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Heating Water with the Sun
by Doug Pushard
Heating water directly with solar thermal panels or heating water by generating electricity through photovoltaic (PV) panels and then using this electricity to heat water are two ways to reduce your reliance on the grid and your water footprint. Why choose one over the other? This article will explore these questions and more.
Solar thermal and PV are methods that will cut your footprint. Solar thermal is less popular than solar photovoltaic, but both are proven technologies. They can be used to heat water and heating water is one of the largest consumers of energy (see links below) in a household, so installing either is a great way to cut your footprint.
Producing energy is a large consumer of water, so by reducing your energy footprint, you reduce your water footprint. With solar thermal there are two primary ways to heat water. One rarely used method is heating water directly by circulating it through the sun-facing collectors and then back into a storage tank. The more common method involves heating collectors filled with glycol which then in turn go through a heat exchanger that heats water.
 The direct method is not as common as the collectors get very hot and the possibility of leaching metals into the water. Although extremely popular in the 1970s, solar thermal is a specialty these days so finding a good, experienced installer can be difficult.
There is significant math involved in correctly sizing and plumbing a system. However, a correctly designed and installed system should work for decades and will greatly reduce your energy load. The glycol in the system will need to be recharged or changed, but other than that these systems should provide hotwater for years and years. A large advantage of these systems is they are very effective and efficient even on cloudy days. Over-sizing a system, means you may have more hotwater than required sometimes of the year. This excess heat can be used in hot-tubs or house heating. In the peak of summer, depending on your location, excess hot water may need to be dumped or cooling fans installed on the back of the collectors to reduce heat at night.
 Solar photovoltaic panels generate electricity (see related links below). This electricity is used to then heat the water. It is a two-step process and therefore a little less efficient. However, there are thousands of competent PV installers. If a PV system is going to be installed anyway; having it handle the water heating is just a few more panels. Plus, you would have just one system for generating your household electricity and heating your water. PV panels have been dropping in price and are widely available; even many local hardware stores are carrying small systems these days. They are available in complete plug-in-play kits that can be installed by competent Do-It-Yourselfers.
Heating water consumes a lot of energy and it is required even on cloudy days. Therefore PV-based systems will require more panels and require some type of backup, either battery or the grid. In rural areas they are often tied to the grid, so the grid provides this backup. Another option is a battery-tied PV system. These systems are more expensive and not every installer is going to be competent on installing these types of systems as they are less common. Over-sizing a PV system in a grid-tied system means just sending more power to the grid during the day. In battery-based systems, if may involve larger batteries or automatically shutting down power generation when the demand is not high enough and the batteries are full.
In addition to panels or collectors, keeping the heated water hot requires an upgraded hot water heater. These hotwater heaters are significantly better insulated than off-the-shelf models and, in the case of solar thermal, may have an internal heat exchanger. This upgrade pays for itself in both types of systems in that they greatly reduce the heat loss in the hot water.
These days both systems can be managed via Wi-Fi and the Internet greatly reducing unnecessary duplicate service calls (i.e. a technician coming onsite only to discover they do not have the correct part). In many locales there are tax credits for either type of system. This should be investigated prior to making a decision on which is going to be the best for you. A link to available credits and rebates is in the Link section below.
Heating water requires significant energy and either PV or solar thermal systems are methods to do it. In rural locations where propane is used to heat water, both are better alternatives long-term. Which type of system you choose is a matter of choice. Solar thermal systems are more effective and efficient at heating water but require another type of system and ongoing maintenance. PV-based systems are more widely available but will require more panels to heat water and some type of backup to heat the water on cloudy days. Both will reduce your grid-tied energy load, thus reducing your carbon and your water footprint.
Save water, do one or the other if you can! Watch for the second installment of this article, where I will review how much water you will save!
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