Evacuated tube collectors are generally more efficient and provide higher water temperatures than flat plate collectors, but are also more expensive. Choosing which type of collector to use requires a careful consideration between the outdoor temperature, solar radiation, water temperatures, cost and other variables. For most typical hot water applications, flat plate collectors offer more "bang for the buck" and are also attractive because of their low maintenance, long life, and ability to shed snow (a big consideration here in Alberta). That being said, we like to weigh all the factors before we choose a particular type of collector for our systems.
Solar thermal systems can provide the bulk of hot water demands in a given year, but because of periods of cloudy or rainy days, solar thermal systems do require backup systems to handle those days when solar production is insufficient. Adding collectors or increasing storage is a way to overcome the need for backup, but is usually inpractrical and expensive. It should be noted that even on cloudy days, the solar thermal system is providing hot water, but most likely not enough to meet the hot water demand; hence the need for a backup system.
Solar thermal systems work extremely well in cloudy as well as cold winter conditions. The collectors rely on solar radiation for heating, which is present even on those cold and cloudy days. Of course, the production will be lower on cloudy days, and the number of hours of production drops as the number of daylight hours decreases, but solar thermal systems provide hot water year round. Because of the shorter days and lower solar radiation in the winter, the traditional backup system is utilized more often during the winter months.
In general, while solar thermal systems can augment another source of space heating, they are not typically used to provide all of the space heating requirements. The reason for this is that the time when we need the most heat (coldest days in the winter) is also when we typically have the shortest days and therefore lowest solar production. Without oversizing the number of collectors as well as storage capacity, this is usually not a practical application.
The short answer is yes, but usually you don't want to. Solar thermal systems often provide too much hot water during peak production periods and therefore need to dump this excess heat somewhere. In the fall, a good place to put it would be into the ground loop, heating up the ground and increasing the efficiency of the geothermal system. In a single residence, or heating dominant building, this might be a viable option. However, most commercial buildings are cooling dominant, which means you are usually cooling with the geothermal system at the same time you are trying to dump heat. Because in cooling we want low temperatures in the ground to increase efficiency, we wouldn't want to dump heat into such a system because we would ruin the cooling efficiency. However, in an application where cooling is not present, the ground loop is definitely a good place to put excess heat.
Quity simply, solar thermal systems are used for heating hot water, whereas solar photovoltaics (PV) systems generate electricity.
As with anything in life, you get what you pay for. There have been a lot of collectors (especially evacuated tube collectors) coming onto the market at very low prices. We would advise our clients to proceed with caution as the glass and seals used in some of these collectors don't match up with the top quality collectors. These cheaper collectors will often lose their vacuum sealing and/or have issues with glass permeability. Some of the collectors on the market utilize Borosilicate glass for the evacuated tube collector as opposed to soda-lime glass which is much less permeable to helium (helium is present at atmospheric pressures and wants to equalize with the vacuum on the inside of the tube). Studies have shown that within 4.5 years, borosilicate glass would let in enough helium to effectively increase heat loss by 20%. Because of these problems, we recommend clients to be very careful when choosing a manufacturer. We use only certified and high quality collectors in our designs which are backed by solid warranties as well as decades of proven service.
A typical solar thermal system is designed to handle 50-70% of the yearly hot water demand which means that the cost of heating hot water is reduced by this same amount. This means that the bigger your hot water heating demands are, the more money can be saved on energy (you would of course need a bigger solar thermal system).
Solar thermal systems are by nature very passive and thus require very little maintenance. Because solar thermal systems typically require a backup boiler system, the maintenance costs compared to a larger boiler system are expected to be smaller, but not significantly so.
While this answer obviously depends on the size and type of system being installed, because of the currently low price of natural gas, payback periods are typically in the 15-20+ year range. In situations where natural gas is not available, the payback period can be drastically reduced. In both cases, the payback period can be significantly improved by taking advantage of the accelerated capital cost allowance for clean energy generation, as discussed below.
While in Alberta there aren't any government rebates or grants available to help pay for solar thermal systems, Canada does have a special CCA Class (43.2) available for depreciating clean assets. It is called the "Accelerated Capital Cost Allowance (ACCA) for Clean Energy Generation" and it allows the depreciation of green assets (Geothermal (including heat pumps), wind, solar thermal etc...) on a 50 percent per year declining balance basis. Basically, the capital cost allowance allows clients to depreciate their assets much quicker than normally allowed under the tax code. If we take for example a $100,000 solar thermal system, under normal tax codes, after 3 years, only 10% of the asset would be depreciated (2% in the first year, 4% every year after), equating to a tax credit of approximately $3,400. Under the CCA Class 43.2 however, 25% of the asset is depreciated in the first year (first year rule) and then 50% every year after, meaning after 3 years, 81% of the asset is depreciated, equating to a tax credit of $28,400! We have found that the accelerated capital cost allowance can sometimes halve the payback time period and we strongly recommend taking advantage of it. Clients should speak to their accountants, but more information can be found here.
Assuming a proper installation with regular maintenance, a flat plate collector system should last in excess of 30 years, even up to 50 years. Evacuated tube collectors would be expected to have a slightly shorter lifespan as they are inherently more complex. As with any asset, proper maintenance and care are essential for the longevity of the system. We also cannot stress enough how important the proper design and installation are to the success of the system; even simple design errors can have a strong effect on the life of the collectors.
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We very much welcome furthering the discussion on these topics so please feel free to post comments and/or questions below and we will be happy to respond to them.
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