Thermally-Activated Technologies

Advanced Thermal Recovery Cycles

New approaches and applications are needed to address situations in which more thermal energy is available than can be practically used for conventional heating and cooling functions. To capitalize more fully on an otherwise wasted energy resource, for example, thermal energy might be used to treat water or sewage on site; generate hydrogen; generate shaft power for driving pumps or blowers; or generate electricity. Many other approaches are possible as well.

Thermodynamic Cycles
Organic Rankine cycle equipment is emerging from research and development laboratories and into field tests. Operation of these cycles can extract heat energy from a source in the range of 250-800°F and convert it into electricity. Power system efficiency of 8-15% is expected depending on feed heater options and ambient conditions. Lower cost organic Rankine cycle systems could be targeted for combined heat and power bottoming cycles to increase net electrical output or toward low/moderate temperature waste heat streams.

A novel combination of liquid desiccant and synthetic membrane separation technologies makes possible a desiccant energy recovery system with separated ventilation air pre-treatment and exhaust air recovery stations. This enthalpy pump design, which has the energy efficiency advantages of a direct-contact mass and heat exchanger, also allows for combined enthalpy exchange and active-desiccant dehumidification operation. This system can evaporatively cool, provide summer and winter energy recovery without wheel frosting problems, is easily integrated with conventional AC, and is a convenient retrofit to older buildings.

Stirling engines are classed as external combustion engines. They are sealed systems with an inert working fluid, usually either helium or hydrogen. They are generally found in small sizes (0.5 - 50 kW) and are currently being produced in small quantities for specialized applications.

Thermochemical Active Working Fluid Cycle
The concept of using a thermochemically active working fluid (AWF) in a closed cycle gas turbine system is theoretically attractive and has been evaluated independently for DOE. It involves the use of a high density working gas which, on heating, breaks into simpler molecules, with an increase in total gas volume. Therefore, the turbine operates with a larger gas volume than the compressor, and generates proportionately more power than the compressor absorbs. This leads to higher net output and significant gain in efficiency. Proof-of-principle experiments have been carried out with an AWF which would not be acceptable for DE applications. Once a suitable working fluid is identified and its properties characterized, a design schematic should be developed and modeled in as much detail as is reasonable to address fluids/materials compatibility issues, potential safety issues, and to reassess the technical performance of the cycle in light of the initial findings.

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