On average, 9% of a home’s electricity usage goes toward heating water, so homeowners who are looking to save money and shrink their carbon footprint often consider installing a solar hot water collector.
Solar hot water systems are pretty intuitive. Rather than photons from the sun knocking electrons off of silicon (as we explain in Solar 101), which is the extremely complex process of photovoltaic solar panels, energy from the sun can simply get absorbed into an atom. We experience that increase in energy as heat. The darker the coloring of the surface, the less light is reflected. That means more photons are absorbed and more heat is generated.
Solar hot water equipment uses this heating principle in one of two ways. Either the water is run through black tubing which is set up to get as much sunlight as possible; or, for colder climates, a fluid like anti-freeze is run through the black tubing. The warm anti-freeze is then circulated through copper tubing inside an insulated tank of water, warming the water while keeping the anti-freeze isolated so the water doesn’t get tainted.
These heaters, known as solar collectors, can have a tank of water above the panel, in which case the warm water rises and the cool water falls into the black tubing to get warmed. In colder areas, the water tank would freeze when exposed to the elements, so it’s placed inside the home, and a pump keeps the water moving between the tank and the solar collector.
In Minnesota, the ideal angle for solar thermal is quite steep because the sun stays so close to the horizon in winter. In case temperatures drop too low, or the skies are cloudy for too many days, many systems include a small gas or electric heater that kicks in automatically when needed.
Another design variation replaces the flat panel with glass cylinders. The black tubing is coiled up inside, and as much air as possible is pulled out of the cylinders to make use of the universe’s best insulation: vacuum. However, this design is generally not recommended for Minnesota because they do not handle our climate well.