It was a wonderful, spring-like day in Denver yesterday...75 degrees, sunny, the perfect excuse to sit on the back patio. With a diurnal temperature swing of 38 degrees at night, 75 in the day, heating and cooling my house seemed like distant concerns. Still, sitting out in the sun makes for a fine environment to brainstorm: How to best retrofit our nation's huge investment in suburbia?
I don't feel too hypocritical about my house. It isn't the straw-bale/adobe hybrid, passive-solar driven, food-forest enveloped home that I eventually hope to have, but it isn't exactly a McMansion either. It's 2300 square feet (while that may not seem small to most people, many of my neighbors come in at well over 6000 square feet of semi-custom goodness), and while it is in suburbia, it's very close to a light-rail station. I have a relatively efficient, natural-gas powered forced-air heater, and a high-efficiency air conditioner, but no matter how much they may have "efficiency" on the label, they are a problem. My utility bills are not exactly cheap in peak heating or cooling months (though quite affordable compared to the average around here). What to do?
The primary energy demands of my home--like most suburban homes--are for heating, cooling, and hot water, in that order. How can I retrofit my house to 1) save me money, 2) make me immune to future energy supply disruptions or price spikes, and 3) provide a positive example? Today I'll look at some solar options that I'm considering.
As people who read this blog regularly know, I'm not a fan of solar photovoltaics--I think they provide a very poor energy return on energy invested, and are only economical for the consumer when highly subsidized by tax incentives. Tax subsidy only helps some people at the expense of many--it does not make PV a solution for the masses.
Solar hot water, however, is a very promising means of capturing solar energy. Solar hot water systems can provide 75% of my year-round water heating energy for an initial investment of about $3000 (Colorado is an excellent solar location due to our high number of sunny days, especially in winter). Without considerint tax credits, that investment pays itself off in a little under 8 years with my rate of useage, and at today's energy costs. That's a winner.
But the water-heating component of my energy useage is far less than my home heating. How well can solar hot water meet that need as well? It is quite conventional to use solar hot water for in-floor radiant heating or with conventional radiators--but this is not a perfect solution. It can provide much of the day-time heating needs (when the sun is shining), but does nothing to address the more significant night-time heating needs. What is needed to effectively retrofit a standard suburban home with solar-hot-water space heating is to integrate thermal mass into the equation. As far as I know, this really isn't being done at all. Rather than pumping very hot water through radiators or sub-floor tubing, solar hot water can heat high thermal-mass water walls inside the house. This is a better solution for two reasons: 1) solar hot water has difficulty reaching high temperatures necessary for standard radiators during the dead of winter, even in Colorado, but it can still provide sufficient BTUs to raise the temperature of a thousand gallons of water (in several thermal-mass banks) from 60 to 80 degrees F, and 2) the high-termal mass of that volume of water will slowly dissipate heat for hours, providing radiative heating throughout the night. Isolated-gain solar-hot water heating is one of the most efficient ways to capture solar energy--the only remaining issue for applying this energy to heating is to correctly design a thermal mass based system...
Thermal mass can have its drawbacks, however. Increasing the mass inside my house will have the effect of requiring more energy to cool in the summer. The thermal mass can be exposed to cool night breezes when it cools enough to open my windows, but it will still have the effect of slowing the overall cooling effect of that breeze. So, some solution must be developed that makes the same thermal mass that is so useful in winter into an asset in summer as well. One solution is simply to drain the water in the thermal mass when it is no longer needed for heating (and turn off the associated plumbing loop). This eliminates the problem, but doesn't leverage the existing investment in plumbing and mass banks work to help with summer-time cooling. Instead, it should be possible to use the same winter-time system as a radiative cooler. In the winter, water (or an anti-freeze solution in closed-loop systems) is pumped through the solar collector during the day when the sun is shining, but turns off at night to avoid bleeding heat from the hot water storage. It should be possible to use the same solar collector that gains heat during the day in winter to dissipate heat at night in the summer. Sky-facing night radiators can rapidly lose heat, nearing freezing temperatures even in the tropics. The fundamental design of a solar collector and a sky-facing night radiator are quite similar--it should be possible, perhaps with minor alterations, to utilize the same system for both tasks. If this works, then in summer the thermal mass water banks can be cycled through these sky-facing night radiators after the sun goes down, and help to actively cool the house--by sunrise the next morning there could be a thousand gallons of 40 degree F water radiating cool for the remainder of the day.
While the trifecta of using a single solar-water system to heat water, heat my house, and cool my house is quite exciting, the concepts require a little more work. If anyone has heard of people using solar hot water space heating, or sky-facing night-radiators, please let me know in the comments. In addition, any other thoughts on retrofitting suburbia are welcome--hopefully this will become a bit of a series as I explore and implement various strategies.