Re-using lost efficiency from Solar Panels
I will start building a Passive House within a few months. Location is far North West Washington State close to Vancouver, BC but on US side. This means there are many heating days and virtually zero cooling days.
The house is being built using PassivHaus and Pretty Good House design principles. It is 1800 Sq Feet and uses ICF footers with SIP walls, Intus windows and my builder has shown good air sealing on previous builds. The house design was modeled using PassiHaus software and is close to the requirements for PassivHaus. I decided not to put in the extra insulation required to meet the standard because the weather at my location is mild and this is where I followed the Pretty Good House principles.
Washington state has a good Solar rebate program, 54 cents per KWh generated, so it seems worthwhile going for Net Zero energy using a solar array. I have calculated that 4-5KW will suffice if not be overkill.
The inverters for the panels have efficiency losses and although they claim operating inefficiencies of 97% this is under ideal conditions and as an engineer I anticipate that real world inefficiencies are a little lower at say 95% average. This means that with a 5Kw system I would be throwing away 250 Watts while the system was running.
Given that I will have many heating days and no cooling days why not put this to good use and place the inverters inside the house to heat it? This is my primary question.
I plan to get double duty from them as I will not have a clothes dryer and instead will build what in Europe is called an airing cupboard. This is a small closet where normally a hot water tank is placed to raise the temperature and is used to dry clothes. I lived with this system for many years and liked it. I would place the inverters in this closet.
I would get triple duty by placing a large water holding tank in the closet. This would act as thermal mass to prevent any overheating and the water would be pre-heated before going to the electric tankless heaters and therefore slightly reducing electricity use for water heating.
Maybe I could by a slightly less efficient inverter, hopefully at a lower price.
Are there any flaws in my logic? I do realize that I don’t get to save 250 watts three times.
Are there any code issues?
Steve Carlton
http://www.KissPassiveHouse.com
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Replies
Steve,
As long as your local electrical inspector has no objections, I don't see why you couldn't install your inverter within your thermal envelope. My inverter is indoors.
The main problem (for me) is the fact that my inverter has an irritating hum. I urge you to find a local homeowner who has the same brand of inverter that you want to install. Visit the home at noon on a sunny day, and listen closely. If you think you can live with the noise, go ahead.
Stephen : is it losing 5% as standby losses ??
or only 5% of power generated when the PV system is under load?
i ask because it seems not much to dry clothes and be absorbed by a water vessel
if it is only in work during PV loads, and goes on with the vaying loads...
Martin....Hum is definately a factor and thanks for bringing that up. It would be really annoying. Modern inverters switch at much higher frequencies than older models and should have less hum but I will definately check into this.
Jin...I haven't chosen an inverter yet and was just using an average efficiency number. I realize that this is not much power but I am not relying on it to do all of the drying or water heating. I just do not want to waste the heat generated by the inverter. There will be secondary heater sources for both of these applications.
Also consider that the inverter will be in a humid environment often if installed that way.
Probably most are rated for exterior ..just mentionning :)
Hi Steve: I found CEC (California Energy Commission) Efficiency to be the most helpful metric re choosing among different manufacturer's inverters for my PV installation. You should be able to find quality, cost competive grid tied inverters with 96.5% or better CEC Efficiency. SMA notes their Sunny Boy 4000TL-US and 5000TL-US inverters can be installed "in living areas" for example, but cautions they "make noises". Offhand I doubt you will achieve significant additional net heating gains by adding a (costly) water tank near the inverter, especially during winter heating season when daylight hours are shorter at your latitude. I found that slight partial PV panel shading issues to be more important re annual yields for my rooftop installation, so I chose Enphase Energy M215 microinverters. Enphase also offers a 25 year warranty vs. a typical 10 year central string inverter warranty. Your site may or may not have any panel shading, note that even a partial shading of one single panel can lower the yield of the entire string of panels when using a central inverter