Topical Article: Simulations of Grid-Responsive HVAC Cooling Measures via Ice/PCM storage


Electricity is commonly used to cool buildings. And cooling of residential houses also occurs when the overall electricity demand is high. Thus, cooling adds to peak demand. In the case of differentiated electricity prices, peak-hour cooling also has a negative impact on the energy bill. Both these effects could be avoided by moving the need for electricity from peak hours to off-peak. Such measures are called grid-response.

This article describes simulations made in the USA of two types of grid-response measures. The first one focuses on a heat pump with a two-speed cooling coil, reducing its capacity to approximately 75% of its maximum capacity during peak hours. The other measure is using ice or other phase-changing material (PCM) for cooling during peak hours while also turning off the electric cooling coil.

The impact of these measures on overall energy need, peak hour energy demand, comfort level, and total energy cost was evaluated using simulations. These tests were carried out on two single-family homes to simulate two different climates. Atlanta, Georgia, represents the southern climate, and the northern climate is represented by Indianapolis, Indiana. Annual energy simulations were carried out for the two alternatives and were compared to a baseline with a cooling system in normal operation.

Three scenarios were included in the simulation with reduced cooling coil capacity. The first was the baseline scenario, and the second was the scenario with reduced compressor speed during peak hours. In the third scenario, the compressor speed was reduced in tandem with the indoor airflow rate, resulting in improved dehumidification. Both alternatives reduced the peak power production by 26-28% in both cities compared to the baseline. At the same time, the total amount of cooling (in kWh) remained relatively constant, and the only significant reduction of comfort was seen in the dehumidification scenario in Indianapolis. The downside was an increase in annual electricity need and cost for the dehumidification scenario, therefore a lower seasonal cooling COP.

Three scenarios were considered in the PCM simulation: the baseline, ice, and other phase-changing material. During the latter two scenarios, the heat pump compressor was fully switched off during peak hours. The cooling is instead provided by the phase change material used. The storage is recharged during off-peak hours, using electricity. In all three cases, the total amount of cooling was very similar. Compared to baseline, the total electricity consumption increased for both scenarios, but someone primarily moved it to off-peak hours. This also caused a lowered electricity cost. The electricity consumption and cost of the PCM storage is lower than that of the ice storage due to different COP for the two methods.

Bo Shen, Jian Sun, Oak Ridge National Laboratory

 

This text has been shortened by the HPC team 

Click here to read the full article >

Click here to read the full HPT Magazine >