APPLYING CHP TO THE VENTILATION AIR OF BUILDINGS

There is a strong industry focus on packaged CHP systems for small scale applications where the design time for unique installations cannot be justified. Distributed generators such as microturbines, reciprocating engines and fuel cells can all now be purchased as CHP products. The development of these products will bring the energy, environmental and economic savings realized in larger applications to the smaller consumers. CHP systems traditionally operate most effectively and give the shortest payback when operated continuously at full output in a baseloading application. This is in conflict with a typical commercial building whose energy requirements vary extensively over daily, weekly and seasonal time periods. Just as CHP is not expected to supply the entire energy requirements of the industrial sector, so CHP should be looked at as merely part of the energy mix for the commercial sector as the capital cost of CHP equipment is typically higher compared to its alternatives and there are technical complications to supply a heating or cooling to power ratio away from design values. An economic CHP system must therefore have a capacity much lower than the peak load of the building to ensure high utilization of the system so that the larger capital investment can be recovered through energy cost savings as quickly as possible. In the absence of a year round continuous demand for either hot or chilled water a commercial CHP system must offer a diverse range of outputs so that the waste heat from the generator can be utilized as mush as possible particularly since the generator component is likely to dominate the capital cost of the installation. This paper proposes that the outdoor, or ventilation air stream into a building provides an excellent capacity match for CHP equipment packaged as a CHP Dedicated Outdoor Air System (CHPDOAS). Ventilation air has the largest temperature and humidity difference with indoor air of any stream of air in the building and so reduces the heat and mass transfer surface areas in the equipment. Also since the ventilation air is only a fraction of the total air flow rate that is being conditioned the CHP system can overcool the air in the summer or overheat the air in the winter and the effect is simply the reduce the cooling or heating workload of the conventional equipment since the ventilation air is then mixed with the bulk of the air remaining in the building before being conditioned. This means that the CHP system can run its generator for longer hours and at higher loads than would have been possible if the outlet conditions were set at space neutral or space supply conditions.Copyright © 2003 by ASME