Last month my post about long term storage of solar energy using hydrogen generated a few interesting questions about why we are not using heat pumps, solar thermal collectors, and heating in general. And these questions are certainly relevant. Part of the reason I have been reluctant to discuss thermal energy is that it is a huge field, and one in which there has been a great deal of research over the years, particularly by in the 1970s and 80s when the OPEC oil embargo really demonstrated to many just how dependent we are as a society on the supply of relatively cheap oil as our primary energy source.
So in this blog post I will cover some basic ground and share my thoughts and answer Gord’s question about heat pumps. In the next blog post, I will cover some possible solutions and maybe even future plans. I will try not to go too heavy on math and physics, partly because this analysis would require more space than a single blog post. But mostly because so far I am just dealing with rough back-of-the-envelope thinking remembering that I am not a mechanical engineer.
Heating is a major consumer of energy in homes. In our house, we use about 500 to 600 gallons of diesel per year for space heating, and another 400 to 500 gallons of propane a year for domestic hot water, cooking, and our gas clothes dryer, at roughly $4.50 per gallon that represents a not insignificant heating cost of around $5000 per year at current prices. We all know that the cost of fuel is increasing, I would guess that this amount will double over the next ten years. So why not go electric, you ask? There is the rub. One gallon of diesel contains about 137,000 BTUs or a little over 40 kWh of energy. Burning fuel in in a space heater, like our Toyo, is about 87% efficient, so each gallon of diesel consumed in the Toyo is equivalent to about 35 kWh of energy. So that means my 500 gallon fuel tank stores about 17.5 MWh of energy when used with our Toyo space heater. That is a lot of energy! But, if I use that 500 gallons to generate electricity with a diesel engine connected to a genset, then my efficiency drops significantly. I can only get about 10kW per gallon of diesel with my generator so the 500 gallon fuel tank only stores about 5 MWhs of energy if used to power an electric space heater with my generator. So, without solar, I would need 2000 to 2400 gallons of diesel to heat my house.
Of course, we do have solar PV, but the time of year I need the most heat (when the temperature drops as low as -40 degrees), is the exact time of year when I have the least solar radiation. So at least of for the winter months, if it is a choice of burning fuel in the generator to power an electric space heater, or burning fuel in the Toyo, the choice is obvious, the Toyo is 3 to 4 times as efficient as converting the diesel to heat energy, converting the heat energy to mechanical rotational energy, converting the rotational energy to electricity, and then converting electricity back to heat.
Now, you might ask, why not use a heat pump since they are significantly more efficient after all than a resistive heating element? Here the math still does not work out, at least in winter for air-source heat pumps. The efficiency of heat pumps is measured in the so-called coefficient of performance or COP. The COP is the ratio of heat to work. Work here is the energy input to run the heat pump. So, a COP of 2 would mean that for every kW of energy input to the heat pump, the heat pump would be able to move 2kW of heat. Based on the above, to beat the Toyo, the heat pump would need a COP greater than 3.5 to break even. But there is another problem, the COP of heat pumps decreases at the temperature difference between indoor and outdoor temperature increases. Generally, heat pumps become significantly less efficient below freezing, so a heat pump with a relatively high COP of 4.5 may drop to a COP of 2 or even lower as the temperature drops. Typically, most heat pumps switch to a supplemental source of heat, often an electric element to improve the heating performance as the outside temperature drops into double digit subzero temperatures. And of course, these very cold temperatures occur both in winter, when there is very little sun, and a night, when there is no sun.
So, at least for the three coldest and darkest months of the year, when a larger portion of energy is likely to be produced by a diesel generator, a typical air-source heat pump is less efficient than the diesel powered Toyo. Now, of course, if there was a sufficiently large amount of long-term energy storage (such as the hydrogen storage system discussed last month), then the inefficiency might be tolerable, but of course, this increases the cost and complexity of the storage system.
So a second alternative may be a ground-source (or geothermal) heat pump. These work by extracting heat from the ground, where the earth maintains a relatively stable temperature throughout the year. This solves the problem of the temperature difference, but these systems are much more difficult to install, and computing the COP of the entire system is more complex. Remember to goal is a COP of 4 or greater to break even.
There is another, more subtle issue with ground-source heat pumps in a northern climate. That is there is little to no heat recharging in the summer. What I mean is that in a more moderate climate, the heat pump is used for heating in the winter and cooling in the summer, with the heat transferred about equal. In Talkeetna, on the other hand, the ratio of heating and cooling days is very lopsided. Consider the mean average heating degree days compared to the mean average cooling days in Talkeetna.
“Degree days are based on the assumption that when the outside temperature is 65°F, we don’t need heating or cooling to be comfortable. Degree days are the difference between the daily temperature mean, (high temperature plus low temperature divided by two) and 65°F. If the temperature mean is above 65°F, we subtract 65 from the mean and the result is Cooling Degree Days. If the temperature mean is below 65°F, we subtract the mean from 65 and the result is Heating Degree Days.”
https://www.weather.gov/key/climate_heat_cool
Looking at weather statistics accumulated by the National Weather Service and reported by the Alaska Climate Research Center, the Heating Degree Day Normal from the period 1991-2020 was 10592.3 mean annual heating degree days. By contrast, the Cooling Degree Day Normal for the same period with the same base of 65°F is 16.8 mean annual cooling degree days. We are not in Texas, that’s for sure!
Over time and over seasons, the available heat from the ground source becomes less, and so the efficiency of the system drops over time.
This all seems pretty dire, and it would be if it were dark and cold year-round. But of course, that is not the case. Outside of the three months of the winter, there is a good solar resource here, and much of the year is in the sweet spot of having more than 7 or 8 hours of day light, and having temperatures above freezing. Heating in the six months of Spring and Fall is a different problem. One is generating most of the required energy from the sun, and second is storing the heat so that it can be used on cool or cold nights. One of my “bar napkin” computations seemed to indicate that on average, our heating load is about 35kWh per day during the non-winter period. Of course, some days this number could be much higher, when the temperature dips, and some days a bit lower. This could be way off, so I would want to check with someone who actually does this for a living, but it represents about a gallon per day of diesel used in the Toyo, which seems about right.
So, now assuming that we could generate about 35kWh a day from PV, the problem becomes one of storage as the energy is produced during the day but is needed primarily at night. One option would be to add an additional 35kWh of battery storage. This is certainly doable, however, at current prices (for at least my system) this would cost over $20K just to store energy for a few hours. Another concern is with lithium battery efficiency, it is much better than a generator, or long term hydrogen storage, but there is still around a 10-15% loss in my system (between battery charging and discharging efficiency, voltage drop on my low voltage cables, and the standby losses of the battery management system).
Other factors not yet considered include the current uses of propane: for water heating, cooking, and clothes drying. Clothes drying is somewhat optional now, since we do also own an electric tumble dryer that is used primarily when the sun is shining and the batteries mostly charged for the day. So the question to consider is whether it is worth it to move this usage off of LPG and on to a renewable resource. A second factor to consider is whether or not it is worth maintaining a non-electric (or at least a low electric power) source of heat for redundancy in case of a system failure in cold weather.
Next time: PVT and hot water
