[Greenbuilding] is it ever sensible to use PV to heat water?

[Michael O'Brien]

Hi, Steve--

First of all, I want to validate your decision. Since we are in a time of rapid innovation and change, it's important that people try different solutions and keep the rest of us informed about their experience and outcomes. It's too soon to conclude that one solution is the best for every need.

Charlie Stephens is a good friend and I respect his opinions. However, in his enthusiasm for bleeding edge innovation, he is sometimes wrong. He is fearless when it comes to complexity, for example, and will recommend solutions that only highly specialized technicans can install or maintain. I'd suggest listening carefully to his recommendations and do your own due diligence. He speaks forcefully so it's easy to think he must be right.

Here in Oregon there have been thousands of solar hot water systems installed on homes, and they are generally reliable and trouble-free. We have one on our house that is five years old and works fine. The net installed cost to us was under $2000, so the payback is about 8 to 10 years.

An air-to-water heat pump sounds good in terms of efficiency, but consider: most models draw heat out of the ambient air where the heat pump/tank combo is located, which is usually inside the house. If the house is in a heating climate, the heat pump is going to take heat out of conditioned air. That means the heat pump will also cool the house during the heating season, which is counter-productive. This idea makes sense for a split system like the Daikin Altherma with an outdoor unit, but the cost estimate was $22,000 for our house. Granted it has a COP of around 4, but even running it on low-cost PV, that's a big investment and there are  as yet no financial incentives or tax breaks.

I believe simplicity and reliability are essential to long-term energy savings and reduced costs. If equipment has high efficiency but you can't find anyone who knows how to maintain or repair it, the initial savings can be eaten up quickly. Not to mention the hassle factor.

Best,

Mike O'Brien




On Mar 23, 2012, at 9:50 AM, molasses at q.com wrote:

> On this topic: http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead
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