Why Thermal Storage?

 If demand reduction and off-peak power consumption continue to command substantial discounts in electric power costs, the question arises, “Why will thermal storage continue to be an attractive method of achieving load shifting in the deregulated energy market?” Several reasons include: Thermal storage systems target the most egregious contributor to poor load profiles-commercial cooling systems. Also, the technology exists and is proven. Thermal storage represents one of the few legitimate tools for shifting load. Energy efficiency benefits society and the customer, but thermal storage also benefits the industry setting the price for that energy.

1. Thermal storage systems are designed for the commercial customer (who always pays the highest time-dependent rates).
2. Storage systems do not negatively impact a facility’s operation, as other load shedding or load control programs almost always do.
3. Existing thermal storage technology is easily adaptable to central chilled water plants. Even though centralized chillers only serve about 25% of commercial floor space, Thermal storage systems can make a significant difference in relatively few installations.
4. Thermal storage is versatile. Other than the certainty that on-peak power consumption will continue to command a premium, there is little assurance concerning the form those rates will take. In many cases customers will have a choice as to the structure of the demand penalties. Traditionally, a simple demand charge (kW) and energy charge (kWh), often including a time-of-day differential, have been used to discourage on-peak electrical use. Rate design will surely be more exotic in a deregulated environment as providers maneuver to offer the most competitive plans possible. Real-time rates, often superimposed on a traditional demand structure, and interruptible rates, a fairly common tool in natural gas pricing, will also grow in availability.

Thermal storage is also cost effective. DSM programs have helped to foster the growth and acceptance of thermal storage. The generous terms of these programs often made it economical to install storage capacities capable of avoiding all the on-peak chiller operation. This is referred to as “full storage.” Often forgotten is the fact that if the goals are more modest, thermal storage can be installed with little or no cost penalty as compared to conventional chiller systems. DSM incentives are certainly welcome, but not necessary to make thermal storage a good investment. There are no defined limits on the quantity of storage that can be theoretically applied to a building.

An alternative referred to as “partial storage” minimizes or eliminates any additional initial capital investment. By operating a chiller for the entire day, on-peak at standard conditions and off-peak at ice-making conditions, its size is usually reduced to 40% to 50% of the conventional design.

Storage is only needed for about 40% to 45% of the required ton-hours. Both chiller and storage are greatly reduced in size, compared to the “full storage” design. Peak demand savings of 50% to 60% of the standard chiller demand are usually achieved.

Many examples exist of effective thermal storage systems that were installed for little or no additional cost over their conventional alternatives and that also provide significant energy and energy cost reductions.

The economics of thermal storage can usually be justified under any power rate that significantly penalizes on-peak power consumption.

As Laurence J. Peter once said, “An economist is an expert who will know tomorrow why the things he predicted yesterday didn’t happen today.” The same can easily be said of electric industry analysts. Engineers will take refuge in whatever facts they can grasp within the confused and nebulous nature of today’s electric power industry.

The benefits of thermal storage systems for air-conditioning

Thermal storage, introduced in building air-conditioning, is the system which levels the power load by storing the daytime power demand for air-conditioning, pushing the peak power demand up, in the form of heat in advance.

Energy-saving effects of thermal storage systems -Comparison with conventional air-conditioning systems-

(1) Improved efficiency of heat pumps using rated operation

In thermal storage systems, heat pumps always can be run at high-efficiency rated outputs without emulating the air-conditioning load variations.

This is because it is possible to control the amount of heat produced by increasing or decreasing operation hours of heat pumps not only during peak air-conditioning load periods but during light load periods.

(2) Improving efficiency of heat pumps using outside air temperature difference

As the outside temperature drops in night, heat pumps for storing heat become more efficient for cooling, but efficiency is reduced for heating.

For office buildings in which the annual cooling demand surpluses the annual heating demand largely, since the energy-saving effects for cooling excel, the efficiency of heat pumps operation improves over the year.

(3) Heat losses arising from the thermal storage tank

While the efficiency of heat pumps improves, heat losses occur from the thermal storage tank since there is a time lag until the chilled (or heated) water produced during the night is actually used for air-conditioning. Therefore, it is common to insulate inside the thermal storage tank for cutting down these heat losses and taking security of the energy-saving characteristics of thermal storage systems.

(4) Increase in carrying power due to thermal storage

In addition to requirement of two pumps in thermal storage systems, one is for thermal storage between heat pumps and thermal storage tank and another is for air-conditioning between storage tank and air-handling units, storage water is open to the atmosphere in many cases, the pump head and pumping power increases.

Energy efficiency comparison of thermal storage systems with other heat source air-conditioning systems

In air-conditioning systems for heating and cooling with absorption refrigerating machines and boilers as the heat generators using gas or oil rather than electricity for the heat source are widespread, particularly in the area of centralized air-conditioning. Recently also, co-generation system is becoming more widespread.

Recently in Japan there have been moves to promote the spread of co-generation which demonstrates overall efficiencies of 60 – 70%. However, particularly in commercial use, since annual load factor for heat demand is extremely low, approximately15-20%, while electric power demand such as lighting and motive power used firmly over the year, it is difficult to recover the exhaust heat as expected. Therefore there are many cases which can’t complete the overall efficiencies as the catalogue shows.

Even supposing the exhaust heat in co-generation is completely recovered, the energy efficiency in thermal storage system will be 10% better than that in co-generation, and in the future, with the introduction of the latest Advanced Combined Cycle power stations which have thermal efficiency of 50% at generating end, thermal storage systems will demonstrate 30% higher energy efficiencies compared with co-generation.

And comparing the amount of CO2 emission in “thermal storage systems at rated operation” with that in co-generation, thermal storage systems can reduce it for 40%.

As described above, it has been proven that thermal storage systems are extremely dominant as technology for efficient use of existing energy resources.

Therefore, it becomes important to devise the overall efficient use of energy resources with two wheels, one is the effort to improve thermal efficiency at power supply side and another is the high efficient use of energy by thermal storage system at demand side.

Shifting peak power demand by thermal storage systems also contribute to reduce the cost of electricity charges by controlling and cutting the cost for installing new power stations and to maintain mid/long-term energy security increasing the weight to nuclear power which carries a base load.

Benefits to users

The key to come thermal storage systems, which are superior in energy saving characteristics and the effect of power load leveling, into wide use depends entirely on the economic benefits for users.

It is said that the biggest problem of thermal storage systems are to increase the construction cost by installing thermal storage tank and complication of its design.

The benefits which improve this difficulty are as follows.

(1) Cost reduction of electricity charges

Our company has expanded the range of electricity charges, including discount schemes for nighttime electricity charges (1/3 – 1/4 of daytime charges). And in addition to this, we are actively tackling the promotion of widespread use through schemes such as the payment of fixed incentives to the manufacturers of ice storage systems.

Calculations of annual electricity charge benefits for a thermal storage system compared with a conventional system based on a typical office building, show economic benefits in a 53% reduction of electricity costs.

(2) Reductions in equipment size and cuts in machine room space

With thermal storage systems, size reductions can be achieved on substation, pipes in a machine room, and control equipment since the heat pumps are reduced in size or number, and this enables machine room space to be reduced in size. The provision of sufficient machine room space is a problem in buildings in the Tokyo area, so the reduction in space will be a major benefit to customers.

(3) Operational convenience

With thermal storage systems, sudden air-conditioning loads such as in the morning can be met by utilizing the thermal storage tank, but with conventional or other heat source air-conditioning systems, it is necessary to start up the heat source equipment to cope.

Thermal storage systems have also received high acclaim from operating staff for their ability to give peace of mind in air-conditioning operations since temporary back-up is possible with the chilled/heated water stored in the thermal storage tank even if an equipment failure should occur. By the way, the unmanned operation for thermal storage in the night can be achieved by the progress of control technology.

Thermal Energy Storage

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.

Other Benefits

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.

 

Build Thermal Storage, Not Power Plants

The following is part of a report written by: Scot M. Duncan,

The concepts discussed herein apply to any utility, state or region facing pollution issues, power generation capacity shortages, or transmission and distribution congestion and having the desire to improve power plant efficiency and reduce the need to build new power plants.

 

There is no one silver bullet, there are many proven technologies and business models that applied collectively or individually can bring real improvements. One such technology that can provide immediate benefits is Thermal Energy Storage (TES).

 

• Thermal Energy Storage is the simple process of cooling (or freezing) water during the evening hours and storing it for use the next day to air condition large commercial, industrial and institutional buildings.

• TES is the rough equivalent of building electricity generating power plants in the sense that TES taps the unused capacity in our existing power plants at night when they are typically operating at very low output levels.

• Using TES in lieu of building new power plants allows for an effective increase in capacity during the peak usage hours, without any of the negative environmental impacts associated with building new power plants.

• TES simultaneously increases the efficiency of existing transmission and distribution facilities in addition to the benefits it provides for generation plants.

• With properly crafted incentives and rate structures, the private sector could be enticed to contribute a significant percentage of the capital required to build a very large number of TES systems, essentially subsidizing other ratepayers.

 

TES is a fully-proven technology that can improve power plant efficiency by 20% to 43%, improve cooling system efficiency by up to 25%, and reduce cooling system related peak electrical demands by 60% to 80% on the hottest summer afternoons, by shifting major air conditioning related electrical loads to the night instead of the afternoon.

 

Properly designed, implemented and commissioned TES systems provide long term benefits to all ratepayers and the environment. It is a simple concept on its face – if a power plant can be made more energy efficient, it will use less fuel. If it uses less fuel, it will produce fewer emissions and the environment will be improved from the resulting reduction in harmful greenhouse gasses, acid rain, and toxic pollutants such as mercury. Sustainable peak demand reduction will reduce the potential for costly brownouts or blackouts as well as reducing the need for expensive infrastructure improvements.

This improves the reliability and affordability of electric power for all ratepayers.

 

One Engineer has been quoted as saying “Think of TES like a 1,000 MW power plant that consumes no natural resources, and reduces pollution from other power plants, and you can persuade building owners to pick up half the cost of the system that benefits all of the ratepayers.”

Coal Power or Ice Power?

Make ice when electricity is cheap.  Melt the ice for air conditioning when electricity is expensive or in high demand.  Pretty simple alternative to spending billions building new coal fired electric power plants. 

 

Thermal Energy Storage, TES systems have been in use for almost a hundred years.  One of the original applications was to use a small inexpensive compressor to make ice all week long and then melt all that ice to cool the sanctuary for two hours on Sunday.  We have been using tank type water heaters (hot thermal storage) for years to avoid having large instantaneous gas or electric water heaters. 

 

So why don’t we find a TES air conditioner in every house and small business?  The answer is also pretty simple:

·          Most electric rates are averaged so it is not less expensive to buy electricity when it should be cheap and it is not more expensive to buy electricity in high demand periods when the price should be exponentially higher.

·          In very round numbers it costs thousands of dollars per kW (or ton of A/C) to fund the construction of electric generation plants, transmission and distribution (TD) infrastructure.  There is no mechanism to divert funds from building these coal fired construction projects to fund installing a TES system in your home or business.  The current conservative estimate of avoided costs to build generation, transmission and distribution infrastructure to serve a three ton air conditioner is $45,000. over the 15 year life of the TES system.

 

Should we invest in new coal generating plants or invest a fraction of that in your home?

 

If the above economic rationalization isn’t enough to convince you consider the following.

 

·          Running your air conditioner at night is much more efficient because the ambient outside temperature is much lower and you’re a/c unit operates more efficiently.

·          Running the generating turbine at night is much more efficient for the same reason, lower night temperatures.

·          All power plants run more efficiently when they are fully loaded and demand is predictable.

·          Transmission and distribution is more efficient at night.

 

A massive deployment of TES will postpone the need to build additional power plants for many years.  We can land on the moon.  Why can’t we make ice?