Performance of an air-to-air heat pump heating and recovery unit at high ventilation rates

Abstract

This thesis reports on design and performance studies of a prototype combined air source heat pump and storage system, retro fitted to heat occupied spaces subjected to high ventilation rates. The source of heat is from the air in the extract duct. Two limiting thermal conditions exist. The first is the thermal capacity of air passing over the ducted heat exchangers. The second is the dew and freezing points of the exhaust air, because of the insulating effect of ice on exchanger fins and tubes. Both are alleviated to a significant extent with high mass flow rates passing down the duct, since more heat can be extracted for a set decline in exhaust air temperatures.
This study identifies reasons for ventilation and building strategies involving high ventilation rates, including the physiological and emotional needs of people and the various economic, climatic and Governmental polices (climate change levy, public
health legislation) that impact upon heating and ventilation design. The study recognises the need for reduced carbon dioxide emissions and explores issues of indoor air quality and sick building syndrome and how increased ventilation rates can address them. The proposition investigated in this thesis is that air source recovery and heating by heat pump systems, combined with a heat storage system, can economically allow increases in ventilation rates to well above current standards without incurring great increases in energy use and carbon emissions; and in some circumstances reducing them.
The thesis discusses in depth and detail, the advantages and disadvantages of possible alternative methods of heating a building and ventilation recovery, comparing their effectiveness and cost. A prototype system has been designed and field trials of a retrofit application have produced performance data that has subsequently been used in a long term cost comparison.
The rig's design and construction are fully documented and its function over a full heating season is comprehensively explained (recording methods, types of calibration, control choices etc). A theoretical estimate of the energy requirements could have been attained using simulation and degree day information, however, a real like-for-like comparison using field trials prepared and a model was developed which allowed test data to be used to predict costs. The rig was tested over two heating seasons and compared with actual reading from alternative heating systems, degree day calculations are discussed but the reliance is on the actual live data gathered. (although summer cooling is achievable with the test rig no readings were recorded or comparison made).
The work shows that heat pump heating and recovery systems and combined storage ability out-performed the other systems investigated. The crucial elements of its functionality are the high temperature of the heat source and the vast volume (and thermal capacity) of air being used, extracting at 24 °C and delivering at 35°C. The Coefficient of performance varies through the heating season but, synthesis of theory with test rig performance demonstrate that the longer term cost of the system is attractive; and its attraction shall probably grow with anticipated future trends in consumer demands for comfort and air quality coupled with fuel costs and a philanthropic social and political attitude to emissions control.