Yes. You read that correctly. This is the story of one of the most exciting upgrades we will be making to our property this year. It will allow us to continue storing our vegetables on a year round basis while also making our house more energy efficient and comfortable to live in. You might be aware that we made a big investment in a solar photovoltaic system a few years back to offset our electricity consumption for grow lights and refrigeration, but that investment doesn't give us permission to waste energy. In fact, to maintain any hope of actually approaching net zero, we'll need some significant upgrades to our home heating systems as well, so on we go. As I write this, the plan has not yet been fully executed, but most of the equipment has already arrived and I will be hooking everything up over the next month or two. Read on for all the details.
The plan started with a problem, as many do. Our problem was that our walk-in cooler pumps heat into the basement of our home. It uses a common window air conditioner to extract heat from the cold room, and since it is installed on an interior wall, that heat is exhausted to the interior of our home. I could have installed the air conditioner on an exterior wall, but I figured that keeping more heat inside our home would be generally better since we are needing to heat our home at most times in the year. Since first building our walk-in cooler in 2014, we have also insulated our basement thoroughly, so now when temperatures rise in summer and our cooler air conditioner needs to work more, that extra heat no longer wicks through the once bare concrete walls. Instead, our newly insulated basement walls now keep all of that heat inside. It's not cool, literally!
Some of you will I'm sure be wondering why we don't just cool our walk-in cooler with outdoor air. The average annual temperature in Saskatoon is only around 3ºC after all. Well, we do actually have a simple thermostat and ventilation fans wired up for this purpose (see below), and it works wonderfully from November through March, but in all of the other months we can't count on this ventilation to provide enough cooling power.
So back to the dilemma of what do to with all that heat we pull out of our walk-in cooler. I could resort to installing the air conditioner on an exterior wall, but as an energy conscious Canadian trying to make the best of things in this oh so cold climate, I tend to look for ways to preserve my heat like my grandmother preserves her plastic bags. I don't just want to pump that heat energy out into the open air in summer. Surely, we can make use of it somehow! Enter the air to water heat pump.
Thankfully, we do still have a need for heat in summer, because we regularly use warm water for dishes and bathing. This is where the air to water heat pump can help us. Like our window air conditioner, an air to water heat pump is designed to extract heat from air, but instead of dumping that excess heat into the outdoor air, it dumps it into water. I don't even have to MacGyver something together to make this work for us, because there are air-to-water heat pumps on the market already that are appropriate for our scale.
If you've replaced a water heater recently, you might have seen or considered an electric heat pump water heater like the one shown in the drawing below (or like this one). It extracts heat from the room it's in and uses that heat to warm up the water in the tank. A regular electric water heater gives you 1kw of heat for every 1 kw of electricity used for the heating element, but a heat pump water heater can give you 2-3kw of heat energy for every 1kw of electricy used to power the heat pump. That extra heat doesn't magically appear out of nowhere. It is extracted from the heat in the air around the unit. This means that the efficiency of these units will depend on the temperature of the air. The warmer the air, the easier it is to extract heat to warm up the water in the tank.
So naturally, one of my first thoughts was to put one of these heat pump water heaters in our walk in cooler and let it do its thing. I would just need to add a thermostat to turn on the water heater when our cooler was too warm and turn off the water heater when the cooler was cool enough. However, after looking deeper into the specifications of these units I decided against this. They are designed to function well at an average room temperature, but efficiency really plummets when the air temperature drops. If I put one in our 4ºC cooler and challenged it with the task of extracting heat, I don't think I could count on great results. Add to that the fact that it makes absolutely no sense to store a hot water tank inside the room that you are trying to cool. On to the next idea...
It turns out that there is a device perfectly designed for just the task I was imagining. The Sanden CO2 Heat Pump Water Heater extracts heat from the surrounding air and transfers that heat to water. Just how well can it perform this task? For that answer, we turn to an independent team of researchers from the Washington State University Energy Program who conducted several trials with the Sanden heat pump and published this report with their findings:
One of the key pieces of information in this report was their table of results showing the heat pump efficiency at different temperatures. The variable of particular interest is the coefficient of performance (COP), which tells us the ratio of output capacity to input power. For example a COP of 2 would mean that a heat pump would deliver 2 kW of heating energy for every 1 kW of electrical energy supplied. Note that the COP numbers are all well above 1 even down to the 17ºF (-8ºC) air temperature tested.
What's missing from this data is a test at the 4ºC (40ºF) air temperature, which would be the consistent air temperature inside our walk-in cooler. Not to worry though. A simple graph can reveal the trend here and allow us to estimate the COP for that 40ºF air temperature.
Now with that data on a graph, we can see that the there is a pretty linear correlation between the outside air temperature and the COP, so a line of best fit can help us figure out the corresponding COP for an air temperature of 4ºC. According to the dotted line drawn there, we can expect a COP of 3.1 when our heat pump is working away in our 4ºC walk-in cooler. That means, when we use 1 kW of electricity to power the heat pump, we can extract 3.1 kW of heat energy out of our walk-in cooler and store that heat in our hot water tank. That sounds pretty darn fine to me! Best of all, the heat removed from our walk-in cooler will no longer be added directly to our interior living space and that will really help keep our home cooler in the summer months.
In order for this system to regulate itself based on the temperature of the walk-in cooler, I will be wiring in an external thermostat to tell the heat pump when to turn on. If the cooler air temperature rises above 4ºC, we want the heat pump to start extracting heat, and when the temperature dips below 4ºC, we want the heat pump to turn off. It is highly unlikely that our hot water usage will perfectly match the heat output of the heat pump so we also need to build in the following adjustments.
One possibility is that the heat pump will cool the walk-in cooler to 4ºC to easily, meaning that it won't actually end up heating much water. To respond to this scenario, I will wire a water thermostat ( labeled T in the diagram below) to some ventilation fans and set up the system to vent the cooler when more heat is needed in the hot water tank. In other words, if the hot water in the tank is below 40ºC in the summer time when we would appreciate a cooler house, the fans will turn on, inviting cool air into the living space and pumping warm air into the cooler. The heat pump will proceed to extract that excess heat, giving us more hot water and a cooler house.
There could also be a scenario when the water tank isn't warm enough but we don't want to extract any more heat from our cooler or living space using the heat pump. In this case, we will need a supplemental source of heat that can respond on demand. We can solve this problem by installing a tankless water heater after the hot water storage tank as shown above. This tankless water heater will draw power when we call for hot water and it can be easily be set to increase the incoming water temperature to 40ºC for use at all of our hot water fixtures.
So there's the plan, and that is where I need to end this story for now. Our new Sanden heat pump patiently awaits me in our walk-in cooler and I intend on making time to install this system before the heat really settles in this summer. At that point, I will add some photos, and eventually do a follow-up on the performance of the system in different seasons as well.
UPDATE: The heat pump is fully installed now and has been successfully pulling heat from our cooler and using it to heat our hot water tank. You can see the basic components of the system in the video below, which shows you all three of the cooling methods we have used to cool our walk-in cooler: automated vents, a window air conditioner, and an air to water heat pump.
