Martin Holladay, Green Building Advisor resident ‘Energy Nerd’ recently posted an article with the very pugnacious title, “Solar Thermal is Dead,” referring to solar hot water. Now energy guys will wage verbal wars for years over the merits of different types of low-dust cellulose insulation … you can imagine how heated things might get by suggesting holy solar was anything but perfect. A bit like throwing on a Derek Jeter jersey and striding down Yawkey Way. So what was so controversial?
Solar Thermal is Dead
Well, maybe not controversial much as evocative. Holladay’s article asserts that solar thermal systems providing hot water may be (or already are) surpassed by combining a hot water heat pump with a solar electric system.
A quick overview of what each item referenced is: Solar thermal or solar hot water is a solar thermal system for heating water. It consists of a solar collector on the roof and a storage tank connected via an insulated hose to transport the heating medium, usually ethylene glycol.
A solar electric or photovoltaic system is one which produces electricity. Solar electric (usually called PV for short) panels are located outside and tied into the building as well as potentially the electric grid.
Hot water heat pumps are hot water heaters which pull ambient heat from the surrounding air and concentrate it. This is essentially the opposite of a refrigerator or air conditioner which pulls heat from an interior space and expels it. The heat pumps are driven with electricity.
The idea advanced in Holladay’s articles is that the cost of PV has come down so drastically that it makes more sense to install more PV panels and produce hot water with a heat pump than to install a solar hot water system. It’s an intriguing idea and one which flies in the face of decades of green building thinking.
Advantages and Disadvantages over Solar Thermal
This conceptual shift has a lot of advantages over traditional solar hot water.
- The hot water production of traditional solar thermal is directly tied to available sunlight. A photovoltaic system indirectly produces the electric fuel for a heat pump. With the system tied into the grid (assuming grid access), there’s no chance of losing hot water. The heat pump will produce hot water as needed and then the solar contractor would simply install a PV system large enough to meet year-round total demand.
- Hey, no anti-freeze in your house!
- Which is needed to combat the possibility of a solar thermal system freezing. D’oh!
- Larger PV systems can sell their electricity back to the local utilities in areas which have net metering.
- Solar electric systems have no moving parts and generally require less maintenance than solar thermal systems.
There are drawbacks as well
- The cost of electricity and hot water production vary enormously by region so detailed ROI calculations should be made before committing.
- The cost of installation varies a great deal as well.
- There may not be space in the building envelope to locate the heat pump.
I strongly encourage checking out the original article and especially the comments section. It’s nearly 100 comments string by top-notch efficiency pros picking over the idea in detail.
Is Solar Thermal Dead – Audits
Part of what I find intriguing about this idea comes from my experiences on audits. Oil is the heating fuel of choice in Maine by an overwhelming margin (over 80% of all homes).
And a huge percentage of those homes have tankless coil hot water system which produce hot water from the main boiler. This is massively inefficient as the giant cast iron boiler must run year round to produce hot water. Many Maine homeowners gnash their teeth in frustration when their heating system kicks on in July.
Another frequent finding is damp basements and dehumidifiers. Most homes in Maine have basements and being a state made mostly of granite, clay and mud, they tend to be wet. The common response to a wet basement is a trip to Home Depot to buy a dehumidifier. However, dehumidifiers chew through loads of electricity, making them a less than perfect solution.
A solar electric and hot water heat pump rather neatly addresses each of these issues. Because of the huge inefficiency of using the full heating system to produce hot water, we often discuss options during audits. Like noted earlier, solar hot water systems address only a portion of total use necessitating a supplemental system.
A hot water heat pump/solar electric system is a good solution. The hot water can be moved entirely off the heating system without intermittency issues. With a decent sized PV system and grid tied electric, it can be net positive. An added wet basement benefit is that heat pumps dry out the air in the area. While they may not replace a dehumidifier, they can help minimize their use, another reduction of electrical use.
Solar hot water has been the green community’s woobie since forever. The collapsing price of solar PV systems has made an intriguing idea but challenging idea much more feasible.