Making hot water systems in the age of Solar Electricity


For many years the best way to capture the energy of the sun, other than simply by inviting it through south facing windows, was to use it to heat domestic hot water. It was only about ten years ago that the Plymouth Area Renewable Energy Initiative (PAREI) sparked Solar Raisers such as this one organized by the Tin Mountain Conservation Center where 40 people helped install an evacuated tube system on a home in Madison, NH.

The advantage to solar hot water is that it does not require very sophisticated technology to capture the energy of the sun and transfer it into water for either residential or commercial use.  Indeed, there are lots of do-it-yourself schemes such as bread box heaters that involve almost no technology by simply placing a black painted water tank inside of an insulated box glazed with any of several available transparent or translucent glazings, sometimes as simple as old storm windows.  This writer had a hose connected to an old water tank with no glazing but just fully exposed to the sun for use as an outside shower for summertime.  Sometimes the water would almost reach scalding temperatures on warm sunny days.

On the other end of the scale are hot water systems that are used for space heating with storage tanks connected to pumps and controls that circulate hot water through pipes embedded in a floor or other distribution system as in the Tin Mountain rooftop system that heats the building along with a wood fired backup boiler.  These are generally engineered systems requiring a lot of technology and hardware.

The most common application has been for residential hot water  using evacuated tube systems or simple “flat plate” collectors that consisted of glass or other type of glazing covering an insulated box with an “absorber plate” generally made of copper with a series of pipes running through it that connected to a storage tank much like a conventional water heater. I will explain the system that has successfully served my own home since I installed it in 1978.

As my wife and I operated a business we called Alternative Systems in the mid ‘70’s, we built a house that incorporated much of the technology we sold at the time.  A Fisher woodstove, still in use, a composting toilet that served for many years, a story in itself for another time, Window Quilts, and a Daystar solar hot water system.  Daystar, located in Burlington, MA, made very high efficiency flat plate collectors. Curiously, Daystar was actually owned by Exxon, hinting that the oil companies maybe actually have realized that fossil fuel’s days were numbered.

The Daystar system came as a complete package and I sold several of them, some to DIY folks who set them up themselves, and other to customers who had plumbers do the installation.  A few were roof mounted but several were mounted closer to the ground, generally at a 45 degree angle to optimize their ability to capture the sun year round.  An engineer friend mounted his so that they could be tilted with the seasons to gain even more sun, and, I add from my own experience having had to use a very long handled snow rake on my own collectors, to simplify removing snow that crusted over with ice on a very few occasions.

My own system was the subject of a federal monitoring program that consisted of metering hot water and backup electricity and a run time meter on the circulator pump.  A researcher visited my house periodically to take readings that showed that our system provided 85% of our hot water even with two little ones in reusable cloth  diapers!  To optimize the system we even had a switch that would allow us to turn off the back-up electric power on mornings where the sky was clear to the West, indicating that the day’s washing would be in sun-heated hot water.

The system did require some maintenance, however, as the water from our municipal system tends to rot out water heaters very quickly, so our biggest expense has been to replace the stone line tank a couple of times until we finally installed a “lifetime” high performance stainless steel tank.  One replacement control module and one or two circulator pumps were the other plumbing components replaced in over forty years of service, while the original collectors still gather the energy from the sun every times it comes out.  The bigger issue is replacing the roof shingles on the south side of our roof, because the collectors will have to be removed and, hopefully replaced with a couple of spare collectors that I have from the old Alternative Systems days.  I’ve been able to get by with the original shingles by painting them with aluminum paint every few years that has been surprisingly successful but has finally run its course.  Note that hot water collectors are MUCH heavier than the solar electric PV panels that are beginning to take their place which has been much of the reason for my procrastination.

It is not so much that today’s solar electric Photovoltaic or PV panels are so much lighter than solar hot water collectors as it is that their cost has plummeted and their efficiency increased while the cost of electricity is constantly climbing.  These are the reasons that our Solar Water Raisers of ten years ago have given way to a greatly increased interest in solar electricity. There are many advantages to PV’s over solar hot water systems, not the least is that there are no mechanical components to wear out or a heat transfer fluid to leak or need changing.  PV’s can be installed almost anywhere within reasonable reach of wiring, either from the roof down or underground from a ground mount system.

PV’s however are not generally a DIY installation unless you are like my engineer friend who installed his solar hot water system and also his own PV system years later so he could follow the sun with them.  PV installations usually will at least need the services of a licensed electrician to make the proper connection into the structure’s wiring system in conjunction with the local utility’s requirements, at least in grid-tied systems (not off-grid with battery backup).

Heat pump water heaters

A 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 only use a built in electric heating element as do most electric water heaters. As with other heat pump systems, this make 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 most of the mechanical part of the system, the compressor, outside the house, and one or more “heads” that transfer the heat or cooling remotely inside.

During periods of high demand the water heater can be set to rely on a standard electrical coil built in to the tank whereas mini-splits heating a building might have to rely on fossil fuel backup heating. However, when a mini-split heat pump is working in an extremely cold outside environment it drops back to performing like any other electrical resistance heating unit with one unit of energy in and one unit out. Because water heaters are not usually operating in sub-freezing temps, there should never be a time that a heat pump water heater should never lose an effective level of efficiency.

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 utilize electricity from a building’s PV system resulting in another way of using the sun to heat water.  In addition, 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 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 the failure 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.