Performance and Cost Discussion

The most productive hours of sunlight for PV systems are from 9 AM to 3 PM. Before and after these times, electricity is generated, but at much lower levels [8]. In addition, an afternoon thunderstorm will severely reduce local PV output before it will indirectly reduce the load by cooling ambient temperatures and suppressing solar heat gains. This ha s profound technical impacts that can negate some of the benefits associated with distributed, grid-connected PV. A n hour of energy storage can alleviate this problem [9].

INDICATOR

NAME

UNITS

Base Case 1997

| +/- %

2000

| +/- %

2005

| +/- %

2010

| +/- %

2020

| +/- %

2030

| +/- %

Plant Size

kW

30

30

30

30

30

30

Battery Subsystem

Type

Lead-acid

Lead-acid

VRLA

VRLA

Adv. Battery

Adv. Battery

Units Per Year

Each

5

50

200

200

200

200

Performance

Battery Replacement

Years

3

5

5

10

10

10

AC-to-AC Efficiency

%

76

78

78

80

80

80

Discharge

kWh/day

30

30

30

30

30

30

Availability

%

90

90

90

90

90

90

Annual Energy Delivery

MWh

2.7

2.7

2.7

2.7

2.7

2.7

Energy Footprint

kWh/m2

13

13

15

15

26

26

Selling Price

Battery

$/kW

350

200

10

300

15

275

20

300

30

275

30

Power Conditioning

650

600

10

550

15

500

20

400

30

300

30

Max Power Tracker

700

675

10

650

15

625

20

575

30

500

50

Balance of Plant

350

325

10

300

15

275

20

225

30

200

30

Total Capital Requirement

2,050

1,800

1,800

1,675

1,500

1,275

Unit Operations and Maintenance Cost

Fixed Costs

$/kW

Cooling

18

18

18

18

18

18

General Maintenance

33

33

25

25

17

17

Variable Costs

^/kWh

Charging

(delivered)

2.1

2.0

2.0

2.0

2.0

2.0

Battery Replacement

52

44

67

30

33

30

Operations and Maintenance Cost

Fixed Costs

$/yr

Cooling

548

548

548

548

548

548

General Maintenance

1,000

1,000

750

750

500

500

Variable Costs

$/yr

Charging

56

55

55

54

54

54

Battery Replacement

3,500

1,200

10

1,800

15

825

20

900

30

825

30

Annual Operating Costs

$/yr

4,600

2,800

3,200

2,200

2,000

1,900

Notes:

1. The columns for "+/- %" refer to the uncertainty associated with a given estimate.

2. Battery system installation requires several hours.

PV/Battery Sizing

There are different approaches to sizing batteries for PV applications. For stand-alone applications, some syste m developers have sized batteries to provide up to seven days of back-up. Examples include the following militar y installations:

• Navy facilities at China Lake (334 kW PV/3,500 kWh battery) and San Clemente Island (94 kW PV/2,500 kWh battery) in California

• Air Force facilities in Idaho (78 kW PV/700 kWh battery)

• Army training areas in Hawaii (5 kW PV/600 kWh battery)

• Marine tank target range in California (69 kW PV/2,000 kWh battery)

Sizing strategy for grid-connected PV installations depends on the uses of the system and the tariffs available from the local utility. For example, power quality applications require batteries sized to provide nearly instantaneous full-power discharges for only 15 minutes of back-up. A peak shaving application for a PV system may require the battery t o boost the output of the array to meet peak loads for 1-2 hours a day. If the differential between peak and off-peak electric rates is not significant, then the battery can be sized for one hour of operation and the facility owner ca n purchase power from the grid when the PV array is not available. However, if the differential between peak and off – peak rates is significant, then an economic analysis should be undertaken to determine the optimum size of the battery system. For example, the 2.4 kW PV/25.2 kWh battery Salt River Project offered 170/kWh peak, 100/kWh shoulder, and 30/kWh off-peak experimental rates to the PV/battery demonstration it sponsored with EPRI and Sandia National Laboratories. The battery was sized to match the peak electric demand of the home (5 kW) or double the PV output (2.4 kW), in 3-hour load-shifting operations [11]. A number of PV developers optimize the PV installation, but no t the battery system, opting for 7-10 hours of battery back-up power in the event of outages. In many cases, P V installations require only minimal battery back-up to add value to PV-generated electricity. If the transmission system is heavily loaded, batteries can store solar energy which would be lost during hours when transmission service i s constrained, delivering the electricity later [14].

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