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Old 11-30-13, 07:40 PM   #1
Exeric
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Default Solar PV or solar thermal for heating your home?

I think all of us have our favorite implementations of green technology. I plead guilty as charged. I think some of us aren't as conscious of our biases as we should be, especially before influencing people new to this forum. Every climate and home site has advantages that favor one technology over another. One size does not fit all.

One thing I'm beginning to realize is just how important the different home site characteristics are when deciding between solar PV or solar heating. Climate is the controlling factor for both with house orientation to the sun coming in a close second. When I say that I'm considering both the ability to utilize the sun and also the cost and complexity to do that. Here's a map of the concentrating solar insulation values for the whole year in the USA:


First thing I want to point out is that you should always use concentrating solar energy instead of PV solar energy to estimate how your site is going to do if your thinking of solar thermal. Concentrating solar energy is based on heating effect so it is based more on infrared solar energy, whether it is a parabolic mirror or a simple roof or solar heating panel collecting the energy.

Second, you actually can't use that map I just pointed to if you are trying to figure out if your site is good for solar thermal. That map is the Average solar heating effect. You need the map that includes the shortest and coldest days only. Here is the map you need: :

Some areas just are not going to work out very well for solar thermal and should probably be biased toward PV electrical. For instance I grew up in the central valley in California. It regularly hits 110F at least once a summer. You would think this area would be ideal for solar thermal. Also it never really gets below 25F and maintains an average high of 53F in late December, which is the same as where I live now, outside of the central valley. However as you can see the central valley is colored dark, dark, dark. That's because it is socked in with fog for three months each winter. There are microclimates all over California where this will occur. Fog, and most forms of precipitation, seems to be good at absorbing the infrared region of light. That's why areas around large bodies of water tend to be more moderate in temperature. Oceans absorb heat energy and release some of it when it gets colder.

Another comment coming.


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Old 11-30-13, 07:57 PM   #2
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Even if your site looks good for solar heating from a solar insolation perspective with the simplest form of it, utilizing roof heat, it may not be. Like I said, the average high temp where I live now is 53F on the shortest day of the year. I need a 27 degree temp rise to give me a good reserve in which to supply the house with warm air over a couple of hours. The simpler forms of solar heating without heat storage will be the cheapest and easiest to implement. However, if your site is not adequate in the heart of winter then you should probably go with solar PV instead, or a solar heating system that gives a higher temperature like dedicated solar panels of either warm air or warm water construction.

There's one exception. In the southwest of this country you have very high insolation values because those regions are generally considered high plains, or high desert areas. At high altitudes there is not as much atmosphere so it can get fairly cold depending on where you are. The cold air at high altitude is most what makes it cold. But they also don't get a lot of cloud cover and precipitation, and because the air is thinner and doesn't absorb as many infrared photons for a given outside temperature in winter your solar heating panel or roof will get hotter for a given temperature there than other places, like California.

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Old 11-30-13, 08:18 PM   #3
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Once you decide your house must use the more complicated forms of solar heating then it comes down to deciding whether to use that for heating your house or if solar PV makes more sense in your climate. Here's the PV power for various climates in the USA in December:http://www.nrel.gov/gis/images/map_p...er_dec2008.jpg

PV has a big advantage then because PV utilzes the higher wavelength photons. Think of the xray principle where the photons are such a short wavelength they can even penetrate solid objects. Ikea even is getting into the business in Britain utilizing PV panels that work on ultraviolet wavelengths that work well in cloudy places like Great Britain. Ikea's New Solar Panels: Sunshine Optional, Subsidies Required - Businessweek

I leave it you to research if places like the Pacific Northwest might be good places for solar PV now with these new panels. I don't know.
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Old 11-30-13, 08:25 PM   #4
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I don't know a lot about California but I can say this about solar thermal. .... if you have a high heat load building, think twice about trying it. The proportion of the total load provided by solar will be quite small. Stick with solar hot water as it has a year round load and a realistic amount of storage is needed.

As well, the concentrated solar thermal is fine in theory until you actually install them and have to service them as I have. It doesn't matter what the technology, the higher the temperatures the system is capable of providing, the more trouble you are getting into. The most efficient systems use a lot of surface area and lower temp. Remember, we only need 40-50C to heat a house and even less in an efficient house.
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Old 11-30-13, 09:20 PM   #5
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Well, that's my bias too. See this in case you haven't:http://ecorenovator.org/forum/solar-...bottom-up.html

It seems like the roof heating is going to end up working fantastic for where I live. I was just starting to think that I better warn people here about the pitfalls if they don't live in an area conducive to a roof heating system. I'm only getting about 35F temp rises under the very best conditions. That works fine for here but I doubt that would work most places in the Midwest, Northwest and East, as well as almost all of Canada. And that still leaves out places like where I grew up where there just isn't the potential to get a maximum of 35F temp rise in the heart of winter. I just don't want to end up being a destructive force by influencing people to get into what I'm doing only because it works for me where I happen to live. I often wish other people here that are very knowledgable would use this principle also.
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Old 12-01-13, 05:10 PM   #6
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@Mikesolar
I think I just realized we were talking past each other. I referenced "concentrated solar" because it is based on heating effect. You need to use concentrated solar maps, rather than PV maps, because of this. PV doesn't use the heating part of the spectrum as much so you can't use those maps if you're trying to determine how much solar heating potential you have in your local area. I hope that clears things up.
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Old 12-01-13, 08:11 PM   #7
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Quote:
Originally Posted by Exeric View Post
@Mikesolar
I think I just realized we were talking past each other. I referenced "concentrated solar" because it is based on heating effect. You need to use concentrated solar maps, rather than PV maps, because of this. PV doesn't use the heating part of the spectrum as much so you can't use those maps if you're trying to determine how much solar heating potential you have in your local area. I hope that clears things up.
Haha, I didn't catch that at all. Good point.
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Old 12-01-13, 08:17 PM   #8
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What is the expected heat output of a solar air panel?

I have local pyranometer readings for December and January which are my highest heat load months and am trying to figure out what sort of efficiency a solar air panel usually provides so I can get a good idea of what sort of total BTU output I would receive per square meter on a full sun January day using the figures I have for my area. Is there a good rule of thumb multiplier or a ratings database for solar air heaters to look at under some sort of standard test conditions that I could modify to know how large of an area I need to dedicate to solar air heating?
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Old 12-02-13, 12:49 AM   #9
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Mnrenovator, under ideal conditions the references I've seen show a minimum of about 60% conversion efficiency for a flat plate air solar panel. 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

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.

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.

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.

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
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Old 12-02-13, 01:38 AM   #10
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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.

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