Scaling up a solar hot water design

IHDiesel73L · 03-09-10, 10:02 AM

I recently found the side Builditsolar site and I'm glad that I did. I can't post a link to it yet, but I'm sure many folks here are familiar with the site. The $1000 solar water heating system is probably the best designed DIY solar project I've ever seen. I've been researching solar water heating for a long time and was always frustrated by the fact that factory built systems cost an enormous amount of money, and most DIY systems I found were overly complicated and quite frankly, ugly. My wife and I hope build a new home within the next 10-15 years and since the home will be where we remain for the rest of our lives, we'd like to invest heavily in energy saving technologies not only because we will get our money's worth out of them, but also for self sufficiency purposes. Our primary source of heat will be a wood gasification boiler with oil as a backup, but I've struggled with how to provide hot water year round without having to use the wood boiler in the summer. Based on what I think our usage will be though, the system as it's currently designed would definitely have to be scaled up. I also have some other ideas since I intend to make this system a permanent fixture in the home.

In terms of scaling the system up, what would be the point of diminishing returns? How should collector square footage compare to tank size? The collector in this case is 48 square feet and the tank is about 180 gallons. Would doubling the capacity of the system simply mean making the collector size to 96 square feet (I think I would go with two 4' x 12' banks on either end of the house if this were the case) and using a 360 gallon tank? Or would it be a better idea to leave the collector the same size and just increase the heat storage tank to create more "inertia?"

In terms of the tank itself I was thinking of building it into the foundation of the house utilizing poured concrete. It would not be difficult to form a tank along one of the outer walls of the home leaving plenty of room for thick foam insulation on the sides and bottom and an EPDM liner. If a larger tank would be advantageous, something on the order of about 1500 gallons would not be overly large (4' x 4' x 12' inside dimensions), plus it would allow room for a much large preheating coil. The system as designed will hold 9 gallons of 140 degree water, meaning that one would have to use more than 9 gallons before cold water was introduced to the actual water heating unit. At first glance this seems like it could be easily scaled up to 18 or 27 gallons simply by increasing the length of the coil. Is there anything I might be missing here?

Finally, I'm still not sure how the heat exchanger from the wood boiler would come into play. Would it make sense to have cold water from the well first pass through the solar heating tank for preheating and then to an insulated holding tank containing a flat plate heat exchanger fed by the wood boiler? This would avoid the problem of the solar heating tank becoming too hot, but I would certainly have to put an anti-scald valve on the domestic hot water tank. I'd be very interested to hear a critique of this idea.

Solar Mike · 04-24-10, 05:46 AM

Hi

For domestic solar heated water using flat plate collectors around 1 sqf per gallon of water or 50 L per sqm of collector area is commonly used, this will supply you with all your hot water in the summer months. It makes sense to provide for 2 days of storage to gain max benefit from the sun. So the tank size really depends on what your daily draw off is. I dont think bigger is necessary better - higher losses, more tank insulation is required, expense.

If you plan on a 1500 g tank then if it located inside the building envelope any heat lost goes into your house.

With a wood boiler, I would make the main HWC sized for 1 days draw off, then you can fire up the boiler for a couple of hours and have a days worth of hot water to use. Any solar input could also feed into this tank and then when it got to useable temp divert to a larger secondary tank as a pre-heater.

Cheers
Mike

03-09-10, 10:02 AM	#1
IHDiesel73L Lurking Renovator Join Date: Mar 2010 Posts: 4 Thanks: 0 Thanked 0 Times in 0 Posts	Scaling up a solar hot water design I recently found the side Builditsolar site and I'm glad that I did. I can't post a link to it yet, but I'm sure many folks here are familiar with the site. The $1000 solar water heating system is probably the best designed DIY solar project I've ever seen. I've been researching solar water heating for a long time and was always frustrated by the fact that factory built systems cost an enormous amount of money, and most DIY systems I found were overly complicated and quite frankly, ugly. My wife and I hope build a new home within the next 10-15 years and since the home will be where we remain for the rest of our lives, we'd like to invest heavily in energy saving technologies not only because we will get our money's worth out of them, but also for self sufficiency purposes. Our primary source of heat will be a wood gasification boiler with oil as a backup, but I've struggled with how to provide hot water year round without having to use the wood boiler in the summer. Based on what I think our usage will be though, the system as it's currently designed would definitely have to be scaled up. I also have some other ideas since I intend to make this system a permanent fixture in the home. In terms of scaling the system up, what would be the point of diminishing returns? How should collector square footage compare to tank size? The collector in this case is 48 square feet and the tank is about 180 gallons. Would doubling the capacity of the system simply mean making the collector size to 96 square feet (I think I would go with two 4' x 12' banks on either end of the house if this were the case) and using a 360 gallon tank? Or would it be a better idea to leave the collector the same size and just increase the heat storage tank to create more "inertia?" In terms of the tank itself I was thinking of building it into the foundation of the house utilizing poured concrete. It would not be difficult to form a tank along one of the outer walls of the home leaving plenty of room for thick foam insulation on the sides and bottom and an EPDM liner. If a larger tank would be advantageous, something on the order of about 1500 gallons would not be overly large (4' x 4' x 12' inside dimensions), plus it would allow room for a much large preheating coil. The system as designed will hold 9 gallons of 140 degree water, meaning that one would have to use more than 9 gallons before cold water was introduced to the actual water heating unit. At first glance this seems like it could be easily scaled up to 18 or 27 gallons simply by increasing the length of the coil. Is there anything I might be missing here? Finally, I'm still not sure how the heat exchanger from the wood boiler would come into play. Would it make sense to have cold water from the well first pass through the solar heating tank for preheating and then to an insulated holding tank containing a flat plate heat exchanger fed by the wood boiler? This would avoid the problem of the solar heating tank becoming too hot, but I would certainly have to put an anti-scald valve on the domestic hot water tank. I'd be very interested to hear a critique of this idea. Last edited by IHDiesel73L; 03-09-10 at 10:06 AM..

04-24-10, 05:46 AM	#2
Solar Mike Helper EcoRenovator Join Date: Apr 2010 Location: Wellington, New Zealand Posts: 61 Thanks: 0 Thanked 6 Times in 5 Posts	Hi For domestic solar heated water using flat plate collectors around 1 sqf per gallon of water or 50 L per sqm of collector area is commonly used, this will supply you with all your hot water in the summer months. It makes sense to provide for 2 days of storage to gain max benefit from the sun. So the tank size really depends on what your daily draw off is. I dont think bigger is necessary better - higher losses, more tank insulation is required, expense. If you plan on a 1500 g tank then if it located inside the building envelope any heat lost goes into your house. With a wood boiler, I would make the main HWC sized for 1 days draw off, then you can fire up the boiler for a couple of hours and have a days worth of hot water to use. Any solar input could also feed into this tank and then when it got to useable temp divert to a larger secondary tank as a pre-heater. Cheers Mike