Last month I promised a deeper dive into solar thermal and PVT technology and, more importantly, whether or not we could use this technology to off-set some or all of the propane and diesel fuel we currently use to heat our house and provide year-round hot water. In researching options, I found that the Cooperattive Extension Service at our state land-grant college, the University of Alaska Fairbanks, has published A Solar Design Manual for Alaska which contains a great wealth of practical information on both solar PV and solar thermal systems in far northern climates. I also reached out to a vendor of solar thermal systems and have a proposal in hand that would generate enough thermal energy to heat our house and provide hot water for 9 months out of the year and actually produces enough heat for all 12 months, but, again, storage is the problem. More about that below.
This is consistent with what I learned from the extension service. There just is not much sunlight in winter above 60 degrees North latitude, and once solar panels (PV or thermal) are covered in snow, they do not produce anything at all. So that means that an ideal solution would be on that sheds snow, and optimizes production as much as possible. A new challenge for solar thermal is that PV prices are now so comparatively low that for many applications using electricity produced by PV panels to run a heat pump or even resistive heating is still more cost effective than a thermal system.
Trying to even find a local supplier of solar thermal equipment proved to be difficult, not a good sign at all. I turned to Hydrosolar, a Canadian dealer for Abora Solar, a Spanish compnay that manufactures aHtech hybrid PVT panels for some design help and budgetary quotations. Roger at Hydrosolar was very helpful and ran some simulations and proposed a starter system with all of the components needed to get going.
So, what is a PVT panel? They are not mentioned in the Solar Design Guide I linked to above. In short they are a hybrid solar collector that includes both a traditional PV panel to produce electricity and a Thermal panel to collect heat. Before going on it might be worthwhile to discuss Solar Thermal collectors in general.
Here is some background. There are a few common ways to collect solar thermal energy today. One very old technology is a batch heater. That is basically a tank that sits in the sun. Not very common or efficient today.
The evolution of the batch heater is the flat-plate solar collector. This uses a flat surface painted black that absorbs heat and transfers it to coils of piping with a circulating heat transfer fluid that is pumped inside to circulate through a tank where some of the heat is removed and then the fluid then circulates back to the collector where it absorbs more heat.
The other technology is the vacuum tube collector. These use vacuum cylinders or tubes to prevent additional heat loss to the environment and because of their shape, can absorb solar radiation over a wide range of directions.
PVT is simply a hybrid of a PV panel with a flat-plate solar collector. The PV panel acts as the solar absorption plate, performing as usual, but with a heat-transfer fluid circulating behind the panel, cooling it in summer, and transfering the heat to an indoor tank, as with a traditional flat plate thermal collector. Abora Solar claims that their panels are thus able to capture 89% of the solar radiation that falls on the panel, as opposed to about 20% for a typical PV panel. The cooling performance in the summer increases the output of the PV panel (since PV performance is inversely proportional to temperature), and in winter, the heat loss from the fluid can raise the temperature of the panel above freezing to melt any snow that falls on the panel.
That heat loss also means that the PVT panel does not produce as much heat in the winter. Of course that is potentially offset by the increase in PV production caused by snow removal from the panels. In a roof top situation, this could be a significant benefit as we have snow on our roof from November through March which is why we did not mount any of our solar panels on our rooftop. But that also means that in winter, we would need some other heat source to at least support snow melt off of the PVT panels. That heat could come from vacuum tube collectors, or it could be from a heat pump (or even our existing propane hot water heater).
Vertically mounted vacuum tubes are one solution to this issue. By mounting the tube vertically, winter efficiency is increased, while summer is decreased. Since heating load is less in summer, this may not be a problem, especially if the heat produced by roof mounted PVT panels is sufficient to provide domestic hot water in the summer.
So, if PVT panels are able to produce almost 4 times as much combined energy as a standard PV panel, why are the not more popular? Cost is the main reason, PVT panels cost about 4 times as much as a standard PV panel, and they require more infrastructure to be in place (namely a thermal system with tank, pump, and controller to store heat, as well as a battery and inverter for electricity). If space is available, an argument could be made to just install 4x the number of PV panels and use the extra electricity to run a heat pump or even resistive heating. Of course, this also implies not only more PV panels, but change controllers, battery storage, and inverter capacity. In reality, however, space is always an issue, especially space on a roof with good solar coverage.
So with PVT or any Solar Thermal system, heat storage and distribution are issues. The heat from the panels or tubes must go somewhere and be radiated out into either a water tank (for use as hot water), or though some form of radiators, in-floor tubing, or heat pump loop for space heating purposes. Also, heat is needed on demand, not just when the sun is shining. At the very least a buffer tank is necessary to transfer heat from the heat transfer fluid into water. And as with batteries in a PV system, the tank must be sized both to meet the input coming from the solar collectors and the demand over the day, including at night and on cloudy days.
This is where European homes have an advantage over our Alaskan house. Many European homes use hydronic heating using a boiler heats water that then flows through radiators located throughout the house. Adding a solar thermal system requires that the existing heating infrastructure of pipes and radiators be connected to the storage tank that is heated by the transfer of the heat transfer fluid into the water. This is analogous to connecting a PV system up to grid-tied house, there is already a power distribution system in place, so the work is in connecting PV panels to an inverter and connecting the inverter into the existing electrical panel. (Note that I am not claiming that either scenario is “easy” just that there is already infrastructure in place to distribute the hot water, or electricity, where it is needed in the home).
In contrast, our house is small, and while we do have hot water in the bathrooms, laundry room, and kitchen, we do not use hydronic heat. We also do not use heating ducts. Instead, as I mentioned before, we use a single space heater to heat the whole house. So for us to make use of solar heating would require not only the solar thermal collectors and piping from the collectors to a central location, but also one or more thermostatically operated tanks and pumps to circulate the transfer fluid, but also a pump and circulation system for the hydronic heating fluid to circulate though some form of radiators that we would also need to install.
Another factor that I glossed over above is seasonal heat storage. This is starting to get some market attention lately. In the summer, once the buffer tanks have reached their operating temperature, they cannot accept any more heat, must like a battery does not accept any input after becoming fully charged. Unlike with PV panels, however, the solar thermal collectors cannot just shut down, they are still collecting solar radiation, but the heat has no where to go. This can cause an explosion as heat builds up and pressure builds in the system. The extra heat must be dumped somewhere. One approach is to just literally dump hot water on the ground. If we had a pool, we could use it to heat the pool. But another more attractive option would be to dump the excess heat into some form of long term storage in the summer, and then pull that heat out in winter.
Various “sand battery” products are currently in production, but the principle is very simple. Heat is circulated through sand or rock or even a large body of water, that is stored in a very insulated container. Later on, when heat is needed, the heat is collected though coils of tubes in the media. Some sysetms use hot air, some use a heat transfer fluid. If used in conjunction with a ground-source heat pump, the water is pre-heated by the media, and this heat input then improves the efficiency (COP) of the heat pump. This requires a large area, or more specifically volume of storage, typically underground. In order for the system to work, it must also be above the water table, so in that regard, it is similar to installing and plumbing a large septic system (i.e. not cheap). So, in our house, if we wanted to make year-round use of solar thermal energy, we would need in addition to the solar thermal system, we would also need the battery tank, and geothermal heat pump.
So, is it worth it? From a financial perspective, it looks like about a 15 to 20 year payback for solar thermal with PVT and vacuum tubes (assuming a fixed cost for propane and diesel). That is starting to approach the life expectancy of some of the components of the system. On the other hand, much as with all things off-grid, the potential cost savings is not the only factor. It would be more cost effective just to move to a grid tied house if that was the only consideration. Whenever I read about my friends in north Texas experiencing yet another long weather induced power outage, I am reminded of the benefits of energy independence. The fact that electricity from the grid is cheaper is of no consequence when the grid is down.
My current thinking is that the time to install any form of solar thermal system is either when the house is under construction, or during a major renovation or addition when walls are open, and plumbing is exposed. But I also think that PVT is really the only viable option for roof-top PV mounting if year round production is a requirement in a snowy climate. So for us, at least, I think the best course of action is to plan for thermal as a part of our long-range plan to add two more bedrooms to the house. This will also increase the roof area for collectors, and might help inform factors such as roof pitch and overall square footage.
Alternaively, I may just try a small thermal installation just to provide domestic hot water, to get some familarity with the technology and at least reduce our dependnce on propane.
This can all change, of course, but I think the shorter term priority may be to add additional invert and charging capacity by adding an additional stacked Sol-Ark 15 inverter, and doubling the size of our battery bank. That will yield an additional 3 MPPT inputs for PV arrays and provide a total output of up to 24 kW (or 100 A) of 240 VAC power. (Still half of a traditional grid tied house) on batteries, and up to 30 kW (or 120 A) of power when operating off of PV.
