Thermal Energy Storage
Posted July 26, 2008on:
Storing thermal energy for use at a later time is an excellent energy management strategy. Thermal energy storage (TES) systems can store low-cost energy that is generated off-peak as an electrical demand cost-control measure. But TES can also be used to hedge in competitive utility markets for both electricity and gas, to reduce emissions, and to lower energy use.
Frequently, energy is available at one time but needed at another time. TES systems bridge the two times. TES is a mature technology that has been used in a variety of applications ranging from cooling and heating of buildings to cooling of gas turbine inlet air. Some TES systems have been operating continuously and satisfactorily for over 30 years, and some manufacturers and system designers have been in business throughout that period.
A classic TES application collects solar energy during the day for use in heating a building during the night. Recently, it has become common to build cooling reserves during the utility off-peak period for use during the following on-peak period. These applications result in reduced energy cost and, frequently, decreased energy use as well.
When utility energy is used to operate heating or cooling equipment near design capacity and unneeded output is stored for later use, the end user’s equipment often runs at a more consistent and efficient rate. The utility may also be able to optimize the use of its equipment. TES operation that smoothes the load profile also reduces energy use, particularly in the case of cooling equipment, because the chillers are operated more at times when they operate more efficiently due to lower ambient wetbulb temperatures.
Alternatively, energy may be available at the discharge of a device or a process at a temperature that is suitable for heating or cooling a space or another process, but the supply does not occur at the same time as the demand. TES provides a means for storing the heating or cooling capacity that might otherwise be wasted and making it available when it is needed. This application can produce the benefits of reduced emissions, energy use, and cost.
In many installations, TES provides additional benefits. For example, the addition of TES to an existing cooling system highlights the benefits of increasing the difference between chilled water supply and return temperatures. This modification improves operation of the distribution portion of the cooling system, increases thermal storage capacity, and reduces energy use by the chillers.
TES applications for buildings and processes require energy to be stored from only a few hours up to a several days. Daily cycles are most frequently employed, but in some applications heating-cooling units may be available to charge TES on weekends. The storage medium can be designed and constructed to accommodate energy storage for several days.
Costs and Benefits
Utility rate structures offer lower energy prices during off-peak periods when the demand for power is less and the demand for cooling or heating is usually lower. TES reduces operating costs by taking advantage of the lower utility energy rates.
Electric utilities may offer reduced rates during off-peak periods to encourage improved use of their base load capacity, which is more efficient than their peak units. The utility’s off-peak period may not be the same as the facility’s, but they often overlap enough to justify the application. Consequently, the cooling equipment for the facility may be operated at full capacity during the lower rate, off-peak period to charge thermal storage, and partially or completely shut down during the higher rate, on-peak period.
Commercial and industrial rates commonly have peak demand and energy rate components. In many cases, end users can reduce utility cost simply by shifting the operation of cooling equipment partially or completely from the facility peak period to its off-peak period, reducing peak demand and the accompanying demand charges.
Savings in energy cost may be used to amortize any additional capital cost of thermal storage. In many instances, the initial cost of a system with TES is no greater than one without TES. Capital costs of TES are often offset in a variety of ways. For cooling systems, chiller size and cost can be reduced by the chiller’s increased operation at design capacity. Ancillary equipment can be downsized, including pumps, cooling towers, and the electrical service for these items.
The strategies employed in designing and operating a system using thermal storage affect how much capital cost can be reduced. Considering TES early in the conceptual design phase makes capital cost reduction more likely to be realized.
The first cost of additional chillers to expand the capacity of an existing cooling system makes the first cost of TES particularly attractive. Chiller size determines capital cost-the larger the unit, the higher the cost. TES also offers capital cost benefits to systems producing a variety of outputs-heating, cooling, and electrical power.
Heated TES can also offset capital costs. For example, heat recovery chillers may be used with TES to reduce boiler capacity and to produce savings in the costs of both the heating equipment and the associated fuel supply system.
Applications having relatively short periods of high thermal load coinciding with high utility rate periods are ideal candidates for TES. Examples include sports facilities, auditoriums, churches, and some industrial processes. With proper design and operation, these applications will always produce savings in operating cost, and they may well achieve savings in capital cost, too.
The utility that serves a customer with TES benefits from the storage system too. The utility can better utilize its base load electrical generation plants. As a result, load can be met with less generation and distribution capacity.
Thermal storage can be installed at a customer facility at lower cost than the cost to the utility of installing additional generating capacity. This explains why utilities have offered incentives in the form of partial payment of the capital cost of TES installations as part of demand-side management programs.
The electric utility also realizes other energy savings. As stated previously, TES for cooling increases chiller use during the cooler portions of the day and at night, when chillers operate more efficiently. Additional on-site energy savings may be achieved by using heated TES, which reduces both energy use and combustion emissions when heat recovery is employed.
As limits on emissions become more stringent, interest in TES to reduce on-site and power plant emissions will increase. Existing emissions regulations may make it desirable to reduce on-site energy use in new construction. In addition to emission reductions due to increased efficiency, smaller chillers with TES systems tend to lose less refrigerant.
TES produces a more forgiving heating and cooling system and gives the system operator more operating flexibility. Not only can utility energy be drawn at times that are more advantageous for the user, but heating or cooling loads can continue to be satisfied even if a heating or cooling unit is off-line temporarily due to equipment failure or for periodic maintenance. TES may allow a user to take advantage of spot retail utility rates that have been proposed as a means of dampening fluctuations in wholesale electrical prices. With this strategy a facility owner could also consider interruptible power for heating and cooling equipment.
TES tanks containing water can be used as auxiliary reservoirs for fire protection systems. If the reservoir is located at a high point in the distribution system, gravity feed may suffice for this application, thus offering an added level of security. On the other hand, using an existing fire protection reservoir can help reduce the capital cost of a retrofit TES system.