Background
In Switzerland, no uniform definition of a nZEB existed at the time of the field monitoring. However, in March 2011, the MINERGIE-A®-label has been introduced as an nZEB concept requiring a net zero energy balance in the boundary of the building technology. While initially only applied for residential buildings, an extension to offices has been introduced in May 2014. Therefore, not much experience existed and monitoring data were required for office application. Moreover, as innovative technology at the time, a PV/T collector for DHW preheating is installed and a publicly rentable electric car serves as local electricity storage for on-site generated PV-electricity to enhance local self-consumption on on-site PV electricity.
Key facts
Description of the technical concept
The building is equipped with a ground-coupled heat pump with a capacity/COP of 33.1 kW (30.5 kW)/4.6 (3.0) at B0/W35 (W50). 11 double-U-tube ground probes of 80 m depth each serve as heat source. The building has a floor heating with design temperatures of 35 °C/30 °C both for the offices and the 7 flats. In order to meet the nZEB balance a solar PV-system of 128 m2 is installed on the roof. With a peak power of 23.7 kWp., an annual yield of 1007 kWh/kWp has been calculated. Additionally, a PV/T collector of 7.1 m2 of 180 Wel.p/m2 and 310 Wth./m2 is coupled to a 500 l storage to preheat the DHW. The DHW is reheated by the heat pump in a 1000 l storage which also contains a direct electrical back-up for legionella protection. Additionally, an 800 l heating buffer storage is integrated in parallel and a balanced ventilation with 80% heat recovery efficiency is installed. In summer time, the borehole field is used for free-cooling operation. Since only electricity is used as energy carrier, the building can be characterized as all-electric building.
Current Market State
The MINERGIE-A® label has been revised in 2017 to cover the whole energy balance incl. appliances, which has to be compensated with on-site PV electricity on an annual basis. Electricity is weighted with a factor of 2 for the balance. However, exported PV-electricity is weighted with a reduced factor of 0.8, so there is still an incentive for self-consumption.
Field Monitoring Results
The objectives of the field monitoring is the evaluation of the net zero energy balance acc. to the MINERGIE-A®-label requirements. As further aspect the load match of office use and PV yield is investigated in terms of self-consumption. In this context, also the use and availability of the electric car is of interest as local electricity storage option. Moreover, the performance of the PV/T collector for preheating the DHW is evaluated.
The monitoring period lasts from May 2014 to April 2016. The heat pump reached a seasonal performance factor for SH and DHW of SPFhw = 4.3 (4.5) in the first (second) year, with 4.6 (4.9) in SH and 3.5 (3.5) in DHW mode. Including the solar DHW preheating of the PV/T collector, the overall performance is SPFgen=4.8 based on the generated heat. The ground-coupled free-cooling operation reached an SPFc = 17, since only electricity for the pumps is used. According to the user statements, comfort is fulfilled by only using the passive cooling. By the free-cooling operation, the SPF of the DHW mode is increased by 5% to the warmer source temperatures is summer.
The heat pump is with 45% (58%) the largest electricity consumer of the building technology. Based on the used energy, however, the SPF decreases significantly to SPFsys=1.3, which is mainly caused by the accompanying DHW distribution pipe heat tapes, which consumed more than 30% of the balanced electricity in the first year. Despite this large consumption, the MINERGIE-A® requirements could be already reached in the first year with a balance of -8.6 kWh/(m2a), i.e. a PV surplus of 8.6 kWh/(m2a) regarding the consumption of the building technology.
The PV/T collector reached a thermal collector efficiency of 27% and a solar fraction of 13% of the DHW in the second year with a thermal production of 1400 kWhth/a similar in both years. By rejecting the heat from the PV-cells, an increase of electrical efficiency by 1.3% to 10.8% could be reached, which was evaluated by the performance with and without thermal collector operation on single days with similar irradiation and temperature conditions. The electrical yield of the PV/T is 900 kWhel./a, also similar in both years.
The yield of the solar PV was in total in both years about 24,500 kWhel./a which is with 1030 (1040) kWhel./kWp slightly above the calculated value.
System performance and optimization
The overall system performance of the system is with 4.5 in a good range for ground-source heat pumps. By the solar preheating with the PV/T collector the heat pump tends to work at higher DHW temperature for the storage reheating, and therefore, the SPFw of 3.5 is limited.
Based on the monitoring results of the first year optimisation measures have been implemented and approved by the performance in the second year.
The very poor system performance of 1.3 could be improved by the accompanying DHW pipe heating which is operated for DHW comfort reasons. However, by the change of control strategy of the direct electric heating to a scheduled operation, the electric consumption could be reduced by 60% without complaints of the users. Thereby, the MINERGIE-A® balance could be increased from
-8.6 to -15 kWh/(m2a).
The electric car was not so frequently used and could thereby contribute to self-consumption of the PV-electricity, which was limited by the charging capacity, though. In the future, a smart charging control shall be applied, where the charging power and time can be adapted to the instantaneous PV surplus. But this would hard- and software update of the charging station and the car, which, however, are now available.
Economy, ecology, costs and self-consumption
At the time of monitoring, the PV-tariff with cost equivalent feed-in tariff called “KEV” was high, so there is no financial incentive to enhance the self-consumption of the on-site generated PV-electricity.
The PV-electricity necessary to meet the MINERGIE-A® nZEB balance, though, could not be sold under the KEV, but only the surplus of the balance, since the so-called “ecological added value” of renewable energy generation had to remain at the building site to fulfill MINERGIE-A® requirements. Thereby, for a MINERGIE-A® certification, self-consumption was beneficial. In 2017, though, this was changed, so presently, all PV electricity is taken into account. However, weighting factor of 0.8 instead of 2 for the balance still are an incentive for self-consumption.
Moreover, KEV has been replaced by a 30% subsidy of the PV investment cost for systems of 2 to 30 kWp. But, at current PV system prices, it is possible to reach generating cost of PV electricity of 0.15-0.2 CHF/kWh, which corresponds to the electricity price for private consumers in daytime tariff. Common feed-in tariffs of PV electricity without KEV are around 0.04-0.1 CHF/kWh, so there is an incentive for enhanced self-consumption for smaller system. The heat pump operation and the electric car have been investigated in this context.
