Exhaust ventilation air heat recovery by means of heat pumps with natural refrigerants

CO₂ emissions can be reduced by a great deal in the residential building sector. Today, the average final energy consumption of European households is due to space heating by 65 % and to domestic hot water preparation by 14 %. The European Performance Building Directive 2010/21/EU defines the concept of Near Zero Energy Buildings, which aims to lower building heating consumption to nearly zero. Therefore, domestic hot water consumption will play an essential role in the European Union’s 2050 decarbonization objective, as the current percentage of 65 % associated with space heating will be reduced to nearly 0 %, and the share of domestic hot water heating in the total energy use of buildings will consequently increase. Furthermore, Near Zero Energy Buildings are characterized by a large amount of energy wasted to the ambient through ventilation heat losses. This is because high-performance houses, with reduced transmission losses and infiltration, require mechanical ventilation systems to guarantee indoor air quality.

It is well established that heat pumps are a highly efficient technology for water heating, and that they are a key tool for the decarbonization of cities, both now and in the future. Additionally, they could use the heat from the exhaust ventilation air as heat source, avoiding losing it in the outdoor environment. However, most heat pump systems currently employ environmentally harmful refrigerants. As a result, new regulations encourage and force the shift towards heat pump´s working fluids that do not deplete the ozone layer and do not contribute to global warming. Natural refrigerants are the favorable alternatives and Propane is increasingly being favored due to its affordability and thermodynamic properties, although its flammability poses safety concerns.

The objective of this thesis is to address research gaps in the field of heat pumps working with propane and using as heat source the wasted heat contained in the exhaust ventilation air of residential buildings to produce domestic hot water.
In this work, different methodologies of heat recovery from ventilation air to cover the building heating demand were studied by means of simulations, employing a heat recovery heat exchanger and coupling the mechanical supply and exhaust ventilation system of a low-energy Danish house with exhaust air heat pumps. A total of sixteen energy systems (for space heating, domestic hot water, and ventilation), including at least one heat recovery device, were analyzed and compared in terms of primary energy consumption and CO₂ emissions. Conventional technologies (natural gas boilers, air source heat pumps, electric water heaters) were utilized whenever the demand of domestic hot water and/or space heating was not covered by using exhaust air as heat source for heat pumps. For the system solutions having both the heat recovery heat exchanger and exhaust air heat pumps, different configurations were analyzed by locating the exhaust air heat pumps upstream or down-stream of the heat recovery chain, prioritizing heat recovery for space heating or domestic hot water.

An experimental analysis of a manufactured prototype of an exhaust air heat pump for domestic hot water working with propane, coupled with a heat recovery heat exchanger, was carried out to evaluate the system performance according to the current European normative in use. The collected data allowed to identify the performance of such heat pump technology and were used to validate a dynamic numerical model able to simulate the behavior of the system accurately and with reduced computational time. The developed model was applied to investigate the influence of heat source and heat sink conditions on the performance of the system. Furthermore, computer simulations were used to explore potential measures for improvements at system and heat pump-unit level. The storage tank efficiency was varied changing the insulation material, and its impact on the system performance was identified. Different condenser geometries were studied to assess the effects on the heat pump-unit performance and total refrigerant charge of propane. The study showed the relation between charge minimization and performance degradation of the heat pump’s vapor compression cycle.

At last, a study on the assessment of the potential of exhaust ventilation air heat pumps for domestic hot water production in warm climate was carried out. Specifically, a new propane-based exhaust air heat pump to domestic hot water generation system was designed. The system was meant to work at the extraction point of the exhaust air from the building indoor environment. An optimization process was formulated and applied in order to develop two systems working efficiently in an individual and in a collective building installation strategy. The two configurations of exhaust air heat pump for domestic hot water production were analyzed and compared to conventional technologies for domestic hot water generation, such as natural gas boilers and ambient air heat pumps. The results demonstrate the potential of using exhaust ventilation air as the heat pump’s heat source, as well as heat pumps in collective installations as opposes to individual installations, for energy and environmental conservation purposes