Thermal Storage: A Tool For Energy Savings In Hybrid HVAC Systems
For the most part, the TES market has been restricted to larger applications, which often have skilled people capable of investigating and evaluating equipment choices. However, the energy managers of institutions and larger companies are increasingly receptive to equipment with economic advantages beyond "first cost."
Increasing Demand, Dwindling SupplyIn fact, there is a growing trend to go beyond simple payback analysis that is seen as too simplistic. This is especially true given the increasingly uncertain outlook for future energy prices. For instance, the long-term "price-trend line" for natural gas has an upward slope that is increasing at the rate of 400% every eight years, while that of crude oil is increasing at the rate of 100% over the same time frame. These upward trends are expected to accelerate as the economies of previously underdeveloped countries (like China, India, and Pakistan) begin to consume energy the way we in the United States are doing. It is anticipated that by the year 2020, China will have surpassed the U.S. and Europe combined in oil consumption. The implications of this in relation to energy costs are enormous.
What makes the situation particularly daunting is that the same experts who are predicting large increases in the future demand for energy are also predicting peaks in the energy production levels of many oil and natural gas fields around the world. Our own country is projected to virtually deplete its supply of crude oil in 50 years, even if we develop the newly discovered Alaskan oil field at the Arctic National Wildlife Refuge (Figure 1).
Rather than focus on simple "first cost payback," analyses of energy conservation, projects are beginning to look at 20- and 40-year life-cycle costs when evaluating major energy conservation projects, such as TES, since such projects could end up seeing long-term savings in the millions of dollars.
These factors will undoubtedly increase interest in energy-saving technologies of all kinds. But in the case of TES, there are other opportunities, in addition to pure economics, that have the potential for increasing their importance. Most of these involve the integration with other technologies and the identification of multiple functions that can lead to qualitative improvements in the value of a total building system through "hybrid" HVAC.
Currently, TES is used primarily in load-shifting and load-leveling strategies on commercial buildings. Such buildings have electric rate premiums during peak times of day, and owners end up paying 50% to 75% more for the same electrical power as many industrial facilities due to rate structures that penalize poor electric demand profiles. TES can even out the load over the course of a 24-hour period and take advantage of lower off-peak rates and reduce demand charges (Figures 2 and 3). This is especially important for the increasing numbers of building owners who desire to obtain LEEDTM certification, since many of the LEED points are determined by the level of energy cost reductions achieved.
Although the cost of TES equipment is significant, much of the extra cost is offset by reductions in the chilled water system. The premium that remains is then recovered through reductions in electrical demand and in operating expenses through increased operation during the night, when rates are often 30% to 90% less. Although rates vary around the country, in the Chicago area, a typical on-peak charge is $0.0575 per kWh while the off-peak charge is $0.0249, and a typical summer demand charge is $14.23 per kW.
Using A Cold Air SystemA typical thermal storage economic analysis, however, often misses additional installation savings other than the obvious savings in chiller, cooling tower, and pump sizes. For example, the availability of lower chilled water temperatures allows further reductions in pipe sizes and coils because of the greater temperature differentials that are made possible (e.g., 20°F vs. the normal 10°); this can make possible the use of a cold air system. There are also significant reductions in electrical power wiring distribution that shouldn't be overlooked. In addition, the thermal storage tanks have longer life expectancies and depreciate slower than the more complex chilled water system.
It has been observed in recent DOE studies that occupants in cold air systems will raise thermostat settings by 2° to 3° because the lower humidity levels make people feel more comfortable even with a slightly higher temperature. The subsequent reduction in cooling costs is quite significant yet often omitted from the economic analysis.
There are additional missed opportunities that TES and a cold air system make possible. The use of 42° to 47° supply air temperatures reduce supply cfm and that, in turn, reduces all components in the air distribution system, such as air handlers, terminal boxes, diffusers, and the ductwork (which can represent half the cost of an HVAC system). The energy savings in fan horsepower with cold air systems can be almost as much as the chiller savings.
Not to be overlooked is the fact that cold air distribution systems allow for smaller ductwork that can fit into tight spaces. On new construction, this can allow savings in construction costs if lower ceiling spaces are possible, especially on high-rise buildings. It can also allow for smaller equipment rooms. The task of including all the cost savings can be quite lengthy and complicated, but it can often reveal a TES system to be a net first cost saver, in addition to a continuous energy saver. An innovative engineer who understands all the opportunities made possible by thermal storage can often design a system whose first cost is lower than any other alternative.
Controlling The Thermal Storage SystemControl strategies for TES systems are often classified into categories such as partial storage, full storage, demand limiting, load leveling, chiller priority, and ice storage priority. But the same application can often use a variety of those strategies, and they can change over time, even on a daily basis. Let's take a quick look at the two most commonly used strategies.
The goal of the partial-storage, load-leveling strategy is to downsize the chiller so that it will run at full capacity (where it is most efficient) 24 hours a day during outdoor design conditions. During peak cooling hours the chiller continues to run, and its capacity is augmented by the ice (or chilled water) in the thermal storage tanks. During off-peak hours the chillers are kept running, to either store chilled water or to make ice. This is the most commonly used strategy since the size of both the chiller and storage tanks is minimized, and often yields the fastest payback. The fastest payback, however, is not always the lowest life-cycle cost.
A full-storage, demand-limiting strategy has the more ambitious goal of keeping the chiller off-line during the day in order to minimize the electric demand charge and to take maximum advantage of the lower off-peak electric rates. This increases energy savings; however, it does so at the expense of larger chillers and ice storage equipment, because they will not be operating together as they do in the partial storage, load-leveling strategy.
What frequently occurs is that some combination of these two strategies is implemented. Sometimes utilities providers will establish daytime windows at peak demand times with higher rates; in such instances, the chillers will be controlled to cycle off during these small windows. This compromise reduces equipment sizes somewhat, while taking greater advantage of energy saving opportunities. These compromise strategies can take many forms. For example, if the building has two chillers, both may run at night to make ice, but only one will be allowed to operate during the day.
It should also be noted that the terms "partial storage" and "full storage" are actually based on sizing the equipment for design load days. Under part load conditions, a system designed for partial storage may be switched to a full storage strategy if the chiller is able to make enough ice at night to satisfy the entire expected load for the next day without assistance from the chillers during the on-peak hours.
Boosting Energy Efficiency Through TESIntegrating TES with other energy-efficient HVAC technologies is where the whole concept of TES can really pay dividends. Large buildings, for example, often require cooling all year long since they have many interior zones. During the night, double-barrel chillers with heat recovery capability can be used to heat the building while making ice at the same time. The ice storage is then used the next day to cool both the internal zones and the exterior zones having high solar loads. If storage tanks are also used for hot water storage, it may be possible to completely eliminate the use of boilers; this can offer hidden benefits, such as reducing equipment room sizes and decreasing the building's insurance rates.
Aside from the energy saving potential, there are superior control benefits that come from decoupling the heating and cooling loads from the equipment performance, which thermal storage makes possible. One innovative method of utilizing TES is being used in school systems in the Midwest and the East Coast. The system is called the Regenerative Double Duct™ (RDD), and it was invented by Lentz Engineering Associates, Inc. of Sheboygan Falls, WI. It employs two stages of evaporative cooling (indirect and direct) that substantially reduce the size of the chilled water system, and the design also keeps the chiller off during substantial portions of the cooling season. This changes the building's hourly load profile, making the use of TES even more advantageous. In one Wisconsin school application, a 70-ton chiller was used in place of the originally planned 600-ton unit. This chiller plant reduction is typical of buildings utilizing the RDD design.
Another interesting feature of the RDD system is that it operates on all outdoor air to maximize IAQ. The use of advanced evaporative cooling technologies, energy recovery, and TES has made it possible to operate an all-outdoor air system while avoiding the energy penalty normally associated with very high ventilation rates.
The RDD makes use of a unique dual-duct design that allows the entire school to be conditioned with a central 100% outside air system, rather than using a conventional decentralized system with unit ventilators or rooftop units. The use of a central system is a key to employing this energy efficient strategy (although multiple rooftop units can be designed to provide similar performance). The energy penalty normally associated with dual-duct systems is avoided, in part, by extracting heat and cooling energy from the building exhaust to precool and preheat supply air, and to further heat the air in the warm duct.
This system also takes advantage of TES to reduce the supply air duct system by using cold supply air temperatures. Each zone is cooled with the minimum fan energy possible by using air from the cold supply air duct. During low load conditions, when supply air volume becomes too low to meet minimum ventilation requirements, warm air from the warm duct is mixed in, raising the supply air temperature. Since the warm duct also carries all outdoor air, minimum ventilation rates are always met.
Cogeneration applications present many opportunities for integration of TES with other technologies. The key to cogeneration success is making use of the waste heat from the engines driving the generators. This is easily done in the winter by providing heat for the buildings. Using the waste heat in the summer for cooling is more complicated. When high latent loads are present or all outdoor air is required, open desiccant systems can be used in conjunction with indirect evaporative cooling to satisfy the building's cooling needs. Thermal energy can be stored in hot water tanks until needed for regenerating the desiccant during peak cooling periods. In other applications, absorption chillers are favored, especially when higher recoverable temperatures are available.
Ice vs. Cold WaterWhile ice storage has become the most common method of TES, chilled water storage can be more advantageous in many situations, despite the fact that it requires many times the storage volume. Chilled water storage tanks are less expensive, and the use of a glycol solution (which has a significant cost) may not be required. This is an additional advantage because glycol has a lower heat transfer rate, and on retrofit applications, that can make the existing pipe sizes, pumps, and coils too small to be used in the conversion of the system to ice storage.
On retrofit applications, the existing chillers may be too small to make the necessary amount of ice because chiller outputs drop by about one-third during ice making. Or, the chiller may not be able to run at the low suction temperatures necessary to make ice. In these cases, chilled water storage has the advantage because of its ability to smoothly integrate into an existing installation.
Chilled water storage tends to be most economical on systems requiring over 2,000 ton-hours of storage. On large complexes (such as universities), the use of a chilled water tank is often less expensive than a non-storage system, especially when there is plenty of space and the tanks can be designed for maximum efficiency and minimum cost. With the large mass and low volume-to-surface ratio, the tanks do not have to be insulated, and the economics of scale works in their favor. Although the volume of the chilled water storage tanks is much larger than ice storage tanks, they can be located completely underground (such as below parking lots or athletic fields) and essentially take up no usable space at all.
Sometimes, existing large facilities have fire protection reservoirs, and these can be converted relatively inexpensively to do double duty as chilled water storage devices. Conversely, if no fire protection reservoir exists, the installation of a chilled water storage tank can also act as a fire protection system, thus reducing insurance costs.
Refrigeration systems can also be prime candidates for thermal storage. Food processing industries, for instance, require refrigeration systems that often must accommodate the sudden introduction of large quantities of warm, fresh product. This brings about a high, short-duration load on the refrigeration system. TES is a perfect way to avoid oversizing the refrigeration plant that only occasionally requires full capacity. When subfreezing temperatures are required, the ice tanks must be charged with a special low-temperature fluid.
Some industrial applications, such as batch processing of food products, require exceptionally large cooling outputs for very short periods of time. These applications can best be served by an ice harvesting or a slurry system. This is a fairly simple system, but one which can be very effective and frequently has the lowest cost per ton-hour of storage. They essentially consist of a simple open tank that is very large but inexpensive. Mounted on top of the tanks are relatively expensive, but small, icemakers that normally use ammonia refrigerant. They endlessly make small sheets of ice and drop them into the tanks to form a mixture of highly fragmented ice and water that allows for extremely high discharge rates when their infrequent but intensive use is required.
Another special application that is ideal for TES is electric power peaking plants. On hot days, when they are needed the most, the low-density hot air seriously degrades their output. Cooling the intake air to the turbines will significantly increase their output. Since many of these plants only operate about six hours a day, the size of the chiller required can be reduced considerably, and the chiller can even be kept off when the plant is operating in order to reduce parasitical losses and maximize the plant's power output.