To evaluate the potential of daytime precooling and preheating to reduce evening energy use, testing was performed at the Grange house in Stockton, California. The house was extensively instrumented, and continuously monitored. Internal gains to simulate occupancy were provided by electric heaters and humidifiers controlled by the data acquisition system.
- Precool the house during the day by lowering the thermostat setting to 69°F at 8 am and raising it to 76°F at 5 pm. This stored cooling in the walls and ceiling during the day that was then used to cool the house in the evening.
- Follow the same precooling schedule while using Phase Change Material (PCM) to enhance precooling. This was done by adding 1424 square feet of PCM to the inside of the house’s walls and ceiling, and then running the 8 am to 5 pm precooling thermostat schedule.
One load shifting strategy was tested in winter 2018-19 during heating operation:
- Preheat the house, with installed PCM, during the day by raising the thermostat setting to 76°F at 8 am and lowering it to 68°F at 5 pm.
COOLING SEASON CONCLUSIONS
Two types of HVAC systems were tested in the Grange house over three different summers in Stockton, CA: a larger capacity single speed heat pump and a smaller variable capacity heat pump. Testing was performed to evaluate whether a daytime precooling schedule could effectively reduce energy use on summer evenings. Since the testing was done under varying weather conditions, results were normalized to weather data specified by the California Energy Commission’s Title 24 building energy code for Stockton. Relationships between daily energy use and outdoor temperature were found from the test data and used to predict the cooling energy use that would have occurred under Title 24 weather conditions.
Compared to using a constant setpoint, daytime precooling was found to be an effective strategy for reducing cooling energy use in the evening. This testing showed that energy savings between 5 pm and midnight ranged from 51 kWh or 71% for the 1 ton VCHP to 246 kWh or 82% for the 2 ton HP system. Adding PCM reduced evening energy use by an additional 27 kWh or 14% for the 1 ton VCHP, and by an additional 33 kWh or 10% for the 2 ton HP system.
Daytime precooling cools the house’s thermal mass during the day, and therefore uses more energy during the day. Testing found that from 7 am to 5 pm, the 2 ton HP system used an additional 396 kWh with precooling alone, and used 628 kWh more when PCM was used with precooling. This was 121 kWh and 192% more energy than was used under the constant setpoint scenario for a 1.5 ton HP system from the same product line with identical efficiency ratings to the 2 ton unit.
HEATING SEASON CONCLUSIONS
The winter 2018-19 Grange testing was performed to evaluate whether a daytime preheating schedule, along with PCM installed throughout the house to raise its thermal storage capacity, can effectively reduce energy use and electricity demand on winter evenings. This preheating schedule raised the thermostat setpoint to 76°F from 8 am to 5 pm and lowered it to 68°F the rest of the time. Test results are compared to testing done in previous winters where systems were operated with constant setpoints of 68°F.
Three types of HVAC systems were tested in the Grange house over three different winters in Stockton, CA. A 1.5 ton heat pump was tested using a constant setpoint schedule in winter 2015-16, and a 2 ton heat pump was tested in 2018-19 using a preheat schedule with PCM. A 1 ton VCHP was tested in winter 2017-18 under the constant setpoint schedule and tested again during 2018-19 under the preheat with PCM schedule. Electric resistance heating was also run as a reference heat source in 2017-18 and 2018-19.
Since these tests were run under varying weather conditions, results were normalized to weather data specified by the California Energy Commission’s Title 24 building energy code for Stockton. Relationships between daily energy use and outdoor temperature were found from the test data and used to predict the heating energy use that would have occurred under Title 24 weather conditions.
Compared to running at a constant setpoint, daytime preheating was found to be an effective strategy for reducing heating energy use in the evening. Testing showed that energy savings between 5 pm and midnight ranged from 109 kWh or 87% for the VCHP system, to 159 kWh or 96% for the HP systems, to 247 kWh or 94% savings for ER heat.
Heat pump, HVAC, VCHP, all-electric, electrification