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Discussion Starter #21
I have a cool little simulation program for flat plate collector building. It's free from some university or other.
Some of it is too academic for me; but basically you put your dimensions and material properties into the boxes, the angles and so on, and it pops out an efficiency estimate.
total solar power numbers are available online, so that number is simply multiplied by the efficiency.

I should have 120C stagnation temperature in mid winter according the the program; I found that hard to believe so I built a test box. Just a sheet of glass over a black aluminum plate in an insulated plastic box.
The plate hit 120c during my test on December 22 a few years ago.

But as you said, when the fluid flow decreases, so does the efficiency. A lot. From 72% down to 53%.
But I need hot water; warm doesn't do anything for me.

We get a lot of clear sunny days in winter. The sound of the oil burner running on those days is really irritating.
We pay $5.40 per gallon for heating oil here. My boiler is 25 years old and not very efficient; I took it out of my rental property when I put in a modern gas system. The plan was never to keep it running this long.
 

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How hot do you want it to get? I know that my attempt at solar heating drove the pool temperature up to 94° F for a couple of days one summer when the air temp didn't go below 70° overnight. Not bad for an un-insulated, open top, 14,000 gallon container in an area that rarely breaks 90° ambient.

Our sunshine in December usually comes in white and we have to shovel it. You may have to add augmenting devices to your collector to gain additional heat during winter if you want more than 110° F.

We're about even for the price of heating oil. Maybe a nickel either way, depending on the exchange rate for the Canabuck on any given day.
 

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Discussion Starter #23
How hot I want it to get and how hot it will get are two entirely different things...
I expect to need 60C [140F] from the collectors; maybe a lower temp will work. There are so many variables, it's hard to figure out. It will be adjustable so I can always change it.
The lines to the tank are long [loss] then heat will be lost mixing [despite my efforts to stratify] and heating the steel.

If I find I have heat to spare, I can lower the header temperature by increasing the flow, and vice versa.

The collector array is to be 18M2, 190 sq. ft. Even with low flow / efficiency, I should get 10Kw of heat at midday in winter. That will only be on clear days for 3 or 4 hours of course.

Solar radiation is high and heating load is moderate here.

So your question, how hot do I want it to get? 100C, 212F. I could easily achieve that; in summer.

Before I started planning, I went to a local business of high repute; a slick salesman printed out a slick computer simulation with a lot of numbers and symbols, told me for $12,000 they would install a setup that would give me 85% of my annual heating needs [including domestic hot water].
It didn't match my numbers at all; then I realized what they do. Calculate your total annual heat load, then your total annual heat gain, and stick them together.

I pointed out to the salesman that this system was just a $12,000 hot water heater that would deliver little space heating in winter.
"Most of our customers aren't here in winter." he said lamely.

I found other simulations online that use this principle to sell "solar heating" that will not work.

So I know I'm swimming against the tide here. I hope for 80-85% of winter heat load, but I know in reality it may be considerably less.
Night rate electricity is 9 eurocents per Kwh, considerably cheaper than the cost of running my oil burner. Just having a big heat buffer tank will save me money.

The 2.5 ton tank will heat my house for 2 or 3 days if I heat to a high temperature with fuel, like a high output wood furnace.
 

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We obviously have very different heating scenarios. Ten kilowatt hours of heat at midday will about maintain the temperature in my home on a really cold day in January. There wouldn't be much left over for storage. I have 15 Kw of baseboard heaters and my January bill is about 4,000 Kwh just for heat. That's an average of 5.6 Kwh for every hour of the month.

To top it off, we don't see the sun very much in the winter so solar heating is not an option anyway.

Your tank will store 16.1 Kwh of heat for every 10 F° of useable temperature. I'd need 84 F° for one overcast 24 hour day.
 

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Discussion Starter #25
Yes, a rather large difference in climate between Ontario and southern Portugal!

Most homes here don't have central heat at all; they either don't heat [leading to damp interiors] or use plugin resistance heaters. A minority have AC, and all systems sold here are reverse cycle [I think it's by law], so with our mild climate they work quite well as heating systems.

We foreigners like our comforts; most people use a small wood burner in the house, and that works ok.

I've worked out my heating needs by simply keeping a watch on my oil use, although I don't accurately know the efficiency [or lack thereof] of the old system I've been using. I'm assuming 60%.
 

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Wood burners are great, especially if the power goes out.

I had a 21k BTU split system, air to air, heat pump installed about 10 years ago that let's me leave the baseboard circuits off for an extra month or so. I can understand why the law would stipulate reverse cycle for air conditioning.

Your climate sounds like the ideal situation for that. The downside is the cost of installation. The upside is that the solar heater could be used to supply a water to air heat pump with much lower temperature water than 120° F.

It sounds like an upgrade for your oil heater would be about as beneficial, if a bit more pricey, as the solar heat system.
 

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Discussion Starter #27
The oil burner is being retired for sure; I only installed it as a stop-gap measure to get us through our first winter in the new house.
That was 8 years ago.

Portugal has announced an intent to phase our fossil fuels completely in the next 10 years; I don't believe that will actually happen, but I do think they will be steadily increasing the taxes on fuels.

Burning oil for heat in this climate is ridiculous.
 
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