Solar hot water vs. solar hot air
user-30927
| Posted in General Questions on
Apart from the no freezing and cost advantages, does solar hot air have any other advantages over solar hot water?
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We tried solar air heating in the 70s and 80s but gave up on it because the ducts were too leaky, the fans used too much electricity, and it was really difficult to store the heat in rock beds. They were more expensive than hot water systems too.
Even the one-collector-one-room solar hot air systems available today don't make much sense because they are worthless in the summer. A hot water system to heat domestic hot water is useful year-round.
Michael,
A solar hot air system never makes sense.
If you want solar heat gain for a south-facing wall, what you want is a window, not a solar hot-air collector. But don't make it too big. And even if you install a window (which makes more sense than a solar hot-air collector), the window won't ever gather enough heat to justify its cost.
If you are tempted to install a solar hot-air collector on your roof -- don't. The economics make no sense. You can't store the heat cost-effectively. And when the sun is shining, your house doesn't need any supplemental heat.
Never say never- the Echo system (recently acquired by SunEdison) uses ECM drive blowers and heat exchangers to both cool roof top PV solar (increasing it's net efficiency) and apply that heat for hydronic storage (whether distributed with low temp rads or or air-delivery HVAC) &/ or domestic hot water systems. It has a favorable COP compared to heat pumps, and very reasonble solar-thermal efficiency, but it's a highly engineered hybrid system, not an approach well suited for DIY.
https://www.youtube.com/watch?v=QO3msk2Xpe4&feature=player_embedded
The thermal air panels popularized in the 1970s are best mounted on walls, not roofs, but have reasonable economics as a DIY, not so much for commercial installations, and since the heat cannot be stored, it's always a partial solution, covering low double-digit percentage of a typical home's annual thermal load. In an on-grid home applying the money of a commercial thermal air panel to a mini-split heat pump would usually be a better investment. For the DIYer there are several approaches to thermal air panels described in links at builditsolar.com. (Search the site for "thermal air panel".)
The rock storage approaches with big air handlers used in the late '70s early '80s used a lot of power, with a lower net COP than high efficiency mini-splits.
Your home location is critical, you haven't said anything about shading, The simple solar air collectors look really attractive in theory, but that is often before you factor in the price. Sunrooms can add a lot of heat to a house - during the winter time -- that is offset with air conditioning in the summer time. Solar air generally doesn't have a thermal mass or storage - hot water does.. Its the mass that is needed for room comfort and to improve the efficiency. You want the heat in the evenings when the building envelope is radiating heat and leaking hot air through all the building holes -- not just during the sunny portion of the day.
I've looked at them both, I have solarPV and solar h20, I do have heat gain via windows - and aggressively work to control heat gain in summer time with shading.
The problem with solar hot air - (and hot water) is that the costs can be quite high, and when you want both the most -- during the winter time holiday seasons - the production may be quite low -- again -- no mention of location. Big difference between Alaska and Florida.
In most cases solarPV has a higher return than the solar thermal projects. The reason is that solarPV prices have been dropping - while the off the shelf solar thermal equipment remains at the same price or is climbing in price.