100% solar hot water

There are 3 ways to have a nearly 100% solar fraction for one’s hot water heating for the year.
I think for us at least, probably 90% is for showers and baths.

1. Have a large collector area and a large storage area so that you can build up a huge amount of heat to get you through the inevitable days of clouds. If you assume cloudy days are like coin flips, then if you store enough hot water for 5 days, then that’s 1-0.5^5 = 96.875% solar. From there it is mostly about some algebra. Usually it is recommended to do an unpressurized drainback system ala Alan Rushforth http://www.rushforthsolar.com/ if you are oversizing. The problem is that it is hard (impossible?) to find someone that has done a system like this recently in MA or New England!

2. Since #1 gets expensive… (even a more typical 50-70% solar fraction system usually costs around $8500-$9000 in MA I’ve heard) what we decided to do is the following:

2a- heat our water with a hot-water heat pump (an air-to-water heat pump). Also sometimes called a “hybrid” heat pump. Rheem and GE make them. And Airtap and Nyle (Geyser) make add-ons. We bought a Geyser and have it connected to a large 105gallon tank because I like the idea of keeping complicated stuff separate from the uncomplicated stuff (the storage tank) so if the heat pump breaks, I still have a tank. It’s a little loud but I have it on a timer and have tweaked the temps and run times such that we always seem to have enough hot water.

2b- upsize our PV array according to the estimated hot water usage. My estimates are that we will use about 1100-1200KWh in hot water heating. So we upped our PV system by 900W which will cover that (~9000KWh per year is about ~1200 more than our original 6KW system).

So the math here is:
– Upsized PV array by 0.9KW * $6/W = $5400 (this is before tax credits, etc.)
– hot water heat pump ($1200?). This part could actually be dropped if you are in a “normal” house and already have a propane or natural gas HW heater since as I’ve written before, there isn’t a huge advantage in terms of fossil fuel use of using heat pumps vs fossil fuels directly. In our case, we do it because the house is very tight and so we wanted to keep combustion out of the house for IAQ reasons.
TOTAL: Let’s say $5400. And that’s for 100% solar hot water, vs 50-70% (optimistically) for a traditional $8500 system.

3. Do not use hot water! Shorter showers? Low-flow (Bricor) shower heads. Wash clothes on cold. Etc. Negawatts.

4. If you live in a warm place, do this:
Summer Solar Showers

A few final thoughts
I like our PV+heat pump approach so far. No maintenance (Until the heat pump breaks I suppose!). Vs I think it would be nice to keep a little more of an eye on a drainback system like I mentioned, but maybe I am wrong about that.

A drainback system is perfect for DIY, so that would be the one to choose if you have the time and inclination. See builditsolar.com and the Yahoo Solar Heat group. Plus it would maybe work more years than our heat pump without replacement. But unless you are doing DIY, the $8500 is approximately 3-4 times more expensive than the PV+heat pump approach.

What other factors am I forgetting. I think that’s a reasonable summary of the various issues involved. The ideal green technology I guess is:
1. cheap up front costs ($ payback is ideally from day 1 — meaning: no down payment and extra loan payments less than the utility bill savings)
2. long lasting
3. local/low tech if possible
4. high solar/renewable fraction — (well… high renewable KWh/$ spent)
5. ideally has little or no lifestyle change necessary
6. doesn’t take up a lot of room
7. scales to everyone doing it.

I think that last item could be an item that makes our PV+heat pump approach not as ideal as the traditional approach because it is putting extra strain on the grid (we are essentially using the grid as our storage tank) vs a typical solar hot water system that heats the water directly from the sun has very little to do with the grid, expect for powering the VERY low power pumps involved. And those can usually be done with dedicated little PVs if you wish.

Other than that, I think it will score quite well on all the other items above. We’ll see how long it functions reliably. I’ll get back to you in 5, 10, 20 years.


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One response to “100% solar hot water

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