Small greenhouse as a solar heater?

Seeing how solar ovens work, it made me wonder if I could use my cheap, home brew greenhouse as a solar oven in both winter and summer months. I would be interested in heating 50-100 gallons of water in the greenhouse for home hot water use during summer months. During winter months, I'd be interested in getting hot air for the home from the greenhouse..

Possible? I have great sun exposure on my property year round. My cheap, portable greenhouse can be set up right beside the house so hot water or air wouldn't have far to travel.

My thought is the greenhouse is larger than any solar hot air or solar hot water heater I've seen. Wouldn't it heat a greater volume of either air or water if I chose to convert it into one big solar heater? Or, am I overestimating the potential for heat inside a greenhouse?

By convert, I mean putting mass inside the greenhouse & painting it black. Also, I could put a reflective film on the inside back portion to reflect more sunlight on the mass.
As well, I could double wall it, leaving an air gap between the walls, if that would help hold heat inside the greenhouse.

Yes, I can just stick a black hose on the roof and build a solar air heater to hang beneath a window or two. However, I already have the small greenhouse and if it can heat a greater volume of water and air, then I'd be ahead. If I can use it as a solar heater, it would save me money by not having to build both a solar hot air heater for winter use and a solar batch water heater for summer use.

Anyone have any thoughts as to how this could be made to work? Or, is this just me being stupid?

You are not being stupid! This idea you are talking about (solar water heating) is actually used on a massive scale around the world, just without the greenhouse. One good example is China.

Those things on the roofs are solar heaters, and they work essentially the same way your greenhouse and hose would work. This is a more detailed explanation of how solar water heaters work. Overall, water goes through a medium (in your case the hose) and the black coloring of the tubing absorbs the heat of the sun which is conducted into your water, giving you warm water. Then, the addition of a greenhouse would acts as a heat retainer.

I think your idea would work.

I recommend that people decide whether they want a greenhouse (i.e. a place to keep plants warm), OR a solar heating system. Both can work, but trying to combine them leads to failure (in my climate anyway). For a greenhouse, one wants thermal mass storage, and high insulation levels. For a solar heater, low thermal mass and insulation levels, and high heat transfer to the house. A greenhouse will provide *some* heat during the winter, but pulling too much will nullify its other purpose.

Thank You Kindly,
Topher

Topher, I agree that a single greenhouse can't be used to grow plants and as a heat source at the same time. In my particular situation, I have frames enough for several small greenhouses. I'm willing to use one of them as a solar heat source if the idea is sound.

Heating water during summer months in barrels inside the greenhouse seems doable, in theory. The advantage I think I see is the ability to heat a larger volume of water, which would be advantageous for a large family---7 family members in my home. Ideally, I'd like to shut my hot water heater off during the summer and draw from a large reserve of solar heated water, instead. Possible? Don't know.

As for an air heater during winter months, I just don't know if that would work. Maybe not. There is a greater volume of air in a greenhouse, but if most of it is lost due to poor insulation in the greenhouse, then that use is probably not going to work. Just exploring ideas at this time. Fully aware both uses of a greenhouse may not work very well.

Idk. Climate would have a bearing as stated by topher. I don't see water being heated except maybe as a prewarmer. I would imagine heat/air being doable but it's opposite of what he stated (in my mind, for my climate). I would think large thermal storage and good insulation. There again, my climate has to increase Temps 2- 15 degrees for a couple of days at a time to stay above freezing. Which means only a few days a year where the heat is actually needed in the home. Heating is a non issue compared to cooling.

Bill Bianchi wrote:Heating water during summer months in barrels inside the greenhouse seems doable, in theory. The advantage I think I see is the ability to heat a larger volume of water, which would be advantageous for a large family---7 family members in my home. Ideally, I'd like to shut my hot water heater off during the summer and draw from a large reserve of solar heated water, instead. Possible? Don't know.

Possible. The barrels will need to be insulated from the environment. The usual way to do this is called a batch heater, a black barrel inside a glass topped enclosure. Heat the water, then draw hot water out of it as needed. A greenhouse isn't much help in this configuration, too large, too much surface to be insulated. Temperatures in the tanks want to be in the 150°F range. I heat my own water with solar panels, and easily get around 8,000 degree-gallons on a sunny day (from 128 square feet of collectors), call it 60 degree-gallons/foot^2.

As for an air heater during winter months, I just don't know if that would work. Maybe not. There is a greater volume of air in a greenhouse, but if most of it is lost due to poor insulation in the greenhouse, then that use is probably not going to work. Just exploring ideas at this time. Fully aware both uses of a greenhouse may not work very well.

Air heaters for the winter are better optimized with small volumes for a given area of solar-intercept. Properly done you can get a substantial amount of heat out one.

Thank You Kindly,
Topher

If I have these replies straight, a greenhouse gets hot enough in the summer to kill crops if not well ventilated, but not hot enough to heat water for home use, even if I use black barrels and put reflective material on the back wall to reflect more sunlight onto the black barrels.

During winter months, it's not going to be warm enough to heat air for home heating due to poor insulation. I assume putting a more traditional hot air heater inside the greenhouse wouldn't improve the performance of that solar heater, either.

It was just a thought as I noticed how hot the inside got when I left it up this year.

A greenhouse or even your attic can provide a serious amount of heat, even in cold climates. That heat is insignificant in the dead of a cold winter, but can be all you need if temps are around freezing or warmer. I have installed many systems in my area that use 2 thermostats(one in heating mode and one in cooling mode) and a ducted fan. When the heating t-stat in the house says it is too cold and the cooling t-stat in the greenhouse says it is too hot, the fan kicks on and equalizes the temps. One of these systems that I installed in an off the grid home allowed the owners to burn less than half the wood they had the year before.
As for hot water, Legionella grow at greenhouse temps. Either use a good, complete system or stay away.

Bill Bianchi wrote:If I have these replies straight, a greenhouse gets hot enough in the summer to kill crops if not well ventilated, but not hot enough to heat water for home use, even if I use black barrels and put reflective material on the back wall to reflect more sunlight onto the black barrels.

I wouldn't go that far. A black bag full of water on the lawn, will get you shower-hot water. The issue with the greenhouse is that you are heating a large space to a temperature where it isn't useful for anything else, just to get a bit more heat into the hot water system.

During winter months, it's not going to be warm enough to heat air for home heating due to poor insulation. I assume putting a more traditional hot air heater inside the greenhouse wouldn't improve the performance of that solar heater, either.

A greenhouse certainly will heat air for home heating. The question is how much, and are there better ways of doing that.

Thank You Kindly,
Topher

Well, yes, that was the question. Will a greenhouse heat a greater volume of water than a normal batch heater (considering the greenhouse is several times larger) and will it heat a greater volume of air than a solar soda can heater mounted beneath a window or two, even if it does not do so as efficiently in terms of total solar energy in vs hot water/warm air out?

I get that in your situation, growing food is more important than hot water or home heating and you wouldn't dream of wasting a greenhouse for "a bit of hot water."

I have 7 people under my roof---2 elderly and 3 small children---and I'm the only one working. 1/3 of our electric bill is presently for hot water, if the utility company's charts are correct. During the winter, I pay quite a bit for propane to feed the furnace.

I do not have extra cash laying around for supplies to build a solar batch heater and a solar soda can heater from scratch, neither of which are going to meet my needs with just one of each. What I do have is a greenhouse that I don't need for food production and the materials to put 2 more together. (We grow quite a bit of what we eat without the need of a greenhouse, and again, I have other greenhouses to use for growing food and can spare this one)

Can you see how I might be interested in a larger volume of hot water and supplemental heating?

Will it work to heat water, even if that seems a waste to you in your situation, and will it work to heat air for supplemental heating in the winter, even if that also seems a waste to you?

We'll agree to disagree about hot water and home heating being a waste of a greenhouse. If it will make everyone feel better, I'll even stop calling it a greenhouse if it gets used for heating.

Now, after admitting my financial situation, can a greenhouse be made to work or not, strictly from a mechanical POV? If so, what would I need to do to it to get the results I'm looking for?

Thanks.

As I stated earlier, removing the hot air from the greenhouse and putting it in your home is easy, but solar water heating can be problematic if storage temps are not high enough to destroy Legionella bacteria. Your best bet is to use a pre-heating system for your current water heater. This could be accomplished in several ways, but the cheap one would be to run like 500' of black poly pipe between the rafters, just below the glazing, then find the biggest storage tank that will fit(i.e. stock watering tank). Then divert the supply line from your water heater into coils of copper in the tank. Pump the tank water through the poly to heat the tank and then when you need hot water the cold inlet water will be heated and require less energy to shower and lowering recovery time. I like Taco 007 for a pump, they can be found online for $40-$80.