Thermoelectric heating and air conditioning system with double flux ventilation in passive houses

Resumen

The fight against climate change has determined the need to decarbonize our economies. In this context, buildings, which are responsible for 30 % of world energy consumption (40 % in Europe) and 28 % of CO2 emissions (36 % in Europe), will change a lot in the coming years. The new building codes and the massive rehabilitation of envelopes will reduce the specific heating demand (kWh/m2), while cooling demand will be increased, due to global warming and the more stringent comfort demand. This fact, together with the need to phase out fossil fuel boilers, will massively promote the use of heat pumps for air conditioning in buildings. Although the vapor-compression cycle is the dominant technology in this sector, the IEA foresees in the “2030 sustainable development scenario” that 1.5 of the 14.5 TW of thermal power of the heat pumps deployed in the coming years will come from alternative technologies to the vapor-compression cycle, contributing to the phase out of CFC refrigerants. This thesis proposes the use of heat pumps based on thermoelectricity, whose main advantages are related with the absence of moving parts and refrigerants. Specifically, this thesis focuses on the design and construction of an air-to-air heat pump device integrated with double flux ventilation in homes, with a surface area limited to 100 m2, and a highly energy efficient, as “Passive House”, where the reduced heating load (<10 W/m2) enables the possibility to provide enough heating by solely rising the temperature of the ventilation air flow, taking advantage of the residual heat from the renovated air. Given the importance of heat exchangers in the overall efficiency of the system, two alternatives have been tested: finned heat sinks and finned heat-pipes. Heat-pipes have demonstrated to perform best in the temperature and flow ranges of this application; even in the case of the cold air duct, where heat-pipes work contrary to their original design. The prototype with the heat-pipes is lighter and narrower (which facilitates its installation in false ceilings), thanks to the thermal characterization of the heat exchangers, which determined that their behavior in the horizontal position was equivalent to the vertical one. Following the heat exchangers investigation, one prototype of an air-to-air thermoelectric heat pump was manufactured with a heating power of 1,250 W and 375 W for cooling with a variable COP 1.5-4 and 0.5-2.5, respectively. The prototype has been tested with the help of a climatic chamber in different winter and summer scenarios, working with different temperature gaps, air flows and voltages. Based on the empirical results, a fast and reliable computational model has been developed, capable of representing both, the thermoelectric effects in the thermocouples, as well as the variable thermal resistance of the exchangers as a function of the ventilation flow rate and the heat flow, under different operating conditions. Simulation results have been adjusted and validated with the empirical results. In a subsequent computational study, the number of modules of the heat pump was optimized for different heating and cooling needs, obtaining the maximum possible COP and enabling the comparison of two potential integration layouts of the heat pump with the double flux ventilation system. The stand-alone option as active heat recovery unit has been discarded, in favor of its integration with a passive heat recovery unit, commonly installed in “Passive House” buildings. This combination is more efficient in all the cases analyzed, requiring 5 times less modules. Additionally, the optimization of the seasonal coefficient of performance SCOP in three European climates, following the Directive 2010/30 / EU methodology for heat pumps, concludes that the optimal number of modules in the three cases is 15, which is a promising principle for the standardization of a thermoelectric heat pump model that could reach the market, oriented to “Passive House” dwellings. Finally, a pilot case has been investigated in a 74.3 m2 dwelling in a “Passive House” apartment block in Pamplona (Spain). The previously validated model was integrated with a simulation of the building, based on its “Passive House” certificate. The proposed HVAC system maintains stable comfort conditions both in winter and summer, thanks to the precise regulation system allowed by the thermoelectric modules. The system consumes 1,143 kWh/year (15.3 kWh/m2year) of electricity, which can be produced on-site using 4 photovoltaic panels of 250 Wp. This system is then compared with a vapor-compression cycle heat pump. This technology saves 20.9 % of energy with respect to thermoelectricity, but this only means saving one 250 Wp panel per home. The results derived from this doctoral thesis show that thermoelectricity can be a real alternative to the construction of heat pumps for the air conditioning of “Passive House” dwellings, given the big advantages (silent, robust system, lightweight design and easily installed on ceilings false, easy regulation, integrability with photovoltaic installations and potential savings in manufacturing costs) compared with its lower efficiency, that can be easily compensated with the increase of photovoltaic production integrated in the building.