Solar Thermal Upgrade: Heating and Hot Water
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Posted on Nov 24, 2012
Using the sun to heat water is an ancient idea. What could be more simple that letting the sun heat up a liquid under a sheet of transparent material? Although quite popular in countries like Cyprus, Israel and increasingly throughout Europe, solar thermal has had a rocky road to travel in North America. Jimmy Carter when he was the president of the United States during the seventies, when the oil crisis hit, put up solar thermal panels on the white house as a symbol of what the leader of the most powerful nation on earth could do. Before climate change was understood widely to be the most important moral dilemma facing humanity, the solar panels were removed.
Times have changed. Non-renewable fossil and nuclear resources are limited and will become more and more expensive to extract. It is widely understood in scientific circles and by some of the most respected oil reserve experts, that we've reached peak oil. This means that while there may still be half the reserves remaining, the cost of extracting these remaining resources will become increasingly expensive over time. Despite this, governments that are more often than not, politically influenced by the entrenched oil and nuclear industries and their rich lobby groups, continue to dominate our energy world. Like Jimmy Carter, we have a choice. We can switch to solar power on our rooftops, taking back one of our most fundemental rights. By collecting and using local solar energy we create a local energy industry, reduce our economic dependence on a non-renewable resources that come from a few war torn parts of the world, and make true domacracy possible again. Yes, solar thermal on your rooftop, along with solar photovoltaic actually makes that big of a difference. Try it and see for yourself. You might just find yourself with a little more freedom than your neighbour.
In fact, we all can make the switch to solar thermal for hot water heating. A wide array of systems are available, some even at your local Home Depot for instance. What has not always been so clear is how to make better use of solar in colder climates. In colder climates, during the winter, it can be difficult for conventional solar thermal hot water heating systems to reach the high temperatures needed for your domestic needs (your showers, baths and sinks). As it turns out, our biggest energy need in the winter months is actually heating our homes and other buildings. So, despite the lower temperatures available in the winter from solar thermal systems, that often are not high enough for demestic hot water heating, these temperatures can be enough to heat your home. As it turns out roughly 70% of our homes energy demand is heating and cooling. Clearly, in the winter at 70% of our energy demand in the winter, this is by far the largest area for potential energy impact. Since domestic hot water is roughly 10% of energy consumption, addressing the 70% makes a lot of sense in the winter
In cold climates most solar thermal systems use a glycol mixture to ensure that the heated liquid does not freeze in the winter. In addition, solar thermal systems in colder climates also use heat exchangers to transfer the heat from glycol mixture being heated by the sun in the panels on your roof, to the hot water systems used for your domestic hot water heating needs. Newer systems, for colder climates, use a smart controller and some temperature sensors to use more of the heat for different purposes. In the winter, when the solar thermal panels are only able to produce lower temperatures, the smart controller can drive a pump and heat exchanger to pump this heat into your radiant heating system. In the summer, when higher temperatures are being produced, the heating can be used to for your domestic hot water needs since you don't need heating in the summer.
Combining solar thermal for both domestic hot water and heating, makes the system substantially more effective. The lower grade heat that was not able to be used to heat water for domestic use in the winter, can be used to add heat to radiant heating systems. Since domestic hot water is usually set to about 49 •C if the solar thermal system in the winter can't reach those temperatures, it may not be useful for the domestic hot water system. However, by pumping this heat into the radiant heating system, if your home has radiant heating, much of this heat that might otherwise be wasted in the winter can be captured and stored. Ideally, this lower grade heat, anywhere from say 25 •C to 48 •C, can now be captured in your radiant system. For this purpose, it is also helpful to have a lot of thermal mass to store this heat, so hydronic tubing inside thick concrete flooring is potentially the best combination for this purpose.
Wilson Solar House Example
This past summer, we've implemented a solar thermal system. During the entire summer this past year the solar thermal system provided all of our hot water heating requirements. We actually turned off our electric hot water heating tank. With three solar thermal hot water heating panels added to our roof, this system provided more than enough hot water for our family of four. As October arrived, we had the finishing touches completed on the heat exchange system and pumps so that we could use these same panels to heat our radiant floor system during the fall and winter. The Solunik smart controller is now monitoring three temperature sensors to determine how best to direct the heat being collected. While the temperature is below 49 •C, the heat is directed into a radiant floor heating loop in our large basement area. This essentially ensures that heat from 25 •C and 48 •C, is collected and stored in our basement concrete flooring. If and when the temperature from the solar thermal panels exceeds 49 •C, then heat is directed towards pre-heating our hot water heating system for our domestic hot water heating needs.
As I write this article it is a sunny day and about 1 •C outside. As you can see from the picture of the Solunik controller, the solar thermal panels are producing 43 •C heated glycol from the roof. Although not hot enough for the domestic system, this heat is now being directed into our radiant floor heating system in the basement for longer term storage. This helps keep the house warm at night without any supplementary heating need. For us supplementary heating comes in the form of passive solar heating primarily, a wood stove, and an in-line electric heater (keep in mind we supply all of our electricity for the in-line electric water heater from solar photovoltaic, wind or hydro generated electricity), so it is all 100% renewable.
The system is new, so we will see over the next few years how effective the solar thermal system is at lowering our demand for electricity to heat the in-floor radiant hot water heating system. We are hoping for an overall reduction of electricity demand in the winter by approximately 50%. We'll see. If that works out as planned, we should see a return on investment of less than six years. That is a better sounding investment than my stock portfolio