Solar Hot Water Part VII: Retrospective

This post is about  "lessons learned" in the process of building our solar hot water system, discussing problems and some speculations about what might help to solve them.

As mentioned in the last post, a minor problem with using the system is that the gas-fired tank tends to cool off when the gas is turned off in the summer. In the long run, we plan to get rid of the gas-fired tank backup and put in a modulating on-demand electric hot water heater. My original plan was to couple the second heat exchanger in the tank with a geothermal heat pump or high efficiency gas boiler, but neither of these options is ideal. Geothermal heat pumps won't put out more than 140F and you need the heat transfer fluid substantially hotter to get good heating when the water flow is heavy, like when you are doing laundry and taking a shower at the same time. A gas boiler wouldn't allow the tank to derive optimal heating from the sun. If the thermostat is set at 120F (recommended for domestic hot water), then the gas boiler will come on when the water drops below that temperature and the solar collector won't come on. Same is true for solar tanks with a backup electric heater coil inside the tank. The solar tank can't preheat the water. An on-demand electric hot water heater should allow the solar tank to heat up as hot as the sun can get it, and just add enough energy to bring the domestic hot water up to 120F in the winter. In addition, we can add some solar PV to our roof to offset the electricity used to  run the on-demand heater. That should make our domestic hot water supply completely carbon neutral.

Aside from the aformentioned problem with an installer that did not know what he was doing, the primary problem with the system, though, is price. It is simply very expensive for the amount of carbon reduction achieved. Below is a graph comparing the cost per kilogram of eliminated carbon to two other alternatives: the same system but with a glass-lined tank instead of a stainless steel tank, and a cheaper thermosyphon system which a friend who lives in the same town had installed:

I used the estimated figure on gas eliminated from the Performance blog and included the federal tax rebate of 30% off installed system cost. I did not include the cost of the closet modifications for the thermosyphon system, because the storage tank is integrated into the collector so no change would have been needed in the closet. Note that the estimate here on cost per kilo carbon removed for our solar hot water system is too low, it does not include the winter when the solar thermal collector is contributing little.

Our system has a cost per kilo approximately that of the new refrigerator and I think it is for the same reason. I exchanged email with a friend who was surprised by the high price per kilo carbon reduction achieved for the refrigerator (which you can also see here). Normally replacing a fridge that is 10 or more years old by a new one is a fairly cost effective. I'm not sure how old the fridge was since it was there when we moved into the house, but on reflection, I think I know what the problem was. The new fridge was exactly the same model as the old, except updated with energy saving features. In our kitchen, the fridge is built-in, so we either had to get the same model or spend money on getting the space modified to take a different model. But the original fridge and now the new one are both high end models, and therefore very expensive.  It is a Lexus fridge, even though, we would have been happy with something a little less high end. Similarly, the solar hot water system is a high end system, and the cost reflects it.

That explains why the carbon reduction cost for our system is about three times as much as for the thermosyphon system, but it doesn't explain why even the thermosyphon system is expensive, at almost $20 per kilo carbon removed. At that price, even with the subsidy, it is not an affordable option for most homeowners. A high end gas-fired tank costs around $700 new, and a low end gas on-demand hot water heater costs about the same. An on-demand gas hot water heater can also result in substantial carbon reduction, though it won't be possible to completely eliminate carbon of course (it would for an electric on-demand heater though, by offsetting with solar PV). The thermosyphon system is around 10x the cost for an on-demand gas system, and our indirect solar system is more than 20x. Prior the WWII, many houses in California had solar hot water systems, and today, there are cities in China where solar provides hot water for over 90% of the homes. Why is solar hot water so expensive, the energy from the sun is free, it should be cheaper in the long run for a solar hot water system?

Well,  one problem is that natural gas (or, as I prefer to call it fossil gas) is just too cheap and likely to stay that way. Fossil gas releases only half the amount of carbon per therm of energy generated from coal, and something like a third less than oil so in some quarters it rates as a "clean" fuel. In addition, recent discoveries of gas deposits in shale across wide swaths of the Northeast and Mid Atlantic states promise that fossil gas will stay cheap for some time to come. On the one hand, that's good news because if fossil gas becomes cheaper than coal, switching electricity generation to fossil gas from  coal in states like Illinois where most of the electricity is generated by coal could result in substantial greenhouse gas reductions. On the other hand, unlike solar and wind, fossil gas is still putting fossil carbon  into the atmosphere. So to the extent that cheap fossil gas prolongs our dependence on energy technologies that dump fossil carbon into the atmosphere, it is doing us a disservice.

The other problem is that the technology itself is actually quite expensive and difficult to integrate into an existing building. It requires extensive plumbing on your roof and a backup system. Integrating with the most common type of backup system requires fussy yearly action to turn the pilot off and on, and results in suboptimal performance when the gas is off in the summer. Adding a backup system that works better costs more money. Compare this with grid tie solar PV, the kind most homeowners get these days, where the backup comes from the grid and so there is really no periodic maintenance needed or any kind of additional hardware. While solar PV requires structures on the roof (and, indeed, can result in roof leaks if not properly installed) and high voltage DC lines which are not trivial to get right, there is not as much dependence on the exact configuration of the existing structure except for proper solar exposure. A solar hot water system is more like the traditional off grid solar PV system, where the homeowner must install a large bank of batteries to store electricity for times when the sun isn't shining. Non-grid tie systems usually cost double or more what grid tie systems do, and the batteries also require monitoring and occasional replacement.

Since technology is what Silicon Valley is all about, you would think a budding entrepreneur would be studying this problem for the next great startup, but, as far as I can tell, there are no startups looking at how to reduce the cost on solar hot water. The economics seem pretty simple to me: get the cost down around where the payback time (sorry, unfortunately that's what most people look at) is around 3-5 years. Say, maybe the installed cost is 2x an on-demand gas heater, and you don't need a half inch gas main installed for it. Anybody up for the challenge?