Apr. 20, 2022
How to Use PV Systems to Reduce Peak Grid Loads While Doubling Heat Pump System Efficiency
Now that it’s springtime here on Vancouver Island, the days are getting longer, the sun is showing itself more often, and we start to think of the upcoming pool season. Apologies if you had to shovel snow off your sidewalk today, spring is around the next bend, I promise.
As a high efficiency heating professional, I have been fortunate to get experience working on heating systems for many pools – both commercial and residential. Many of them are outdoor pools that are only operated in the summertime. This presents a unique situation where we get to play with daily and annual thermal energy storage.
Simultaneously combining pool heating with cooling or refrigeration is a great way to almost double the heating and cooling efficiency of a heat pump system.
IF it works.
But it’s not as simple as it sounds. I’ve been surprised with the results when I monitor the systems that I have designed, installed, and programmed. Everything is done correctly, to my obsessively high standards, but the system still doesn’t work quite as expected.
Reality steps in and reminds me that perfection usually requires a lot of fine-tuning.
Pool systems can easily be configured with water changeover where chilled water is delivered to the cold side of the heat pump while heating hot water is delivered to the warm side of the heat pump. The chilled water is used in cooling / refrigeration systems, while the heating hot water is used in heat exchangers serving the pools.
If a system is not designed with some type of simultaneous heating and cooling, an important opportunity for energy efficiency is being overlooked.
In heating-only mode, water from the ground loop is delivered into the cold side of the heat pump and heating hot water is delivered into the hot side of the heat pump. In cooling-only mode, water from the ground loop is delivered into the warm side of the heat pump and chilled water is delivered into the cold side of the heat pump.
Ideally, we want to avoid heating-only or cooling-only mode in these systems. We want the simultaneous heating and cooling mode to be used whenever possible. Running the system in cooling-only mode during the daytime when the pool is warm, only to run the heating-only mode later on in the nighttime when the cooling load is smaller runs the compressor twice, meaning the expense of the water change-over valving and controls isn’t paid back as quickly and the equipment wears out sooner.
Pools inherently have a lot of thermal storage capability. By right sizing the heat pump system and using controls programming to avoid unneeded use of the back-up heater, the heating load from the pool can be smoothed out to be continuous throughout the day. This means a slight drop in pool temperature at night, followed by a climb back to the setpoint in the afternoon when the sun is out. The temperature differences are minor, and morning lap swimmers typically like it cooler anyways.
So that works okay for helping to align the loads, just as planned.
But now the heat pump is running in heating-only mode and then the chilled water circuit needs cooling. The heat pump has to stop, wait for the valves to switch over, and then go into simultaneous heating and cooling mode after some delays. Then, when the chilled water circuit reaches setpoint, the heat pump needs to go through another changeover cycle.
This leads to excessive downtime.
Ah, the difference between a project on paper, and one operating in the real world!
I have the moto “solutions, not problems”, so I developed one: I started using the building cooling diversity to lengthen the chilled water cooling call. Peak cooling loads only occur for short times, on specific days. At all other times, the maximum allowable temperature of the chilled water circuit can be set to a higher value than the design temperature (humidity levels permitting). This increases the thermal storage capacity of the chilled water tank and the chilled water piping by allowing a larger per-cycle temperature differential during off-peak days.
On peak cooling days, the pool heating isn’t needed as much and a lot of the energy from cooling goes into the ground heat exchanger, which is okay because that warms it up and makes the heating mode more efficient hours, days, or weeks later.
This solution did not present itself effortlessly.
With my client’s permission to experiment, I spent hours and hours of my own free time fine tuning the system to optimize the simultaneous heating and cooling mode. My goal was to synchronize the systems so that they could take advantage of the times that my client’s and other PV arrays were pumping energy into the grid.
Eventually, I succeeded in my mission, and the system now operates in a synchronistic manner that allows it to re-use its own energy as well as store excess energy for later use.
I’m sharing this experience with the hope that it will be adopted by others, and perhaps even inspire you to think of new ways to look at the potential energy savings that are waiting to be discovered within your systems.