Got a bunch of solar hot water panels, for free!

[Daox]

Yes, as Ben said, the panels will be for heating DHW and will assist in space heating through the use of hydronic radiant floors. As my chart shows, they'll provide quite a bit of heat (20%+ of my gas usage) even in in the worst of winter. My house is not all that efficient either (yet). Rough estimates show it saving me just under $400 a year.

The setup will be kept as simple as possible. It will be a drain back system to a ~400 gallon tank in my basement so no antifreeze. The DHW and space heating loops will need to go through their own heat exchangers inside the tank to pull the heat out.

Since my last post with that chart, I've tweaked it a bit to calculate things with the collectors at a specific tilt and also tweaked the collector efficiency per month. So, it should be a bit more accurate now.

[bennelson]

So all we have to do is keep the cost of materials under $400 and you have a 1-YEAR RETURN ON INVESTMENT!

Heck, even if you spent $800, a TWO-YEAR ROI ain't bad either!

10 solar panels for FREE is a good start. The materials for racking will cost a bit, as will the "water-box", but we're not talking $1000's here.

[Daox]

It'll definitely be interesting to see how much this ends up costing. It looks like $400-500 for the rack, but there is a lot of other stuff to add on. The piping to the tank (which is fairly large diameter, est. ~$250 plus insulation), the tank (~$500 estimated), and the heat exchangers will be another big costs. I wouldn't be too surprised if it got up to around $1500. However, I'll do it bit by bit and I'm sure some things will cost more and some will cost less.

[S-F]

How many gallons is the tank in the $1,000 builditsolar project? How are you planning on building your tank? This is all very important and interesting to me since I just bought a house and am either going to scrap the existing radiant system in favor of two small minisplits or do something like you are.

[Daox]

My tank will be around 400 gallons. I'm planning on making it very similar to Gary's solar shed tank which is also around 500 gallons. This is similar to the $1000 solar project one, just larger.

I do have a few changes I'll be making to the design and I haven't worked them all out quite yet. The tank is the next thing on my list to start working on. But, I know I will be using a different liner than he is. He used EPDM pond liners for his tanks and I am looking into using a paintable liner. The cost is less (~$100 vs $160) and thats the main reason I'm opting for it. It is also probably a bit easier to put in since the pond liner has to be folded. The other thing I'm thinking of doing is insulating more than most of have done before. I think his tank is insulated to around R30. I'm planning on insulating mine to at least R40. My insulation will also cost less because I'll be doing it mostly with cellulose (sealed from water of course) versus foam board.

[S-F]

Originally Posted by Daox

My tank will be around 400 gallons. I'm planning on making it very similar to Gary's solar shed tank which is also around 500 gallons. This is similar to the $1000 solar project one, just larger.

You mean smaller.... right? Yours = 400 Gal. His = 500 Gal? The idea I've been tossing around involves doubling his tank. May quadrupling it. I like the idea of saving as much as possible but i like the idea of 0 energy a lot more than 20%.

You are planning to build a box around the box and dense pack it? If it gets wet you'll be hosed! (no pun intended)

[Daox]

I'd highly recommend this setup for anyone in the same climate as me. I don't really NEED air conditioning so a heat pump isn't going to be used in summer much at all. I plan on using other means to cool the house (this is one example/possibility). Also, solar power is practially free once its installed, heat pumps do have a running cost even though it is pretty small compared to most other forms of heating. So, I think if you need to use a lot of AC, a heat pump would probably be better for you. If you don't need much AC, you can get away with a solar setup and some smaller form(s) of cooling.

[Daox]

Originally Posted by S-F

You mean smaller.... right? Yours = 400 Gal. His = 500 Gal? The idea I've been tossing around involves doubling his tank. May quadrupling it. I like the idea of saving as much as possible but i like the idea of 0 energy a lot more than 20%.

You are planning to build a box around the box and dense pack it? If it gets wet you'll be hosed! (no pun intended)

Smaller than his $1000 solar hot water tank which is around 120 gallons I think.

I discussed larger tanks with Gary quite recently as I too was thinking about a ~600 gallon tank. The problem is that you can only use the water down to about 85-90F before you need to start heating it back up. DHW needs to be warmer than that even. If you have too large a tank, it might never get up to the temperatures that its really useful which is probably 110F+. Also, with a larger tank, you have more heat loss due to more surface area. You can negate this with more insulation, but it does cost more. Plus, my house is using way more than I'm producing daily, so that extra capacity is completely useless. Only if I had enough panels to produce more than a days worth of heat would I need to look into a larger tank. Then, I could save up for cloudy days.

[AC_Hacker]

Originally Posted by Daox

The problem is that you can only use the water down to about 85-90F before you need to start heating it back up. DHW needs to be warmer than that even. If you have too large a tank, it might never get up to the temperatures that its really useful which is probably 110F+.

This is where a small homemade heat pump can really help...

The heat pump can extract heat from your 85-90F storage tank and deliver heat to you in the range of 115-120F, as you probably need. My experiments have shown that the COP of such a heat pump working in the 80-90F range is quite high... on the order of 6 or 7.

As an added bonus, the chilled water left in the storage tank will present a higher delta T to the solar heat gathering part of your system, and increase the efficiency of your collectors when the sun starts shining.

Of course, when the solar collectors are supplying enough hot water to properly run your floor heating, you would not want to divert heated water through a heat pump.

* * *

Most hydronic floors like yours and Gary's are designed and built for cheap fossil fuel and have wide tube spacing, on the order of 12", and also have insulation that seemed quite reasonable when fuel was cheap. If hydronic floors were built to take advantage of lower feed temps (tube spacing on the order of 6 inches), you could still use 80-90 degree water.

Originally Posted by Daox

Also, with a larger tank, you have more heat loss due to more surface area.

Actually, larger tanks have a smaller surface area, per unit volume than smaller tanks, and lose heat more slowly.

This is why a cluster of bees can survive a cool night outside, but a lone bee will die.

-AC_Hacker

[Daox]

Thats all quite true AC Hacker. It made me rethink things (as well as S-F's comments). So, I ran my own numbers on using a 400 gallon vs 650 gallon tank. There are a few effects to consider.

First off, this change lowers the tank temperature. The reduced tank temperature reduces the heat loss of the tank due to the water being cooler. However, the tank is also physically larger, so it has more surface area to loose heat from. In the end, the larger tank does loose more heat, but the effect is very minimal. All in all, the 650 gallon tank looses ~300 btu per day more than the 400 gallon tank. This is nothing when you consider even in December (worst month for solar), my average solar heat input is ~79,000 btu.

Second, the tank change also increases the collector efficiency. This is because cycling hot water through a collector makes it picks up less energy. The panel also stays cooler and looses less heat to the environment. This is where most of the benefit comes from.

Lastly, going to a larger tank supplys you with cooler water. In my case, going from 400 to 650 gallons will likely lower the average tank temperature about 10 degrees (from 115 to 105F), and lower the peak temperature from 140F to 120F. 105F is just barely warm enough to shower, so some additional energy may be needed for DHW. However, 105F is fine for hydronic heating. Considering that space heating is by far the larger load and energy hog, I should primarily design to its needs. In addition for 6 months of the year I'll be collecting more heat than will be used, so hot water won't be an issue then.

All in all, going from a 400 to 650 gallon tank will allow me to collect about 6.7% more heat from the panels. I just need to be aware that I'll be dealing wih a lower temperature water and design accordinly for it.