# Performance of a Mid-Size Net-Zero Energy Solar House
## 6 Actual Performance at the Site  
The house was reassembled in the competition village located in Denver over a period of one week. Several sensors were installed throughout the house by the Solar Decathlon engineers to monitor the thermal comfort performance of the house. The PV power generation system was tied to the microgrid of the village. Net metering was established for the house to account for the balance of energy generated/used by the house. Figure 6 shows how well the HVAC system of the house was able to maintain the indoor temperature within an established narrow band, i.e., 68-74 \({}^{\circ}\)F. The lowest outdoor temperature was 28.1 \({}^{\circ}\)F, and the highest temperature was 79.4 \({}^{\circ}\)F. The figure shows that rapid temperature fluctuations caused temperature overshoot inside the house on two occasions. Figure 5: Monthly simulated AC energy output of the PV system in Birmingham and Denver along with the corresponding magnitude of solar radiation. Bar plots show the generated power, and line plots show the solar radiation. Figure 6: The indoor temperature of the house vs. outdoor air temperature and the acceptable temperature band. The relative humidity (RH) performance of the house is depicted in Figure 7. Maintaining the RH inside the house was a tougher challenge considering the fact that there was no active humidification mechanism in the house. As Figure 7 shows, outdoor humidity reached values as low as 12% at times. This is evident from undershoots of humidity value inside the house on two occasions when there is a rapid drop in outdoor humidity in a period of roughly one hour. Dehumidification, however, was accomplished by condensation on the evaporator coils of the ductless air conditioning unit. It is worth mentioning that the team did not lose points if the overshoot or undershoot of either temperature and humidity happened during defined unoccupied hours. The carbon dioxide level inside the house was mandated to be controlled under 1000 ppm at all occupied times. The main source of carbon dioxide in the house was the occupants, as there was very little cooking activity during the competition. Similar to the temperature and humidity limits, points were deducted from the teams only if overshoot occurred during occupied hours. The energy recovery ventilator (ERV) controlled by the CO\({}_{2}\) sensor was the device that brought fresh air as required to control the CO\({}_{2}\) level. Timed operation of the ERV was at one point considered due to ease of setting. However, CO\({}_{2}\)-controlled operation was decided to be more energy-efficient. Figure 8 shows that the ERV performed fairly well with only a few overshoots which could very well be during the time that the team was in the house performing tasks while not considered contest time. Energy use of the house, in contrast to energy production of the house, was monitored during the competition and the results are presented in Figures 9 and 10, respectively, in the form of power used and produced. Figure 9 shows that on average, the power use of the house is below 3 kW, while the maximum power demand is a little over 7 kW. The highest demand resulted from simultaneous cooking and air conditioning in the house. The PV system installed on the house had a 9.9-kW\({}_{\text{p}}\) capacity. Figure 10 shows that the system reached a peak power production of 7.3 kW. Negative values of power generation indicate that at that period, the rate of energy use by the house surpassed the rate of energy generation due to high demand falling on after-sunset hours. Figure 7: Measured indoor relative humidity and outdoor humidity showing the acceptable band. Figure 8: Measured carbon dioxide concentration inside the house. Figure 9: Measured energy consumption by the house at the site. The performance of the PV system of the house along with its reduced energy use resulted in ranking 1 st in energy generation among the competitors by ending at 74.4 net-positive kilowatt-hours.