How Do I Size a Solar Thermal Collector Storage Battery

24/7...
Hmmm, I'd have thought a solar thermal collector would be radiating heat out rather than in during the hours of darkness... but that's here in the UK.
If you assume that you are only going to get decent thermal output when the sun is out you could dispense with the battery.
Del

Input = output + accumulation; presumably there is no input when it is dark.

So the thing has to be sized sufficiently to support the load during darkness without its terminal voltage falling to unacceptable lows, which period will vary with latitude and time of year.

How about some numbers?

All I asked was a simple question, or so I thought. All I can tell you is that there is more going on than what meets the eye and because of that I will need power through out the evening and on cloudy days. I will be drawing power from the house on many installations and on others I will have have gather and store it at the collector. It can be done either way. I also need to do it this way for my demonstration unit which will be going from city to city.

The question seemed simple because you are asking about a Thermal (heat) absorbing panel with electrical (power in watts, etc) requirements, - a common mis-understanding, despite the very sophisticated lemniscate beside your name. I would guess you want your pump to circulate the water, (or other substances) which the Solar Thermal panel can only heat, although far more efficiently (often over 90%) than a pv (Photo voltaic) (Electricity producing) panel can, (lucky to get 20% and that before heating element losses, if that was what you had in mind, (- don't,) - should you use a pv panel to heat water), which maybe for your solar thermal panel you need to do to demonstrate the hot water or more normally to move the hot water from the panel to a storage tank? - and then you would do that only when the hot water in the thermal panel is hotter than the water in the tank. If the latter, in the Solar Thermal industry, say with a 3 or 4 square meter solar thermal situation, ("average" family) a 20 watt panel would be fine, and no battery as the water in the Solar Thermal will only get hot if the sun is shining. Strongly. Cheers, Geoff Thomas.

Your total loads came to 14W, which at 12V would be about 1.2A. To give some margin, let's say 1.5A (18W). For (5) days of storage, 120hr × 1.5A = 180AH battery.

If your PV array is active 15% of the time, 18W ÷ 0.15 = 120W of solar cells.

I would have the battery always in the circuit, with the PV cells to charge the battery through a diode and possibly a charge controller.

This is all rather rough, but may serve as a start.

You should run your loads off the battery through a load controller (this will keep you from over-discharging your batteries) and charge your batteries with a charge controller (from the pv). If you figure your load is 2 amps at 12 volts (24 watts) a battery with a 250 Ah capacity will provide for the load for 5 days... but, constantly discharging a battery to its capacity will dramatically shorten it life, so you might want to double that....

Thank you all for your help. I have gotten what I asked for (and then some). I am on a self imposed deadline and have little time for levity. Thank you again.

I have built many systems designed to do what you want to do. Traveling from place to place means that your PV production will be inconsistent, as well as many other variables. The answer will be apparent when you gather all of the information required, which with solar, is always based on some scenario of energy (electricity in this case) production and storage which you accept as a design parameter. It's far from a simple question.

Just know that if what you want to do was relatively simple (your words), it would be relatively common. It is not, because it is not usually cost effective. No levity meant.

Thank you (PFR) for your response. Cost is not the issue for my transportable demo collector. It may be an issue for those wishing to live "off grid" ; but then what else can they do? Comparatively speaking, when you consider the costs of having an electrician hook everything up to house electricity, it may not be that expensive at all.

I am not a fan of PV and now find myself needing to fold it into my application; ugh. It seems simple enough on the surface; connect a PV panel of the appropriate size to a battery of appropriate size through a charge controller and then attach my DC devices up to the battery. Do I need some type of controller between the battery and my DC devices as well?

As you have done this before, can you recommend products that will do the job? I need to order get them as soon as possible to meet my imposed deadline.

no, I can not give you a list of products, because I don't know what you need. Tornado made what appears to be a good guess, but you would want to double the size of the battery so that it never goes below 50% depth of discharge. As he stated, it is just a guess, a starting point. We need a schematic of your system, with operating assumptions, and we will try to help.

You must acquire the knowledge, or employ the knowledgeable. In this instance, CR4 is probably not the greatest place to do either.

Morningstar makes a good low power charge controller and load controller combo.

I can second that. I have 5 of the morningstar 24vdc controllers and four have worked without error for 29 months. the 5th is the backup. I tested it and put it back in the box.

they have a large variety of controllers available online.