Carbon Footprint Reduction: Don’t Bother with Solar Thermal
In 2009, as the next step in our quest for radical residential carbon footprint reduction, we decided to replace our gas hot water heating with solar hot water. For most households in the U.S., hot water heating constitutes around a third of their annual natural gas usage, so we saw an opportunity to cut a large chunk out of our carbon footprint. An alternative to solar thermal would have been to replace our tank-based natural gas hot water heater with an electric on-demand or tank-based resistance electric hot water heater, and upgrade our solar PV system to handle the additional load. At that time, residential solar PV was still relatively expensive, around $8.00 a watt installed compared to $3.00 today, and heat pump hot water heaters, which are about twice as energy efficient as direct heating, were not widely available. In addition, we were still happy with our 2.5 kw solar PV system installed in 2004 and wanted to continue receiving ROI on it rather than replacing or augmenting it. So we decided to install a solar thermal hot water system.
Solar thermal systems differ from solar PV in that the energy of the sun is captured directly as heat rather than being converted into electricity. In the most common configuration, the heat capture device is a roof mounted flat plate collector, which superficially looks like a solar PV panel (see above) but actually functions quite differently. Rather than generating electricity from light which is collected and fed into the grid or your house, the flat plate collector has a heat transfer fluid running through it, which is pumped through piping out of the panels (you can see the pipe at the bottom of the panels in the picture), off the roof, and through a heat exchanger in a storage tank located somewhere in the house, then back to the panels. The sun heats up the heat transfer fluid and the heat transfer fluid heats the water in the tank. Flat panel collectors are extremely efficient at capturing the sun’s energy, around 75%, compared to around 22–25% for commercial solar PV panels.
We found a local company, Sunwater Solar which has renamed itself Adroit Energy, with experience in installing solar thermal systems. Because solar thermal systems basically involve running plumbing across your roof, you need to make sure the installer has experience in plumbing. Leaks can be messy. Our system is a German Schuco Slim V Plus collector together with a Schuco PS 1.3 pump station, an expansion tank, an 80 gallon stainless steel storage tank with dual heat exchangers (in case we later want to run some other kind of heating in the winter), and the plumbing to connect the system together. Schuco has an excellent reputation for quality engineering, and many Schuco systems have been in operation for years without problems.
The heat transfer fluid is propylene glycol a.k.a. antifreeze, and the entire heat transfer loop is self-contained. When most people think of northern California, they typically don’t think of freezing temperatures in winter, but a solar thermal panel exposed to the sky loses a lot of heat during a 36 degree winter night, and a water based heat transfer fluid could freeze and rupture pipes and the collector. Propylene glycol won’t freeze, like water would. In addition, we kept our gas hot water heater in the hot water loop, to handle the winter load when the solar thermal system output was minimal.
The system cost altogether around $22,000 before the tax deduction, of which $5,000 was for remodelling a closet to house the storage tank. After the 30% tax writeoff, the cost was around $14,000. Calculating the carbon emissions reduction is a bit difficult because in 2010 we undertook a major systems remodel of our house (about which more in another article) so I do not have a full year of post installation data to compare with. However, I estimate that we saved around 436 kilograms of carbon, whereas if hot water were 30% of our annual gas usage, we should have saved 697 kilograms. Over a thirty year life, that amounts to $1.09/kg.
The difference lies in the fact that we had to use our gas hot water heater from roughly November through March, because the solar thermal panels only receive sun for around 4 hours per day, minimum, on the winter solstice. In other words, for half the year, the solar thermal system would provide minimal to no energy. Seasonal and shading effects can have a big impact on solar energy systems. Even though we sited the panels on a part of the roof that gets a lot of sun in the summer, the 100 ft. tall redwood “hedge” on the neighbor’s property to the south of our house ensures that most of the roof gets very little sun in winter.
Solar thermal systems are not as maintenance free as solar PV systems. Plumbing can develop leaks or, as happened with us, the heat transfer fluid can evaporate. I originally set the storage tank thermostat to 120 degrees F, but after a couple years, the pump started making grinding noises in the summer. I called Sunwater Solar, and they told me that I should keep the storage tank thermostat at 180 degrees F, to avoid excessive “flashing”. Flashing occurs when the heat transfer loops stalls because the tank thermostat shuts down it down and the fluid in the collector flashes into vapor. Sunwater recharged the system, and we have not had a problem since.
But this incident brings up a problem with solar thermal systems: they are almost too efficient in the summer, and not efficient enough in the winter. A collector that just barely heats water in the winter causes it to boil in summer. You need a large thermal sink, like a swimming pool, to keep the solar thermal collector running all the time in summer if you want to avoid the problem with overheating. That is also why solar thermal is never or rarely combined with solar PV. On the face of it, it sounds like it would make sense to wrap a solar thermal collector around the back of a solar panel. The efficiency of solar cells drops as they heat up, and the solar thermal collector could transfer the heat to domestic hot water. But if you are only collecting heat for domestic hot water, the panel will overheat if the heat transfer loop stalls because the hot water storage tank thermostat shuts it off.
Revisiting the decision, it interesting to see how much a solar PV upgrade + electric hot water heater would have cost. As mentioned, solar PV was around $8/watt installed in 2009. Calculating our domestic hot water reduction at 2410 kwh from the average of 2005–2008, which amounts to only 18% of our total hot water energy use, we would have needed 7 extra 230 watt panels (the maximum size available at the time) according to the NREL calculator. At $8/watt, that would have been $12880 for the panels, an extra $5,000 to replace our inverter with a 5 kw inverter (price is today’s price, should be about the same), maybe $800 for a simple tank hot water heater, and $5,000 for the closet upgrade, for a total of $23,680 before taxes or $16576 after. The potential cost is not far off the cost we actually paid for the solar thermal system.
Today, of course, with solar PV at $3/watt installed for 250 watt panels, the cost would be less, around $15,000 before the 30% tax writeoff. Naturally, in winter we would still have to pay for electricity from the grid for hot water heating. But the carbon intensity of grid electricity has been improving in California due to the Renewable Portfolio Standard. Burning natural gas releases 0.181 kg CO2 per kWh while solar, wind, and geothermal release nothing. The carbon intensity of the grid in California is dropping rapidly. As you can see in the graph below, the crossover point where the grid as a whole became less carbon intense than natural gas happened in 2015. So even if we were to power our domestic hot water with grid electricity in the winter today, the carbon emissions would be less than for a natural gas powered hot water heater.
There are also better options today than a tank-based resistance electric hot water heater. For example, if we would install a heat pump hot water heater, we would only need half the energy required for resistance heating. This is 1205 kwh for 6 months or $3750 for 5 x 250 watt panels. Altogether, the cost would be $14550 or $10185 after the 30% tax write off . That’s more than a 37% reduction in price over 8 years. At that price, you could install another 5 panels and cover the full year. There are also better options than heat pump hot water heaters, for example an on-demand electric, and if you want to learn more about renewable energy options for hot water heating, check out Dragon’s article here.
While the decision to go with solar thermal made sense at the time, today there is little reason to not simply install enough solar PV to cover your hot water heating needs, and use a high efficiency electric hot water heater.
