2 - Diamond Hole + Bendals Pool (repaired)

Wallings - Figtree

Brecknocks 1 - Fisher

Wallings - New Dam below FigTree

Brecknocks 2 - Fisher

Wallings - Fisher

Wallings –

Swetes Feines

Hamiltons - Diamond Hole + Bendals Pool

Power (kW)

10,866

1,026

953

1,307

1,579

8,232

878

10,187

9,786

1,449

Efficiency

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

Head (m)

132

33.22

23.17

38.10

46.03

100

12.80

123.75

118.87

30.18

Gravity (kgm/s2)

9.8

9.8

9.8

9.8

9.8

9.8

9.8

9.8

9.8

9.8

Flow (m3/s)

12

4.5

6

5

5

12

10

12

12

7

Working volume (m3)

70,000

15,957

17,275

2,728

24,549

61,827

15,957

61,827

41,369

17,275

Energy storage (MWh)

17.61

1.01

0.76

0.20

2.15

11.78

1.62

14.58

9.37

0.99

Distance (km)

0.9

0.66

1

0.37

1.1

1.5

0.44

3.55

3.05

1.46

Diameter at 5 m/s (m)

1.75

1.07

1.24

1.13

1.13

1.75

1.60

1.75

1.75

1.34

 

2           Background Information

The client has provided information to the study, which is listed below, and summarized in the following chapters:

Table 3:           List of documents provided

No

Name of document

Date

Study conducted by

1

CREDP: Pre-Feasibility Study for a grid-parallel Wind Park at Crabbs Peninsula, Antigua

Aug 2012

Factor 4 Energy Projects GmbH

2

CREDP: Wind Data Evaluation Crabbs Peninsula, St. Antigua

Aug 2012

Factor 4 Energy Projects GmbH

3

Project Identification Form (PIF): Sustainable Pathways – Protected Areas and Renewable Energy

Apr 2013

GEF

4

Various maps and GIS files, location of reservoirs.

Oct 2013

client

5

ToR for Pumped Hydro Expert Consultancy

Oct 2013

client

6

Crabbs Wind Farm Report – 1st draft Nov 2013

Nov 2013

UNDESA

7

Power plant load data series for 2011

-

Client, email 11.Nov 2013

8

Solar and wind data for 2011 (10min average data)

-

Client, email 11.Nov2013

9

Map 69kV system, feeders near Swetes

-

Client, email 11.Nov2013

10

Time Series Analysis of the Antigua Power System with Wind Solar and Pumped Hydro Energy Storage – Interim report

29th Nov 2013

UNDESA

11

Crabbs Wind Farm Report – 2nd draft Dec 2013

Dec 2013

UNDESA

12

APUA power Output Data 2011

-

UNDESA, email 20.Nov 2013

13

Major Project System Data.docx

-

Environment Division, email 4.Dec 2013

14

Civil Work Local Costs.docx

 

Environment Division, email 5.Dec 2013

15

Analysis of wind speed volatility and estimated bridging power requirements

-

UNDESA, email 13.Jan 2014

16

Wind Farm Layout (preliminary)

-

UNDESA, email 14.Jan 2014

17

Freetown Wind Farm and Pumped Hydro – initial investigation

-

UNDESA, email 16.Jan 2014

18

Crabbs Wind Farm Report – 4th draft Jan 2014

Jan 2014

UNDESA, email 16.Jan 2014

19

Pumped Hydro and Wind sites

-

UNDESA, email 22.Jan 2014

 

2.1       System Load 

The load varies typically between 30 and 50 MW. No distinct seasonal variations have been identified during the year 2011.

 

2.2       Renewable Energy Mix Challenge and Power System Stability

The output of solar and wind power generation varies greatly depending on the weather and wind speeds, which can make connecting them to the grid difficult. EES (Electrical Energy Systems) used for time shift can absorb this fluctuation more cost-effectively than other, single-purpose mitigation measures (smart grid).

The purpose of frequency control and regulating reserves is to keep the balance between demand and supply of electricity. The output of solar and wind power generation varies greatly depending on the weather and wind speeds, which can make connecting them to the grid a challenge to the grid stability. Loss of 10 % generation causes a frequency drop of 3-5 Hz within a minute which requires action from regulating reserves to manage the deficit by primary control.

In order to maintain the balance between electricity generation and demand the variations can be handled either by ramping up or ramping down the generation to maintain the balance between supply and demand.

Hydropower plants have the potential to react extremely fast to all grid requirements and grid disturbances. Also load control by smart grid using electrical energy systems (EES) for time shift can absorb fluctuations more cost-effectively than other, single-purpose mitigation measures. In case of the latter this could also be relevant to consider in Antigua although detailed information on loads has not been possible to obtain. Neither can be expected the development of a smart grid system connecting the consumers in Antigua within the next few years. The single load that possibly could be used for balancing variations in wind is the desalination plant at Crabbs. However, due to lacking information the viability cannot be assessed.

In the following table a there is presented a comparison between suitability of thermal and hydro power plants showing characteristics of the alternatives:

Table 4:           Comparison between thermal and hydro power (source: Andritz Hydro)

 

Thermal plant

Gas turbine

Hydro (standard)

Hydro (advanced)

Power gradient (% min)

2 -4

8 - 12

50 - 100

100 % in

Minimum load (% of P)

40

40

40 (F), 20 (K,P)

0 – 5 (all)

Start-up times

2 -5 h

< 15 min

< 10 min

< 5 min

Reduction of life-time (frequent start/stop, part load)

severe

severe

noticeable

acceptable 

Primary control activates reserves within 5 -30 seconds by generator droop control changing the power output.

 

2.3       Description of Existing Electrical Power System

Currently Antigua is supplied by generation from thermal power. The generators are listed in table 5. The installations are divided between different operators. In accordance with the information received there an underground optical fibre cable system is established between the dispatch centre at Crabbs Peninsular enabling remote control and monitoring by a SCADA system. However, the SCADA system seems to be of two different technical generations, and it is not clear to which degree the two parts are completely compatible with each other. The introduction of new electricity generation like wind plant and a pumped hydro will require an upgrading of the SCADA system which may represent a financial and technical challenge not yet clarified.

Table 5:           Thermal power plant installations

Name of Power Plant

Owner

MVA

SN

(MW)

System Volts

(kV)

Wadadli Power Plant (WPP)

APUA

 

 

10.0

 

#1

 

6.25

5.0

11

#2

 

6.25

5.0

11

Baker

-

 

 

5.1

 

#1

 

6.375

5.1

11

Friars Hill

-

 

 

13.0

 

#1

 

8.122

6.5

11

#2

 

8.122

6.5

11

JV Plant

APC / APUA

 

 

50

 

#1

 

21.345

17

11

#2

 

14.26

11

11

#3

 

14.26

11

11

#4

 

14.26

11

11

Black pine

APC

 

 

28.2

 

#1

 

7.938

6.3

11

#2

 

7.938

6.3

11

#3

 

9.801

7.8

11

#4

 

9.801

7.8

11

#5

 

 

 

 

The conductor size of the 69 kV lines is standardized to ACXSR 150 (see Table 6), but APUA has planned to change the conductor to ACSR 300 in order to improve the reliability of the system. The new conductor will then be able to manage the load in an alternative route in the ring Crabbs – Garden – Belmont – Swetes – Lavington in case of a fault somewhere in the sub-transmission lines. If this upgrading also means changing the earth wire to OPGW (optical fibre ground wire) at the same time is not known. Accordingly, although the loads of the substations is unknown, the lines shall be able to transmit additional power to operate the pumps can be transmitted the 7.3 km from Crabbs to Lavington substation and St. Phillips pumped storage plant (PSP).

Table 6:           Transmission lines

Name of Line

Type of Conductor /

Length of Conductor

Real Value

(ohms)

Per Unit Value

(ohms)

Modulus

Ð angle

Fhill – Belmont

ACSR-150/ 8.4kM

 

0.738 + j0.657

2.28 Ð61.290

Cassada #1–Crabbs #1

ACSR-150/ 9.3kM

5.18 + j4.90

0.1088 + j0.103

7.13Ð43.430

Cassada #2–Crabbs #2

ACSR-150/ 9.3kM

5.18 + j4.90

0.1088 + j0.103

7.13Ð43.430

Cassada #2– Fhill

ACSR-150 / 4.2Km

0.90 + j1.62

0.019 + j0.034

1.85Ð60.800

Crabbs - Lavington

ACSR-150 / 7.3km

1.57 + j2.81

0.033 + j0.059

3.22Ð60.780

Lavington – Swetes

ACSR-150 / 7.0kM

1.476 + j2.67

0.031 + j0.056

3.05Ð61.030

Swetes – Belmont

ACSR-150 / 8.5kM

1.809 + j3.24

0.038 + j0.068

3.708Ð60.800

Belmont – Five Islands

ACSR-150 / 5.2kM

1.095 + j1.99

0.023 + j0.042

2.28 Ð61.290 

The transformers of the substations are listed below in Table 7. The transformers in the substations are a standard 10 MVA. In case of a 10 MW PSP at St Phillips, the required new 66/11 kV transformer installation need a capacity above the standardized substation size, eventually also with a spare transformer. The present load at Lavington substation is unknown.

 

Table 7:           Substation transformers 

Name of Substation

MVA

Connection Mode

Voltage Ratio

Ud%

P.U. Value

Cassada Gardens #1

10.0

Y / Y0 – 12

69/11 ± 8 X 1%

7.5

J0.75

Cassada Gardens #2

10.0

Y / Y0 – 12

69/11 ± 8 X 1%

7.4

J0.74

Lavington

10.0

Y / Y0 – 12

69/11 ± 8 X 1%

7.44

J0.744

Swetes

10.0

Y / Y0 – 12

69/11 ± 8 X 1%

7.43

J0.743

Belmont

10.0

Y / Y0 – 12

69/11 ± 8 X 1%

7.435

J0.7435

Five Islands

10.0

Y / Y0 – 12

69/11 ± 8 X 1%

7.46

J0.746

Crabbs #1

11.67

Y / D - 1

69/11 ± 8 X 1.25%

8.67

J0.743

Crabbs #2

11.67

Y / D - 1

69/11 ± 8 X 1.25%

8.83

J0.757

Crabbs #3

--

Y / D - 1

--

7.36

J2.944

Crabbs #4

36

Y / D - 1

69/11

8.0

--

Crabbs #5

36

Y / D - 1

69/11

8.6

--

Crabbs #6

63

Y / D - 5

69/11

11.27

--

Crabbs #7

63

Y / D - 5

69/11

11.27

-- 

2.4       Wind data

The wind measurements recorded (2010 – 2012) at Crabbs, Guinea B, McNish, Freetown and Barbuda show that Crabbs yield the highest values and is accordingly the prioritized site for the planned 18 MW wind farm. Although the optimisation of the site for the wind turbines is clear, this may not be so clear when it comes to system optimisation. It is assumed that separation of the turbines in two sites, for instance Crabbs and Freetown/St Phillips, would certainly reduce the expected sub-minute correlation between wind variations. This implies that combined operations would likely result in benefits and finally reduce the balance requirements. 

2.5       Solar data

A data set of solar measurements for 2011 in 10 minute intervals is made available. The mean daily solar intensity has a clear pattern, and does not change much over the year, however, November to January seem to have lower intensity than the rest of the year in 2011.

3           Proposed Concept Options 

3.1       Wind Power at Crabbs Peninsula 

Wind power density ranges from 350 to 400 w/m2 at 80 m height across the planned wind farm site at Crabbs Peninsula. Measurements have been taken for 2011 with 10 minute average values. However, wind fluctuations are expected to be much more frequent than 10 minutes. 

The simulated fluctuation in power production (Figure 6) shows that the ΔP (generation capacity) varies basically between -10 MW (decrease) and + 10 MW (increase) at 5 seconds averages for the 15 MW windmill plant. The measurements are taken at the 10 m high wind gauge at the V C Bird Airport, and show quite high fluctuations due to the low location of the wind gauge. Still it is expected that the wind farm will have abrupt changes in the wind speed and power.

The distance from one end to the other of the line of planned windmills is approximately 1000 meters (Figure 9). This appears to be very close between each mill. Power output fluctuations are not completely independent, but they are not completely correlated. The degree of correlation between the wind turbines depends on the time interval being used and the physical separation. The faster fluctuations (regulation) are less correlated than slower fluctuations (load following) as shown in an example in Figure 8. 

3.2       Flywheels

Supply of electricity rarely matches the demand and it results in variation of the power system frequency. In Antigua the allowed variation to the 60 Hz is 3 % or 1.8 Hz. The use of flywheels is a mechanical battery (massive rotating cylinder) to regulate the frequency so it keeps within the limits. A major benefit of fast-response fly-wheel based regulation is that it is far more effective than slow-response resources. Although the costs are essentially higher than a pumped storage plant it may be under specific circumstances be a supplementary power system balance alternative. Although 18 MW wind plant installation and planned installation of a 5 MW solar installation in a system having a peak load of close to 50 MW has a very percentage of renewable energy, there are at the moment no indication that such supplementary installation should be necessary. A pilot aspect of the planned wind-hydro project may also be that in case problems may arise later flywheels are fast and easy to install as separate energy storage.

4           Technical Conceptual Layout Optimisation of Pumped Hydropower Scheme

4.1       Technical Approach and Assessment

4.1.1  Regulation Challenges

The integration of renewable energy resources as wind and solar into the power system represents a challenge regarding balancing the power system due to the variability of these energy resources. The introduction of the renewable energy components (solar, wind park and pumped hydro power plant) would increase the maintenance and operational requirements. From the brief visit to the Crabbs Peninsular it seems apparent that the communication and SCADA systems need extension and upgrading.

One of the main questions arising is the wind plant installation at Crabbs peninsular. The estimated distance between the first and second row of windmills is 500 m and with a wind speed (Vestas) of 13 m/s, the delay between the two rows is approximately 40 seconds. To some degree this may reduce the requirements of balancing the power system. With reference to figure 8 (correlation) it is an alternative to assess the possibility to split the wind park between Crabbs Peninsular and somewhere near the pumped storage plant. The wind measurement campaign indicates that the wind conditions may not be very different (Freetown near to St. Phillips). Assumedly the distance between the two places is large enough to reduce the wind correlation essentially resulting in reduced balancing requirements. Further wind measurements with much less time average value logging than the 10 minutes and a stability analysis can finally clarify the feasibility of the alternatives. 

In addition further investigations on the demand side would be required to finally conclude if the opportunity of balancing also could be supported by disconnection/connection of loads. One of the main single loads is the desalination plant at Crabbs Peninsular.

Having investigated these opportunities it would be possible to conclude on needs of additional actions, for instance flywheel installations, although this is not considered probable.

4.1.2  Potential Locations

The terms of reference has identified two pumped hydropower schemes;

  • A pilot pumped hydropower scheme with 1 MW generation capacity, 0.5 MW pump capacity and 1.5 MWh storage capacity, to balance 1.5 MW wind power.
  • A pumped hydropower scheme with 10 MW generation capacity, 5 MW pump capacity and 15 MWh storage capacity to balance 15 MW wind power.

In the terms of reference to the study the existing water supply reservoirs; namely between the Wallings and Fischers reservoirs (10 MW) and Brecknocks 1 and Brecknocks 2 reservoirs (1 MW).

However, during the site visit to the mentioned sites in December 2013 they were not found suitable for pumped hydropower schemes. The main argument is that the waterway (pressurized pipe) length will be very long with high costs and poor hydraulic waterway stability. The waterways (long pipe, low head) are actually not feasible for a fast responsive pumped hydropower scheme; the waterway does not allow fast uploading of downloading without breaking physical laws such as pressure below vacuum.

A further complication was the competition of the use of the reservoirs as a pumped hydropower scheme will occupy two reservoirs which effectively cannot be used for domestic water supply. While the water could alternatively been produced by the desalination plant at an extra cost, it is clearly a disadvantage and potential conflict of interest. The reservoirs are also prone to periodic draughts and no water available for pumping; in fact at the time of the site visit in Dec 2013 most of the reservoirs water was close to empty, in particular Wallings reservoir was very dry. This will imply that a pumped hydropower scheme and therefore the wind power plant will not be in operation for weeks or months in droughts, which is found to be an unacceptable scenario.

Alternative locations were investigated by the joint team of the Client, APUA and the Consultant, and two new locations were found interesting for further studies. These are:

  1. Lower reservoir at existing Dunnings Reservoir and a new upper reservoir at the top of the mountain McNish. The Dunnings Reservoir is a reservoir which catches rainwater for subsequent water treatment and domestic use. At the time of the site visit APUA was not using the reservoir much due to salt intrusion in the water.
  2. A location near St. Phillips which is utilizing the sea (near Lunch Point in Willoughby Bay) as the lower reservoir and construction of a new reservoir on top of the ridge. 

4.1.3  The design challenge 

Pumped storage hydropower schemes can be designed to meet various challenges:

  1. Long terms storage, typically seasonal storage over months
  2. Short terms storage, typically peaking power over a 24 hour cycle
  3. Very short term storage, typically to smoothen out for undulating power supply and demand, in our case wind power. Response time is typically achievable down to 10-30 seconds
  4. Grid stabilizer. The rotating parts of the equipment will act as rotating reserve in the grid and aid in stabilizing the grid. Response time will be in some few seconds.

In our case the overall important task for the pumped hydropower plant in Antigua, is to mitigate undulating power from the proposed 18 MW wind farm. The response time will therefore have to be extremely fast: down to

4.1.4  Turbine and Pump Setup

There are many combinations of pumps and hydro turbines and penstocks which each have their advantages and disadvantages, and each project will have to find the most technical and economic feasible solution while maintain the critical design parameters. 

The critical parameter in this case is to have maximum availability for the pumped hydropower scheme on the grid at full capacity. 

Two principles can be chosen for the pumped hydropower project: 1) only use the generating unit(s) to match the undulating wind power, and top up by using a pump when power is in surplus where the pumps typically have a smaller installed capacity than the turbines, and 2) use the generating unit(s) for stabilizing the undulating wind power until the storage is empty, and then switch to pumping mode which then acts as a dumping load to the undulating wind power until the reservoir is filled up and switching to generating mode again.

In this case the storage volume is relatively small and the alternative of having separate generating mode and pumping mode is chosen.

 Please note that the wind power in the above will have a much higher frequency in the power undulation; the figure has undulations in a lower frequency for readability and showing the principle in alternating compensation by the hydropower turbine and pump.

 

Number of Penstocks

Number of Turbines

Number of Pumps

Availability

Cost

Comments

1

1 pump-turbine

-

Very Low

Low

Reversible Pump-Turbine (RPT). Long ramp-up time between turbine and pump mode (3-5 minutes)

1

2 pump-turbines

-

Very Low

Medium Low

2 RPTs, gives slightly more flexibility for undertaking maintenance, as one unit can be maintained while 50% plant capacity is still available.

Only one penstock greatly reduces flexibility

1

1

1

Very Low

Medium Low

Large and more costly pump and turbine and civil works, with higher availability of potential manufacturers than dedicated RPTs.

Only one penstock greatly reduces flexibility

2

1

1

Medium

Medium High

One penstock for pumping and one for generation allows the transition between pumping and generating very low with overlapping pumping and generating.

Single units will have poor efficiency at the lower power range resulting in costly operation in the low range of operation

2

2 pump-turbines

-

High

High

2 RPTs. Flexible solution, provides very short transition time between turbine and pump mode as one unit can overlap the other. However, in the transition the capacity is only half of full capacity.

2 units give better efficiency and lower operation costs in the lower power range

2

2

2

Very High

Very High

Very flexible solution, provides very short transition time between turbine and pump mode due to the two penstock.

2 units give better efficiency and cost at the lower power range.

 

 

 

It is important to note that the ramp-up time for a pump turbine i.e. changing from pump mode to generating mode will take several minutes. The best industry standard time for switching between full generating mode and full pumping mode is now down to less than 5 minutes for large pumped hydropower plants with help by converters, probably a little faster for smaller units in the range of 10 MW. It is still too long time without stabilizing the undulating wind power.

 

Therefore the layout option of 2 parallel penstocks has been chosen. Only a single unit for pumping/generating is not flexible enough in terms of maintenance and operation cost in the lower power range.

 

The 2 unit reversible pumped turbine setup is chosen which has lower construction costs than 2 units with separate pumps and turbines.

 

However, the conceptual layout choice should be revisited at a later stage when more exact wind power data and power system data is available and the pumped storage plant can be further optimized, inclusive the size of the reservoir.

 

 

 

Figure 16:   Main figures for the simulated 5 MW reversible pump-turbine that has been used for all the simulations in ALAB.

 

 

 

4.1.5  Preliminary Waterway Stability Analysis

 

It is important to understand purpose for the pumped hydropower scheme, which is to work together with the undulating power generation from wind power plants and solar power plants.

 

The pumped hydropower plant therefore has to respond very quickly to change in supply, which in turn requires the pumped hydropower plant to have advanced technical specifications tailor made for very quick response. Reference is made to Table 4. Both the electromechanical design need an advance technical specification but also a fast responsive waterway. The waterway therefore should be as short as possible, i.e. steep and short pipe, and as slow velocity as possible, i.e. big diameter.

Preliminary simulations was been done by Alab. Alab is a program that does preliminary turbine designs and waterway analysis. This program makes is possible to design a specific turbine and water ways in order to assess the stability of a specific power plant. In this particular analysis we have used Alab to try to optimise the number of units and waterways.

 As the input particularly regarding the required response time and response load is not known, a change of load of 5% to 50% has been adopted for comparison of the two sites, only to find out which penstock is most flexible. Further the analysis has been done on isolated grid operation, i.e. with no stabilizing inertia from the grid and equipment connected to the grid, which is not the case for Antigua. Again, the aim is not to do an exact simulation but only to single out the most flexible site of McNish - Dunnings and St. Phillips.

The pumped hydro scheme in Antigua will never operate in its own isolated grid, but will receive help from the main grid in order to keep the frequency at 60 Hz. The wind turbines must be specified to maintain frequency and the pumped hydro will take care of load variations. Because of this, the main concern for the design of the RPT is load variation on the cost of frequency stability.

 The red curve in the figure above is the estimated frequency and the blue lines are the calculated convergence lines. Since the blue lines are converging fast the waterway is stable. At a later stage, more detailed simulations and calculations will optimize the waterway and turbine regulator in such a way that the maximum deviation will be less.

The two sites have both several features that have been evaluated for choosing the best site, see the table below.

Table 9:           Comparison of the two potential sites for pumped storage hydropower scheme

 

Object

McNish – Dunnings

St.Phillips sea water

Waterway stability

Stable, but longer.

In order to make a stable penstock a diameter of 2m is applied over the 1.3 km length. This give a cost of 6 MUSD more for this option

Very Stable and short

A diameter of 2.0m has been applied.

Access to water

Limited.

There will be periods in the year where there is no water in the Dunnings catchment area. Further the APUA Water might in certain situations request for the same water for domestic use. A consequence of the uncertain access to water will result in the pumped hydropower plant to be shut down in periods, and cannot act as a stabilizing factor in combination with the wind plant.

Unlimited

Transmission line and connection

The length of the transmission line to connect to the grid is more or less the same and the upgrades required in the grid inclusive automation (SCADA) is more or less the same.

The transmission line length is about 6.5 km

The transmission line length is about 5.5 km

Geology/topography

No detailed geologic assessment has been done. It is assessed that the geology has less overburden and probably higher excavation costs for the penstock.

Favourable.

No detailed geologic assessment has been done.

The project is more compact.

Electromechanical equipment

Higher costs due to separate pump and generating units.

The gross head of about 275 m has the option of using Pelton turbines for generation and separate pump units for pumping. In such a setup the Pelton turbine can run without load (spinning reserve) and without using water in contrast to a Francis turbine. It can therefore in certain situations use less water. On the other side the powerhouse will have to be bigger to accommodate separate units for pumping and generation.

Higher cost due to sea water.

The gross head of approximately 100 m dictates a Francis turbine, possibly combined with a pump as a reversible pump turbine, which requires less space resulting in less cost for the building. 

The conclusion is that St. Phillips is the best location for pumped storage hydropower scheme in Antigua.

4.1.6  Pilot scheme 

The estimated electromechanical cost of a 1 MW pilot scheme is estimated to be approximately USD 900.000, which is higher than for the 10 MW scheme and less flexible (one unit). Although no detailed cost estimate is made, it is expected that a 1 MW scheme is more expensive.

If a 1 MW plant is built and a decision to build a full scale plant is made at a later time, then the 1 MW plant cannot be up scaled to 10 MW. This is because the sizes of these two plants are so different that it is impossible to install 10 MW in the powerhouse built for 1 MW. If the power house for a 1 MW plant is built large enough to facilitate a future 10 MW plant, then the costs of this powerhouse will be so large that it would be hard to defend.

The 1 MW will yield very little stabilizing power to the grid, being so small. A better option would be to use the existing grid and thermal power to stabilize a small wind farm of 1.5 MW, which in practical sense will be one wind mill and save the high cost of a small pumped storage hydropower plant.

An argument in previous reports has been that the renewable energy of wind in the energy mix should by rule of thumb less than 15%. With a peak demand of 50 MW this translates to 7.5 MW, therefore a pilot scheme of 1.5 MW wind power should be easily managed by the existing grid without a small and expensive pumped storage hydropower plant. By using off-peak power demand of 30 MW the limit is 4.5 MW.

The better option for the pilot project is therefore to install only the wind power plant in the range of 1-5 MW, learn from the operation of the wind mills by recording wind and power undulations and grid stability parameters, and use that data for the next renewable energy implementation stage (10‑30 MW?) which includes pumped storage hydropower project.

4.2       Conceptual Layout

4.2.1  Civil Works

Dam

The dam of earth fill type is made by use of the excavated material in the reservoir. It is important to note that the water level in the reservoir will be raised and lowered fast and often and the water is salty and require proper design measures. The sealing of the reservoir is by 2 mm thick geomembrane and the ground water level is kept stable and low by a grid of drainage pipes under the membrane. The reservoir has to be fenced off to avoid accidents for operation personnel and the community. The reservoir has a 70,000 m3 storage capacity.

No site investigations were made or made available for the reservoir location. Further investigations and optimisation has to be done in the next phase of the study.

Intake

The intake is typical concrete section with two parallel intakes with separate intake gates and trash rack and a joint gate house. A double set of water level gauging instruments will be included for safe operation inclusive too high water level safety systems to avoid overtopping hazards.

Penstock

Two parallel GRP (Glass fibre reinforced polyester) pipes with the diameter of 2.0 m each will connect the reservoir to the power plant. It is preliminary assumed that PN 16 (16 bar) is enough for the site, while PN 25 is now available in the market. The penstocks are buried for foundation in their entire approximately 230 m length.

Power house

The power house is to accommodate 2 reversible pump-turbines of Francis type and 5 MW capacity each, with all auxiliary systems. The current setup has the turbine centre at elevation at 1 m below the tailrace water level. This implies that the generating/pump unit is installed at a deck below the sea water / ground water level and requires appropriate foundation to anchor the power station against uplift. Preliminary design high surge water level is chosen at 2 m above sea level.

The house has a lowered machine hall, an erection bay, storage rooms, control room and outdoor switch gear and transformer.

Access

A new access road has to be built sloping down the St. Phillips / Freemont ridge down the sea shore and power house location. Another road will give access to the dam and reservoir.

Area for storage, camps, workshops and offices for the construction work has not been assessed.

4.2.2  Electro Mechanical Works

The starting current of a motor will without appropriate installations be considerably higher than the normal operating current. In the case of a 2 x 5 MW pumped storage plant where the reversible pump turbine installation is high, the current surge would represent a problem to APUA power system in Antigua. Also frequent start-up and stops imply mechanical strains that should be avoided for operational and maintenance purposes. The design shall accordingly include control and protection equipment to manage the critical values including change-over time in the pump mode – turbine mode process. It is expected frequent stop and start of the units to follows the generation (weather) profiles. This will imply fast change-over times with full load in less than 30 seconds. This means variable speed units and application of converter technology.

Reversible Pump Turbine

The Reversible Pump Turbine (RPT) is a specially designed turbine which can function as either a conventional turbine or as pump utilizing the same waterway. This is a very flexible solution and if used in Antigua, it will ensure a small and efficient powerhouse. The downside to the RPT is that one loses some efficiency at best operating point compared to a conventional Francis turbine. 

The reason that the RPT is recommended for the Antigua Pumped Hydro Scheme is that if the power house is equipped with 2 x 5 MW RPT with individual penstocks, then the turnaround time from generating mode to pumping mode is very short.

If the turbine is equipped with a high pressure air system to force the water away from the turbine runner and the turbine inlet valve is closed, then the generator can be synchronized with the grid and the turbine rotates in air. When the need for power outweighs the need for stability, the air system is turned off and the inlet valve is opened and within a very short amount of time the turbine is producing power. The flexibility of the plant is greatly increased if one of the turbines is equipped with such a system.

 It is recommended that this is investigated more thoroughly in the next phase of the project

Turbine Inlet Valve 

The turbine inlet valve is mounted as close the turbine as possible and makes sure that it is possible to stop and drain the turbine without emptying the penstock. For both proposed sites this valve will be off the shelf type valves. For St. Phillips this valve will be a butterfly valve and for the McNish – Dunnings site, the valve will be spherical valve. The reason for different valves for the 2 locations is just due to the different head of the sites. 

The valves will be opened by a hydraulic actuator and closed by counterweight. This is done to ensure that the valve will close if the hydraulic system fails and thus preventing the turbine to reach runaway speeds.

Intake gates

The intake for each penstock has to be equipped with intake gates in order to drain the penstocks without emptying the upper reservoir. It is recommended that these gates will be self-closing roller gates. This is because in the unlikely event of a penstock rupture, the gate will close.

Tail race gates

Each of the tail races from the turbines should be equipped with gates. These gates can be sliding gates. A sliding gate is not self-closing, but needs to be closed by use of external force. The tail race gates provides to possibility to close and drain on of the turbines regardless if the other unit are running.

Trash racks and fish trap

Both penstocks and the tail race should have trash racks installed in order to keep larger foreign objects out of the water way. It is also recommended that the need for a fish trap in the tail race is investigated. This is because the tail race ends in the ocean and fishes might swim or be force into the turbines where the fish might clog filters or cause other problems.

 

Flywheel on the generating units

It is an option to add flywheel weight on the generator / motor. This, however, has to be assessed at a later stage when the requirements form the wind farm and grid stability analyses are available.

 

4.2.3  Power Transmission Works

With the reservation that the loads of the substations are not known, it is assumed from the transformer installations that the 69 kV lines do not suffer from extraordinary loss or voltage problems under normal operational conditions without faults. The planned upgrading of the lines to ACSR 300 is understood to be general encompassing all lines to meet the n-1 criteria in the 69 kV ring. In accordance with this understanding the transmission capacity to operate the two 5 MW RPTs’ at St. Phillips should not represent any major problems provided the necessary installations at the RPTs are made to minimize the starting current of the pumps.

However, to transmit power to and from the pumped hydro plant via Lavington substation, additional transformer capacity is needed. It is consequently expected to construct a 69 kV tee-off line with the approximately length of 5.5 km from Lavington to St. Phillips and a 69/11 kV transformer at the power station see also 4.1.2. The sizing of the transformer depends on the maximum current of the pumps, possible installation of wind turbines in the area, see 4.1.1 and one or two transformers taking into consideration spare requirements.

5           Cost Estimate and Energy Production Estimate

5.1       Cost Estimate

The plant is designed to match very undulating flow from wind power, which includes a favorable site, twin penstocks and advances specification for the electromechanical equipment for fast and flexible operation.

The cost estimate is based on the consultant’s data base for hydropower projects and civil unit rates from Antigua.

 

Table 10:         Cost estimate for a 10 MW pumped hydropower project at St. Phillips

Item

Cost

(mill. USD)

Civil works

 

Access roads and preliminaries

0.5

Dam and reservoir works (70,000 m3 / 15 MWh reservoir volume and 6 m regulation height)

3.8

Intake works, inclusive hydro mechanical works

1.1

Penstock, inclusive the pipes for 230m two parallel Ø2.0m GRP pipes

1.0

Power station

1.8

Subtotal

8.2

Electromechanical works

 

Complete 2 x 5 MW reversible pump-turbine units, for both generation and pumping inclusive step-up transformer to 66 kV, inclusive inlet valves and tailrace gates

7.6

Transmission works

 

5.5 km 66 kV transmission line to Lavington Substation, line feeders and sub-station works

1.3

Subtotal construction costs

17.1

Administration, design, supervision (12%)

2.0

Contingencies (20%)

3.4

Construction cost

22.5

The cost estimate does not include any financial costs, compensation or taxes.

The cost does not include any upgrade on the existing 66 kV transmission grid if needed, or any upgrade to the required SCADA communication of the transmission system.

A preliminary cost estimate for the McNish- Dunnings option gives a construction cost of approximately 30 MUSD, i.e. 30-40 % more expensive and less flexible.

5.2       Energy Generation and Consumption 

5.2.1  Wind Power Stabilizer 

There is no net energy generation in the chosen project as it is no natural inflow to the reservoir.

There is no catchment and natural inflow to the reservoir, the scheme is purely a pumped storage hydropower project for short term peaking power and grid stability. The plant has efficiency loss in both pumping and generating mode which results in a net loss of energy in one cycle of filling and emptying the reservoir.

The benefit of the plant has to be seen as a package with other renewable energy plants (i.e. wind power) as a necessary power stabilizing component for larger wind farm installations. There is no net energy generation but net energy consumption. 

5.2.2  Energy Consumption 

The energy consumption is not

The reservoir size is 70,000 m3, which translates to 16 MWh generating storage capacity and approximately 22 MWH pumping storage at full capacity. 

It should be noted that in order to at all times have ready capacity to stabilize the undulating power form the wind plant, the pumped storage hydropower plant has to be either in pumping mode or in generating mode:

  1. Generating mode: The wind farm produces as much as there is wind available. In case the wind drops down the turbine will compensate by increasing generation. It is important to note that this will have to happen very fast, within seconds, and the therefore the turbine has to be rotating and synchronized to the grid ready for action.
  2. Pumping mode. The wind farm produces as much as there is wind available, with part of the wind power runs the pumps as a dump load. Even here it is necessary for the pump to be rotating and ready to take on load, if there is surplus wind power.

Table 11:         Relationship between storage capacity and generating/pumping time

Scenario

Time for using the storage

capacity (70,000 m3)

Energy used for the storage

capacity (70,000 m3)

Generating

 

 

Generating constant at maximum capacity (10 MW)

1.6 hours

16 MWh

Generating constant at idle speed, 1 unit (0 MW)

40 hours

0 MWh

Generating undulating between minimum and max (0-10 MW, average 5 MW)

3.1 hours

15.4 MWh

Generating undulating between minimum and 25% power (min to 2.5 MW)

10.7 hours

13.4 MWh

Generating undulating between 75% and max power (7.5 - 10 MW)

1.8 hours

16.1 MWh

Pumping

 

 

Pumping constant at maximum capacity (10 MW)

2.2 hours

- 22 MWh

Pumping constant at minimum capacity, 1 unit (2 MW1))

20 hours

- 42 MWh

Pumping undulating between minimum and max (2-10 MW, average 6 MW)

5.4 hours

- 23.5 MWh

Pumping undulating between minimum and 25% power (2 to 4.5 MW)2)

8 hours2)

- 26.4 MWh2)

Pumping undulating between 75% and max power (7.5 - 10 MW)

2.6 hours

- 22.3 MWh

Generating and Pumping

 

 

One cycle with maximum undulation (min – 10 MW)

8.5 hours

- 8.1 MWh

One cycle with using a span of 2.5 MW undulation (generating 0 - 2.5 MW, pumping 7.5 - 10 MW)

13.3 hours

- 8.9 MWh

1)     Lower pump capacity can be achieved by installing an additional smaller pump(s) or a water bypass which leads the water back to the sea during very small loads (no pumping of water to the reservoir, it will act as a dump load)

2)     With current setup between 2.0 and 4.5 MW, this can be improved by means described in note 1) above.

It is very difficult to know how the wind undulates and how much the pump/turbine will have to work and use of the storage. As can be seen in Table 11 a theoretical full cycle time is between 3.8 hours and 60 hours. In a period with a lot of undulating wind and full use of the pumped hydropower plant a cycle can take 8.5 hours or 3 times a day. 

A more comprehensive study has to be done to confirm how many cycles the reservoir will be emptied and how much energy is used for it. As an experiment we can guess some input data and find the corresponding energy use:

Assumptions: The plant’s capacity is fully utilized 10% of the time in a year from minimum capacity to 10 MW capacity, while 90% of the year the plant is only operating in a 2.5 MW span. This will imply 876hours / 8.5 hours x 8.1 MWh + 7884 hours / 13.3 hours x 8.9 MWh = approximately 6 GWh/year. For comparison a wind plant of 18 MW is assessed to produce about 70.1 GWh (reference: Pre-Feasibility Study for a grid-parallel Wind Park at Crabbs Peninsula, Aug 2012, Factor 4 Energy Projects). 

Comment to the above calculation: There are most likely other combinations that are more optimal in the combination of pumping and generating mode: e.g. shorten the time for the pump mode and lengthen the generation mode to reduce energy loss. But this has to be done in a later stage, as it is very dependent on the wind power undulations, and should therefore be done after better wind power data is available. If it turns out that the wind is quite stable the size and the cost of the plant can potentially be reduced, or the pump units are separated and smaller units.

6           CO2 Emission and Climate Change

Replacing a certain annual thermal energy production with wind power production, will result in a net reduction in annual CO2 emissions. As the purpose of the pumped hydro system is balancing the wind power output, the wind power plant and the pumped hydro are viewed as one power generating unit in this context.

 Estimating the net reduction in CO2 emissions resulting from the net reduction in thermal generation, requires some basic input data and some general assumptions. The most basic input data required are listed in Table 12. Here, somewhat conservative values (with respect to the level of CO2 reduction) for each of the input parameters are assumed. An important note is that the annual net production delivered to the grid by the wind/hydro system is still subject to a significant level of uncertainty. Hence, the net reduction in thermal production is also uncertain. In addition, the actual reduction depends on the quality of the fuels used and more importantly: the efficiency of the thermal plants that are shut down (fuel/kWh) due to the wind power capacity. This again, depends on which of the existing thermal plants will be held back and to what extent. Despite this range of uncertainties, the estimated total reduction (Table 12) provides an indicator for the size order of CO2 emissions reduction one might expect from an eventual implementation of the wind/hydro concept.

Table 12:     Basic assumptions and estimated annual CO2 emissions reduction due to replacement of thermal power generators (Reference: Engineeringtoolbox.com)

CO2 emissions, annual reduction estimation

 

 

Wind power: Net production (GWh/yr)

70-6=64

GWh/yr

Replacement degree for thermal (%)

100

%

Net thermal production reduction (GWh/yr)

64

GWh/yr

Average fuel consumption* (l/kWh)

0,35

l/kWh

Average emissions (kgCO2/l)

2,7

kgCO2/l

Total fuel consumption (l/yr)

22.4 million

l/yr

Total emissions reduction (tons CO2/yr)

60,000

tons CO2/yr

 *Diesel is the assumed fuel for all thermal generating capacity 

In 2010 the total CO2 emissions from Antigua and Barbuda was estimated to an annual total of 513 000 tonnes (Wikipedia). With reference to Table 12, implementation of the 18 MW wind power plant can reduce total emissions for Antigua and Barbuda in the range of 10 to 12 %.

In addition to the CO2 reduction resulting from replaced thermal power generation, climate gas emissions are significantly reduced from:

  1. Reduced shipping activity in relation to diesel transport
  2. Reduced onshore diesel transport

 An important note is that reduced fuel transport, both at sea and land reduces the overall risk for (and actual) fuel spills to the environment.

 Furthermore, several diesel-based power plants perform on-site fuel processing (refining of industrial diesel oil). Reduced thermal capacity reduces the need for this costly and potentially environmentally hazardous activity.

7           Conclusions and Way Forward

7.1       Conclusions

Pumped storage hydropower plant is a feasible option for compensating for undulating renewable power such as wind power.

A feasible site has been identified near St. Phillips and is utilizing sea water as the lower reservoir and a new upper reservoir of 70,000 m3 is to be built to yield minimum 15 MWh for the 10 MW pumped storage hydropower project. The plant can be connected to the exiting grid at Lavington substation. The project will have to be looked at as a power compensation for the wind farm and not a standalone project. The cost estimate for the pumped storage hydropower plant is 22.5 MUSD for a 10 MW pumped storage hydropower plant with a 15 MWh storage capacity.

To identify the design criteria for the pumped storage hydropower plant more accurate wind data is necessary together with energy system stability analysis and requirements. It is in general advised to spread the wind mills locations to avoid the same wind gust to hit the mills at the same time but generate a smother wind power from the combined wind mills in the wind park(s) reducing the need for compensation by pumped hydro or other means.

A 1 MW pumped storage hydropower pilot scheme is expensive and has limited capacity for stabilizing the grid being a small installation. An optional pilot scheme is to install a few wind mills (2-5 MW?) without compensating means (without flywheels, pumped hydro) but stabilized by the existing grid and thermal plants. All relevant and high quality data will be recorded from the wind mills themselves and the grid to be used for planning of a bigger wind farm paired with a pumped storage hydropower scheme.

7.2       Further Work

Further optimisation of the pumped storage hydropower project is necessary. While a lot of work is needed in topographic survey, geologic investigations and optimisation and design, the most important is to get the design criteria for the plant.

It is necessary to measure wind with a much higher frequency (5 seconds is preferable) and convert this data to the power output of a wind mill park. 

Secondly it is necessary to undertake an energy system analysis including the entire transmission grid, electricity demand and generating capacities, in order to determine the required stabilising criteria for a pumped storage hydropower plant.

An optimisation of a pumped storage hydropower project is likely to include optimisation of the generating and pumping units, both in number and type, and storage capacity. It is noted that there is physical space for a bigger reservoir at the St. Phillips site. A correct optimisation will require better wind power data with higher resolution than what is available today, to determine max capacity but also how often the pumped hydro scheme will have to operate on low load.

 


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Gilbert Agricultural and Rural Development Center (GARD)Unep UnepCaribbean Agricultural Research and Development Institute (CARDI) Antigua & Barbuda National Office of Disaster Services Environmental Awareness Group Antigua
 
National Parks Antigua Caribbean community climate change centre
 
Antigua $amp; Barbuda's Environment Division