Thermally-activated technologies include a diverse portfolio of equipment that transforms heat for useful purposes such as heating, cooling, humidity control, thermal storage, and shaft/electrical power.
Thermally-activated technologies are essential for combined heat and power (CHP)-integrated systems that maximize energy savings and economic return. Thermally-activated technologies systems also enable customers to reduce seasonal peak electric demand and future electric and gas grids to operate with more level loads.
Absorption cycles have been used for more than 150 years. Early equipment used a mixture of ammonia and water as an absorption working pair, with ammonia as a refrigerant. This working pair is still in use today in a range of applications, from small refrigerators of less than 25 refrigeration tons (RT) of cooling capacity to mammoth heat-recovery machines installed with power plants.
Ammonia is an excellent refrigerant with a high latent heat and excellent heat transfer characteristics. However, because of its toxicity, it is often restricted to applications in which the equipment is outdoors to allow natural dilution of any leaks.
Another absorption working fluid came into widespread use after 1945 when aqueous lithium bromide was introduced for building air-conditioning applications. In this working pair, water is the refrigerant.
Water is an excellent refrigerant because it has a high latent heat, but it is restricted to applications in which the cooling requirements are above its freezing point (0°C). Another challenge of using water as a refrigerant is its very low vapor pressure, which causes absorption cycles based on aqueous lithium bromide to operate at sub-atmospheric pressure.
One way to explain absorption technology is to compare it with traditional vapor compression technology.
The basic cooling cycle is the same for lithium bromide absorption and electric chillers. Both systems use a low-temperature liquid refrigerant that absorbs heat from the water to be cooled and converts to a vapor (in the evaporator section). The refrigerant vapor is then compressed to a higher pressure by a compressor or generator and converted back into a liquid by rejecting heat to the external surroundings in the condenser section. Next, it is expanded to a low-pressure mixture of liquid and vapor (in the expander valve), which boils in the evaporator section, absorbing heat and producing the cooling effect. Then the cycle is repeated.
The combination of the generator-solution heat exchanger-absorber is often referred to as a thermal compressor. The net effect of the thermal compressor is to raise the pressure of the working fluid and drive it through the condenser and evaporator. In the vapor compression cycle, this is done with a machine that directly compresses the vapor. Such compression processes require considerable work because the working fluid changes density and stores energy internally. In contrast, the thermal compressor first absorbs the vapor refrigerant into the liquid absorbent in the absorber. The mixed working fluid is then pumped up to a higher pressure in a liquid pump. (Because the liquid working fluid is largely incompressible, this pumping process does not require much energy compared with the vapor compression process). In the generator, the refrigerant is then boiled out of the solution at the higher pressure. The end result is that the refrigerant is compressed with only a fraction of the mechanical work that would be required to compress the vapor mechanically. This is accomplished essentially with a heat engine, which requires heat at a high temperature and rejects heat at a low temperature.
The basic difference between electric chillers and absorption chillers is that electric chillers use an electric motor to operate a compressor to raise the pressure of refrigerant vapors. An absorption chiller uses heat to compress refrigerant vapors to a high pressure. The rejected heat from power generation equipment (e.g., turbines, microturbines, and engines) may be used with an absorption chiller to provide the cooling in a CHP system.
Ammonia-Water Absorption System
Ammonia-water absorption chillers and heat pumps are being developed for residential and light commercial applications. Generally, ammonia-water absorption chillers and heat pumps are 2 to 10 RT and can be modularized into larger systems.
Compared with the lithium bromide single-effect absorption water cycle, the ammonia-water single-effect absorption cycle requires two additional components. These are a rectifier, which is needed because the absorbent (water) is volatile at generator conditions, and a condensate precooler. The rectifier is designed to strip some of the water out of the vapor stream.
One design option for increasing the coefficient of performance (COP) that is available when ammonia-water is the working fluid is the generator-absorber heat exchanger (GAX). GAX is not possible in lithium bromide systems because of the crystallization characteristics. The basic feature of GAX is an internal heat exchange. This allows the heat to be input at a higher temperature such that it is then reused internally. A temperature overlap between the generator and the absorber can be used to move some of the heat normally rejected by the absorber back to the generator, thereby reducing the heat input required and increasing efficiency. This overlap will only exist if the temperature difference between the condenser and evaporator is relatively low, which occurs in most air conditioning applications.
Desiccant systems are widely used in industries in which humidity control is a concern and low humidity has an economic benefit. Since the 1980s, concern about humidity control in commercial buildings with refrigeration processes, such as supermarkets, has led to the transition of the technology from the industrial to the commercial sector. Now concerns about outside air laden with moisture in highly ventilated buildings has accelerated the evolution of desiccant equipment and expanded its use in commercial and institutional markets.
A desiccant dehumidifier uses a drying agent, or sorbent, to remove water from the air used to condition building space. Desiccants can work in concert with chillers or conventional air conditioning systems to significantly increase overall system energy efficiency by avoiding overcooling (and reheating) air and precluding oversized capacity to meet dehumidification loads. Desiccants can run off the waste heat from distributed generation technologies, with system efficiency approaching 80% in CHP mode.
The desiccant process involves exposing the desiccant material (such as silica gel, activated alumina, lithium chloride salt, or molecular sieves) to a moisture-laden process air stream. Allowing the sorbent to attract and retain a heated regeneration air stream drives off the retained moisture from the desiccant.
A solid desiccant dehumidifier is most commonly placed on the surface of a corrugated matrix in a wheel that rotates between the process and regeneration air streams. On the process side, the desiccant removes moisture from the air while releasing heat resulting from the sorption process into the process air. As the wheel rotates onto the regeneration side, natural gas, waste heat, or solar energy can be used to regenerate the desiccant material.
Liquid desiccant dehumidifiers spray the process air with a regenerated desiccant (lithium chloride or glycol solutions) to remove the moisture in a conditioner. The diluted desiccant solution is pumped to a separate regenerator, and heat is applied to the solution to release its sorbed water into an exhaust air stream.
For information about current DOE efforts in this technology area, please visit the Research and Development page.
For information about DOE accomplishments in this technology area over the last decade, please see Combined Heat and Power: A Decade of Progress, A Vision for the Future.
May 23, 2013