Is Solar Hydronic Heating Feasible?
The gathered data confirmed that it is possible to provide heating to The Green Power office using the heat pump hydronic system. Nevertheless, the installed PV system itself was not sufficient to power the HP during the winter period. Therefore, the biomass boiler must be added in order to make the system run completely off-grid. The choice of the boiler was determined by the design heating load from the LoopCAD software. For the extreme thermostat temperature of 22⁰C, the total heating load calculated by LoopCAD for stand-alone building is 7.3kW. In order to provide sufficient heating in case of no solar radiation, the biomass boiler with at least 7.5kW capacity must be incorporated into the system. The higher capacity is not necessary, as the hot water tank retains some of the heat inside. Furthermore, the office is placed in semi-detached building, therefore the real heat losses would be lower.
The chosen boiler for this purpose is the smart pellet boiler ETA PU 7 made by the brand ETA. This boiler can be controlled with smart devices and work on either partial or full load capacity. Its nominal capacity is 7.7kW, therefore it would be sufficient to heat the office when there is no production of electricity from PV panels and when the temperature is the dimensioning temperature. Nevertheless, no PV production is unlikely to happen, as there should be at least some PV electricity, and the comfortable temperature in the room could be less than 22⁰C.
Alternatively, more PV panels could be installed, with the additional capacity of at least 7.3kW, but this would only be an appropriate alternative in constant sunny weather.
Therefore, in order to achieve an optimal solution for system operation, a smaller heat pump with capacity of approximately 7.5-8kW should be installed instead.
The underfloor heating system with a heat pump is able to cover the heating demand of The Green Power Company. However, the currently mounted photovoltaic panels are not sufficient to power the HP in winter. The research included the proposal of the smart pellet boiler ETA PU 7 with nominal capacity 7.7kW to be incorporated into the system enabling it to run completely off-grid.
Considering thermal comfort, it is concluded that the most comfortable temperature in the office was 21⁰C. Moreover, the optimal heat pump setting was found to be 35⁰C, taking into account lower electricity consumption. For the temperatures to be stable and comfortable, the system should be controlled automatically by NEST thermostat on a daily basis based on the schedules of The Green Power Company’s employees. The settings should be changed manually using smartphones or other remote controls if the schedule is changed, or in case there is nobody in the office. Moreover, the system control should be linked to the storage tank. The NEST thermostat connects directly to the HP and gives signals to the HP based on the room air temperature, thus ignoring what heat is stored in the water storage tank.
The calculations in LoopCAD program were found to contain some errors, as the heat output from the wooden floor is higher than the heat output for cement floor according to the software calculations. Further investigation is required to identify these errors.
Future testing will include installing double glazing structures on the windows, as well as mounting a draft seal to the bottom of the door to cover the leakage. This is scheduled for 20th of August 2018 onwards. The current research will be continued for a couple of weeks, this time with fully automated NEST thermostat settings with no change in behavioural patterns of users. In this further research, the optimal solution for the thermostat settings with the least HP energy consumption will be found. Moreover, the lower heat losses stemming from the additional glazing and covering the hole beneath the door will be evaluated.
In practice, the Coefficient of Performance of the heat pump decreases by 0.6-1 for every 10⁰C difference, giving 0.6-1kW less heat output per kW of electrical input. Therefore, since ∆T should be as small as possible, relatively cool heating loop and warm external loop are desired.