EcoRenovator - View Single Post - When solar PV or solar heating for your next renovation

MN Renovator · 12-02-13, 01:38 AM

Mnrenovator, under ideal conditions the references I've seen show a minimum of about 60% conversion efficiency for a flat plate air solar panel.

Thanks for that.

Conditions are never ideal however, especially in Minnesota. To get the actual efficiency you have to downrate that according to the performance parameter:

(Collector inlet temperature - ambient outside temperature)/insolation

Okay so if my inside temperature is 60 degrees and it is 30 outside and it's Jan 4th and the sun looks like this.

Let's say 11am-12pm 450 watts per sq m.
(60-30)/450=0.0666667

I think I did the math wrong. I'm thinking I'd expect more than that or something is wrong with my execution of the equation. I'm also not understanding how the outside temperature and inlet temperature have that much to do with it. I understand you'll lose some out of the front glazing, a little out of the back and some through the sides if they aren't insulated but usually I would think you've got the air traveling through at a pretty good rate to catch the heat.

There isn't a lot of insolation in MN and the outside temperature is usually pretty low also. I think insolation tables for your area in BTUs per hour can probably be found on the internet. If you can locate the collector inlet vent (inside the house in a closed loop system like you are talking about) so its drawing air at a time and a place when its the coldest you can maximize actual efficiency. You still have an uphill battle I think.

I think I have all the insolation data I need to get a decent model for my goals. I just need to do the math to see how much I need together with the details of my particular criteria of expected performance.

For instance, to get maximum efficiency would require one to let the temperature drop in the house to a point you probably wouldn't be comfortable in. Most solar heating does not start kicking in until about noon. Assuming you work and there's no one at home to complain (big "if") then you could probably get a reasonable amount of efficiency out of it on workdays when you're at work.

Not too big of an 'if' for me. I am currently sharing the house with someone else but once I have my place insulated well and they are gone I can go back to dropping the temperature much lower than 68 while they occupy the house but with the added insulation, the drop will slow down considerably.

For most people I don't think that would be reasonable. I don't know how well you're insulated but my preference would be to just insulate as well as can possibly be attained. Then even if you don't maintain maximum efficiency the temperature in the house will have such a small drop from the last heating period that any low actual efficiency will be sufficient to heat the house. I think you will still need a relatively huge flat plate array to get the output you need at your location.

I'm weird, my January 2012 gas bill was under $50, less than 50 therms. A good part of that was keeping the house cold(I love cold). For insulation, I'm trying to decide on whether to add 4"-6" of either XPS or polyisocyanurate rigid. The goal is to get my heat load at -20f to be under 10k BTUhr and 4" of XPS should get me there when added to my current insulation. This will also require triple pane windows to reach this goal. I think this gives me a fighting solar heating chance on the non-cloudy days with probably 64sq ft(~7 sq m) of solar air heating. I'm trying to ditch natural gas and I'm okay with large temperatures swings and plan to use mini-split heating in conjunction to this(with 1.5-3kw electric resistance as the occasional backup if the mini has trouble).

A good older book that will tell you everything you need to know and what you might be up against is "Solar Air Heating Systems" by Kornher and Zaugg

Thanks I'll check it out.

12-02-13, 01:38 AM	#10
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	Mnrenovator, under ideal conditions the references I've seen show a minimum of about 60% conversion efficiency for a flat plate air solar panel. Thanks for that. Conditions are never ideal however, especially in Minnesota. To get the actual efficiency you have to downrate that according to the performance parameter: (Collector inlet temperature - ambient outside temperature)/insolation Okay so if my inside temperature is 60 degrees and it is 30 outside and it's Jan 4th and the sun looks like this. Let's say 11am-12pm 450 watts per sq m. (60-30)/450=0.0666667 I think I did the math wrong. I'm thinking I'd expect more than that or something is wrong with my execution of the equation. I'm also not understanding how the outside temperature and inlet temperature have that much to do with it. I understand you'll lose some out of the front glazing, a little out of the back and some through the sides if they aren't insulated but usually I would think you've got the air traveling through at a pretty good rate to catch the heat. There isn't a lot of insolation in MN and the outside temperature is usually pretty low also. I think insolation tables for your area in BTUs per hour can probably be found on the internet. If you can locate the collector inlet vent (inside the house in a closed loop system like you are talking about) so its drawing air at a time and a place when its the coldest you can maximize actual efficiency. You still have an uphill battle I think. I think I have all the insolation data I need to get a decent model for my goals. I just need to do the math to see how much I need together with the details of my particular criteria of expected performance. For instance, to get maximum efficiency would require one to let the temperature drop in the house to a point you probably wouldn't be comfortable in. Most solar heating does not start kicking in until about noon. Assuming you work and there's no one at home to complain (big "if") then you could probably get a reasonable amount of efficiency out of it on workdays when you're at work. Not too big of an 'if' for me. I am currently sharing the house with someone else but once I have my place insulated well and they are gone I can go back to dropping the temperature much lower than 68 while they occupy the house but with the added insulation, the drop will slow down considerably. For most people I don't think that would be reasonable. I don't know how well you're insulated but my preference would be to just insulate as well as can possibly be attained. Then even if you don't maintain maximum efficiency the temperature in the house will have such a small drop from the last heating period that any low actual efficiency will be sufficient to heat the house. I think you will still need a relatively huge flat plate array to get the output you need at your location. I'm weird, my January 2012 gas bill was under $50, less than 50 therms. A good part of that was keeping the house cold(I love cold). For insulation, I'm trying to decide on whether to add 4"-6" of either XPS or polyisocyanurate rigid. The goal is to get my heat load at -20f to be under 10k BTUhr and 4" of XPS should get me there when added to my current insulation. This will also require triple pane windows to reach this goal. I think this gives me a fighting solar heating chance on the non-cloudy days with probably 64sq ft(~7 sq m) of solar air heating. I'm trying to ditch natural gas and I'm okay with large temperatures swings and plan to use mini-split heating in conjunction to this(with 1.5-3kw electric resistance as the occasional backup if the mini has trouble). A good older book that will tell you everything you need to know and what you might be up against is "Solar Air Heating Systems" by Kornher and Zaugg Thanks I'll check it out.