T & D - 43) Reliability of Compressed Air Energy Storage Plant

Compressed air energy storage (CAES) plant

Introduction to CAES

As the name implies, the compressed air energy storage (CAES) plant uses electricity to compress air which is stored in underground reservoirs. When electricity is needed, this compressed air is withdrawn, heated with gas or oil, and run through an expansion turbine to drive a generator. The compressed air can be stored in several types of underground structures, including caverns in salt or rock formations, aquifers, and depleted natural gas fields.

Typically the compressed air in a CAES plant uses about one third of the premium fuel needed to produce the same amount of electricity as in a conventional plant.

A 290-MW CAES plant has been in operation in Germany since the early 1980s with 90% availability and 99% starting reliability.

In the U.S., the Alabama Electric Cooperative runs a CAES plant that stores compressed air in a 19-million cubic foot cavern mined from a salt dome. This 110-MW plant has a storage capacity of 26 h.

The 290 MW Huntorf CAES station

The fixed-price turnkey cost for this first-of-a-kind plant is about $400/kW in constant 1988 dollars. The turbomachinery of the CAES plant is like a combustion turbine, but the compressor and the expander operate independently. In a combustion turbine, the air that is used to drive the turbine is compressed just prior to combustion and expansion and, as a result, the compressor and the expander must operate at the same time and must have the same air mass flow rate.

In the case of a CAES plant, the compressor and the expander can be sized independently to provide the utility-selected “optimal” MW charge and discharge rate which determines the ratio of hours of compression required for each hour of turbine-generator operation. The MW ratings and time ratio are influenced by the utility’s load curve, and the price of off-peak power.

For example, the CAES plant in Germany requires 4 h of compression per hour of generation. On the other hand, the Alabama plant requires 1.7 h of compression for each hour of generation.

At 110-MW net output, the power ratio is 0.818 kW output for each kilowatt input. The heat rate (LHV) is 4122 BTU/kWh with natural gas fuel and 4089 BTU/kWh with fuel oil.

Due to the storage option, a partial-load operation of the CAES plant is also very flexible.

For example, the heat rate of the expander increases only by 5%, and the airflow decreases nearly linearly when the plant output is turned down to 45% of full load. However, CAES plants have not reached commercial viability beyond some prototypes.

Resource: Saifur Rahman – Advanced Energy Technologies

Wind Variability

One of the most critical features of wind generation is the variability of wind. Wind speeds vary with time of day, time of year, height above ground, and location on the earth’s surface. This makes wind generators into what might be called energy producers rather than power producers.

That is, it is easier to estimate the energy production for the next month or year than it is to estimate the power that will be produced at 4:00 PM next Tuesday. Wind power is not dispatchable in the same manner as a gas turbine. A gas turbine can be scheduled to come on at a given time and to be turned off at a later time, with full power production in between. A wind turbine produces only when the wind is available.

At a good site, the power output will be zero (or very small) for perhaps 10% of the time, rated for perhaps another 10% of the time, and at some intermediate value the remaining 80% of the time.

This variability means that some sort of storage is necessary for a utility to meet the demands of its customers, when wind turbines are supplying part of the energy. This is not a problem for penetrations of wind turbines less than a few percent of the utility peak demand. In small concentrations, wind turbines act like negative load. That is, an increase in wind speed is no different in its effect than a customer turning off load. The control systems on the other utility generation sense that generation is greater than load, and decrease the fuel supply to bring generation into equilibrium with load. In this case, storage is in the form of coal in the pile or natural gas in the well.

 

An excellent form of storage is water in a hydroelectric lake. Most hydroelectric plants are sized large enough to not be able to operate full-time at peak power. They therefore must cut back part of the time because of the lack of water.

A combination hydro and wind plant can conserve water when the wind is blowing, and use the water later, when the wind is not blowing. When high-temperature superconductors become a little less expensive, energy storage in a magnetic field will be an exciting possibility. Each wind turbine can have its own superconducting coil storage unit.

This immediately converts the wind generator from an energy producer to a peak power producer, fully dis-patchable. Dis-patchable peak power is always worth more than the fuel cost savings of an energy producer. Utilities with adequate base load generation (at low fuel costs) would become more interested in wind power if it were a dis-patchable peak power generator.

The variation of wind speed with time of day is called the diurnal cycle. Near the earth’s surface, winds are usually greater during the middle of the day and decrease at night. This is due to solar heating, which causes “bubbles” of warm air to rise. The rising air is replaced by cooler air from above. This thermal mixing causes wind speeds to have only a slight increase with height for the first hundred meters or so above the earth. At night, however, the mixing stops, the air near the earth slows to a stop, and the winds above some height (usually 30 to 100 m) actually increase over the daytime value. A turbine on a short tower will produce a greater proportion of its energy during daylight hours, while a turbine on a very tall tower will produce a greater proportion at night.

As tower height is increased, a given generator will produce substantially more energy.

However, most of the extra energy will be produced at night, when it is not worth very much. Standard heights have been increasing in recent years, from 50 to 65 m or even more. A taller tower gets the blades into less turbulent air, a definite advantage.

The disadvantages are extra cost and more danger from overturning in high winds. A very careful look should be given the economics before buying a tower that is significantly taller than whatever is sold as a standard height for a given turbine.

Wind speeds also vary strongly with time of year. In the southern Great Plains (Kansas, Oklahoma, and Texas), the winds are strongest in the spring (March and April) and weakest in the summer (July and August). Utilities here are summer peaking, and hence need the most power when winds are the lowest and the least power when winds are highest. The diurnal variation of wind power is thus a fairly good match to utility needs, while the yearly variation is not.

TABLE 1
Monthly Average Wind Speed in MPH and Projected Energy Production at 65 m, at a Good Site in Southern Kansas

	10 m	60 m	Energy		10 m	60 m	Energy
Month	Speed	Speed	(MWh)	Month	Speed	Speed	(MWh)
1/96	14.9	20.3	256	1/97	15.8	21.2	269
2/96	16.2	22.4	290	2/97	14.7	19.0	207
3/96	17.6	22.3	281	3/97	17.4	22.8	291
4/96	19.8	25.2	322	4/97	15.9	20.4	242
5/96	18.4	23.1	297	5/97	15.2	19.8	236
6/96	13.5	18.2	203	6/97	11.9	16.3	167
7/96	12.5	16.5	169	7/97	13.3	18.5	212
8/96	11.6	16.0	156	8/97	11.7	16.9	176
9/96	12.4	17.2	182	9/97	13.6	19.0	211
10/96	17.1	23.3	320	10/97	15.0	21.1	265
11/96	15.3	20.0	235	11/97	14.3	19.7	239
12/96	15.1	20.1	247	12/97	13.6	19.5	235

The variability of wind with month of year and height above ground is illustrated in Table 1. These are actual wind speed data for a good site in Kansas, and projected electrical generation of a Vestas turbine (V47-660) at that site. Anemometers were located at 10, 40, and 60 m above ground. Wind speeds at 40 and 60 m were used to estimate the wind speed at 65 m (the nominal tower height of the V47-660) and to calculate the expected energy production from this turbine at this height. Data have been normalized for a 30-day month. There can be a factor of two between a poor month and an excellent month (156 MWh in 8/96 to 322 MWh in 4/96). There will not be as much variation from one year to the next, perhaps 10 to 20%.

A wind power plant developer would like to have as long a data set as possible, with an absolute minimum of one year. If the one year of data happens to be for the best year in the decade, followed by several below average years, a developer could easily get into financial trouble. The risk gets smaller if the data set is at least two years long.

One would think that long-term airport data could be used to predict whether a given data set was collected in a high or low wind period for a given part of the country, but this is not always true. One study showed that the correlation between average annual wind speeds at Russell, Kansas, and Dodge City, Kansas, was 0.596 while the correlation between Russell and Wichita was 0.115.

The terrain around Russell is very similar to that around Wichita, and there is no obvious reason why wind speeds should be high at one site and low at the other for one year, and then swap roles the next year.

There is also concern about long-term variation in wind speeds. There appears to be an increase in global temperatures over the past decade or so, which would probably have an impact on wind speeds. It also appears that wind speeds have been somewhat lower as temperatures have risen, at least in Kansas. It appears that wind speeds can vary significantly over relatively short distances. A good data set at one location may under-predict or over-predict the winds at a site a few miles away by as much as 10 to 20%. Airport data collected on a 7-m tower in a flat river valley may underestimate the true surrounding hilltop winds by a factor of two.

If economics are critical, a wind power plant developer needs to acquire rights to a site and collect wind speed data for at least one or two years before committing to actually constructing turbines there.

Solar Power Tower

Solar tower - Power plant. In solar power stations, mirrors are used to concentrate sunlight and convert it into thermal energy). This process enables temperatures of more than 1000 degrees Celsius to be achieved, which can be used to generate electricity, among other things.

Solar power towers generate electric power from sunlight by focusing concentrated solar radiation on a tower mounted heat exchanger (receiver). The system uses hundreds to thousands of sun-tracking mirrors called heliostats to reflect the incident sunlight onto the receiver. These plants are best suited for utility-scale applications in the 30 to 400 MWe range. In a molten-salt solar power tower, liquid salt at 290ºC (554ºF) is pumped from a ‘cold’ storage tank through the receiver where it is heated to 565ºC (1,049ºF) and then on to a ‘hot’ tank for storage.

When power is needed from the plant, hot salt is pumped to a steam generating system that produces superheated steam for a conventional Rankinecycle turbine/generator system. From the steam generator, the salt is returned to the cold tank where it is stored and eventually reheated in the receiver.

Figure 1 is a schematic diagram of the primary flow paths in a molten-salt solar power plant.

Determining the optimum storage size to meet power-dispatch requirements is an important part of the system design process. Storage tanks can be designed with sufficient capacity to power a turbine at full output for up to 13 hours.

Top

Solar tower configuration

Figure 1. Molten-salt power tower system schematic (Solar Two, baseline configuration).

The heliostat field that surrounds the tower is laid out to optimize the annual performance of the plant. The field and the receiver are also sized depending on the needs of the utility. In a typical installation, solar energy collection occurs at a rate that exceeds the maximum required to provide steam to the turbine.

Consequently, the thermal storage system can be charged at the same time that the plant is producing power at full capacity. The ratio of the thermal power provided by the collector system (the heliostat field and receiver) to the peak thermal power required by the turbine generator is called the solar multiple.

With a solar multiple of approximately 2.7, a molten-salt power tower located in the California Mojave desert can be designed for an annual capacity factor of about 65%. (Based on simulations at Sandia National Laboratories with the SOLERGY [1] computer code.) Consequently, a power tower could potentially operate for 65% of the year without the need for a back-up fuel source. Without energy storage, solar technologies are limited to annual capacity factors near 25%.

The dispatchability of electricity from a molten-salt power tower is illustrated in Figure 2, which shows the loaddispatching capability for a typical day in Southern California. The figure shows solar intensity, energy stored in the hot tank, and electric power output as functions of time of day. In this example, the solar plant begins collecting thermal energy soon after sunrise and stores it in the hot tank, accumulating energy in the tank throughout the day. In response to a peak-load demand on the grid, the turbine is brought on line at 1:00 PM and continues to generate power until 11 PM.

Figure 2. Dispatchability of molten-salt power towers

Because of the storage, power output from the turbine generator remains constant through fluctuations in solar intensity and until all of the energy stored in the hot tank is depleted. Energy storage and dispatchability are very important for the success of solar power tower technology, and molten salt is believed to be the key to cost effective energy storage.

Power towers must be large to be economical. Power tower plants are not modular and can not be built in the smaller sizes of dish/Stirling or trough-electric plants and be economically competitive, but they do use a conventional power block and can easily dispatch power when storage is available.

In the United States, the Southwest is ideal for power towers because of its abundant high levels of insolation and relatively low land costs. Similar locations in northern Africa, Mexico, South America, the Middle East, and India are also well-suited for power towers.

System Benefits -Energy Storage

The availability of an inexpensive and efficient energy storage system may give power towers a competitive advantage.

Table 2 provides a comparison of the predicted cost, performance, and lifetime of solar-energy storage technologies for hypothetical 200 MW plants [5,6].

	Installed cost of energy storage for a 200 MW plant ($/kWhre)	Lifetime of storage system (years)	Round-trip storage efficiency (%)	Maximum operating temperature (C/ºF)
Molten-Salt Power Tower	30	30	99	567/1,053
Synthetic-Oil Parabolic Trough	200	30	95	390/734
Battery Storage Grid Connected	500 to 800	5 to 10	76	N/A

Thermal-energy storage in the power tower allows electricity to be dispatched to the grid when demand for power is the highest, thus increasing the monetary value of the electricity. Much like hydro plants, power towers with salt storage are considered to be a dispatchable rather than an intermittent renewable energy power plant.

For example, Southern California Edison company gives a power plant a capacity payment if it is able to meet their dispatchability requirement: an 80% capacity factor from noon to 6 PM, Monday through Friday, from June through September.

Detailed studies have indicated that a solar-only plant with 4 hours of thermal storage can meet this dispatchability requirement and thus qualify for a full capacity payment.

While the future deregulated market place may recognize this value differently, energy delivered during peak periods will certainly be more valuable.

Besides making the power dispatchable, thermal storage also gives the power-plant designer freedom to develop power plants with a wide range of capacity factors to meet the needs of the utility grid. By varying the size of the solar field, solar receiver, and size of the thermal storage, plants can be designed with annual capacity factors ranging between 20 and 65% (see Figure 6).

Economic studies have shown that levelized energy costs are reduced by adding more storage up to a limit of about 13 hours (~65% capacity factor). While it is true that storage increases the cost of the plant, it is also true that plants with higher capacity factors have better economic utilization of the turbine, and other balance of plant equipment.

Since salt storage is inexpensive, reductions in LEC due to increased utilization of the turbine more than compensates for the increased cost due to the addition of storage.

Figure 6. In a solar power tower, plant design can be altered to achieve different capacity factors

.

To increase capacity factor for a given turbine size, the designer would (1) increase the number of heliostats, (2) enlarge the thermal storage tanks, (3) raise the tower, and (4) increase the receiver dimensions.