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

[Steven Tjiang]

Reuben

You use the excess PV generation from the summer to heat the water in the
winter.  It's not economical to oversize a solar thermal system.

I still about 50% of my summer PV output in the winter.  But SF bay area is
more sunny than NW.

---- Steve (KZ6LSD)


On Fri, Mar 23, 2012 at 11:28 AM, Steven Tjiang <steve at tjiang.org> wrote:

> +1 for simplicity
>
> I located our heat pump water heaters in my unheated workspace.  I have
> several months of temperature measurements down there.  It has never gone
> below 55F in the winter. I know other climates may not be so lucky.
> I use solar preheat with an ICS (Sun Earth Copperheart).  There are no
> pumps, just a few valves.  The water heater is effectively off all summer.
>
> Having said that I'd have to say that DIY may make an active solar thermal
> system worthwhile but let's face it....this is not what most people do.
>
> ---- Steve (KZ6LSD)
>
>
>
> On Fri, Mar 23, 2012 at 11:13 AM, Michael O'Brien <obrien at hevanet.com>wrote:
>
>> 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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