Solar Hot Water in the Age of Solar Electricity: Heat Pump Water Heater – Part 2
Other than capturing the sun through windows for passive solar heating, for many years the most effective way to utilize the free energy of the sun was with a solar hot water system. For years organizations sparked by the Plymouth Area Renewable Energy Initiative, PAREI, helped local homeowners install solar hot water systems in barn raiser fashion.
With the changing emphasis to solar electric installations, solar hot water systems have given way to a new technology that can harness the energy of the sun to produce hot water in a very different manner.
This new twist with heating domestic hot water is electric water heaters with heat pump technology built in. This means that the heater uses electricity to take heat from its immediate environment and transfer it into the water rather than use only a built-in electric heating element as do most electric water heaters. As with other heat pump systems, this makes the system two to three times more efficient than a traditional electric water heater by using one unit of energy to produce two or three units. It uses the same technology that is popping up everywhere in the form of “mini-split” heat pump systems with the largest component of the system, the compressor, outside the house, and one or more “heads” that transfer the heat or cooling remotely.
During extremely cold periods, mini-splits heating a building might have to rely on fossil fuel backup heating, because when they are working in an extremely cold environment their efficiency drops back to performing like any other electrical resistance heating unit with one unit of energy in and one unit out. Because water heaters seldom operate in sub-freezing temps, there should never be a time that a heat pump water heater should ever lose an effective level of efficiency. During periods of high demand, heat pump water heaters can be set to supplement that demand with a backup standard electrical coil built into the tank.
There are several advantages to this type of water heater besides its increased efficiency including that it can help cool the inside of a building in summer by taking the heat from the room where it is located and transferring it into the water, and also that it can use electricity from a building’s PV system resulting in another way of using the sun to heat water. It is worth considering that several states and utilities provide rebates for heat pump water heaters.
A disadvantage of a heat pump water heater is the fact that it is taking heat from its environment which might be fine if it is in a room with a wood stove or ‘alternative’-fueled furnace, but not if it is stealing heat that is generated by an expensive non-renewable energy source. I’ve also heard that these systems they can actually chill an already cold basement to the point that pipes can freeze!
As for the future of stand-alone solar hot water systems, the complexity and expense of a system appears to make little economic sense when compared to a PV powered heat-pump system. On the other hand, keeping an existing solar hot water system operational makes abundant sense so long as major components do not need replacing. Unfortunately, there are existing solar hot water systems that are not functioning simply because of a minor component that a subsequent homeowner or an unfamiliar plumber does not have the wherewithal to troubleshoot.
We’ve found that solar raisers from years ago served to take away the mystery of solar hot water for the several plumbers who took part as they came to realized that solar hot water systems are little more than any other domestic hot water system except that the source of the heat was the sun beaming on solar collectors instead of some kind of gas or oil-fired system or some wires hooked to the grid.
Russ Lanoie is a long-time solar proponent in New Hampshire’s White Mountains and operated his Alternative Systems business in the 1970s—80s selling solar hot water systems, composting toilets and Window Quilts®. He lives in a passive solar home which has had Daystar solar hot water for forty years and 11kW of PVs on his barn since 2015. www.RuralHomeTech.com.