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Solar HW system: Comments?

Last Post 08 Apr 2008 08:21 PM by Topgas. 10 Replies.

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bartman99

New Member

Posts:57

26 Feb 2008 09:29 AM
Hi all, We live in central Virginia and have 3200 sqft house (well insulated). We already heat with hydronic radiant heat and with propane as the power source. We want to add solar HW to the system to help with winter heating and DHW, especially in light of the tax credits, the skyrocketing cost of propane and other tangibles and intangibles. Anyway, we have gotten a quote for a system. I am pasting the text below of the system: The solar system provides active solar hydronic heat collection and storage using American Energy Technology 4’ x 10’ solar panels with black crystal coating, and a drainback heat storage tank with DHW heat coils. The system uses a drainback design to eliminate potential for overheating, freezing and nighttime losses. The heat storage module is an unpressurized, polyethylene tank with copper coils for the domestic hot water and integrates with the radiant system. The price includes 5 AET panels with flush mounting hardware for direct mounting to the roof and solarsil flexible couplings, the Delta T control, the storage tank, bronze solar pump. and tubing & insulation from the roof to the mechanical room. The panels are planned to be flush mounted to the garage roof. Included is a 1 ½ton heat pump to raise the solar supply temps to make optimal use of solar panels in the heating season. *Cost: $11,000 (NOT* INCLUDING LABOR)** Now some questions: 1) General thoughts on this system? 2) Are 5 4x10 panels enough? Too much? 3)What about the heat pump? Seems like overkill to me, though our water is from a well and pretty cold. 4) Is the cost reasonable for this system? 5) Is a simpler solution practical? Thanks, Bart

Topgas

New Member

Posts:63

27 Feb 2008 08:15 PM
Bart, this is my first post on this site so here it goes. I have 6 AET 4x10 panels on my house up here in Maine. I've done something few people have done with a system like this, I've kept day by day records. Everything gets back to BTU's. I've found that the Btu harvest from my panels has less to do with the angle of the sun and the amount of daylight hours than if there is any sun at all. To put it plainly, the sun takes a vacation from Nov. to Feb ( so far). I'm 11 1/2 months into my system and it's great for DHW, but I'm not sure you could heat dog house with it unless you setup for it in the rest of the house. By that I mean I've read on Radiantec website that they did a test house in the early 80's that had eight panels on it and got 90 % of the heat for that house in Vermont from 8 panels. What makes this interesting is that the DOE verified the whole deal. To answer your questions 1) Mine is a drainback and I think they're great. 2) 5 are fine for DHW but I highly doubt your going to heat your house with them but I could be wrong, your down south and there's that heat pump that will make the panels more efficient. 3) I like the idea of the heat pump. 4) Your cost is very resonable, since I did my whole system myself and had 12K into it. 5) oil is the simplist but we all know where that's going. Your AET drainback is the deal from my limited background. My btu harvest has varied from 1,600,000 a month in August to 570,000 in Dec. Check your heat load for your house in btu's and you'll see what I mean. My house needs about 180,000 btu's a day to heat, and that doesn't include DHW. Be careful of the people in the renewables biz, they have a tendency to tell big tales. I have feeling the guys who quoted you know their stuff and you won't hear me say that of too many installers.

bartman99

New Member

Posts:57

28 Feb 2008 09:46 AM
Topgas, thanks for the detailed reply. I am now in the process of getting quotes from 2 other installers in the area. They all (not surprisingly) use different components and differnet technology (ie evacuated tubes vs. flat panel, etc). When I sort all this out I'll repost. B

bartman99

New Member

Posts:57

02 Apr 2008 01:07 PM
Hi, I finally was able to speak at length with the installer/designer of the system. I've attached a diagram that I made up based on our conversations. I think it is correct. Anyway some of the points about the system: The goal is to reduce the need to use the propane water heater as little as possible of DHW or radiant floor heat. Adding the water-->water heat pump basically allows the system to function even on cloudy or very cold nights. The heat pump will take any latent heat from the solar collectors and increase it via heat exchange. The hotter the incoming water to the heat pump, the higher the COP. He quoted a COP from 5-14 depending on water temp. Electricity is MUCH cheaper here than propane (~8-10cents/kwh vs. ~$3/gal). In my mind the heat pump is working somewhat like a geothermal unit (in heating mode only), using the heat form the sun to pre-warm the water, rather than the latent heat in the ground. The storage tank is 300 or 500 gallon and is insulated, but plastic. Max temps in the tank will be 160F. The system will shut down if temps exceed this. The solar system can inject hot water directly into the DHW or the Radiant loop or can inject via the heat pump. This is a gravity drainback system. Without power, the default is for the fluid to drain back into the tank. Also at times of very cold or very hot weather the system will drain. I did not include our current HWH in the diagram, though it is integrated. Apparently this system is similar to one used in the UVa solar decathalon house. Thoughts or comments?

Dayton2

New Member

Posts:7

02 Apr 2008 03:56 PM
I have some questions. What is the maximum entering water temperature that the heat pump can handle? At what outside air temp does the heat pump have to shut down before freezing the collector water? Or will it have that problem in your climate? Do they give a table showing the heat pump COP vs collector water temp? Thanks, I have been thinking of doing this myself.

Topgas

New Member

Posts:63

02 Apr 2008 08:51 PM
Wow. I like the concept but like Dayton2 I have some questions and concerns. First I don't really see pullling enough btu's from 300 gals of water to heat your house, even with a heat pump. I guess it gets back to what everyone says to start with, whats the heat load of the house. Then I would figure the btu's in the tank for a given temp. range and see what you have to work with. I just did a quick calculation and this is what I got: 300 gals.x70 degrees(110 to 40 in tank) x 8.33= 175K btu's. That's all fine and dandy but now your starting with say 40-50 degree water the next morning ( in the big tanks) and I'm not sure what Virginia has to offer during the winter but up here in Maine you might pull 80-90K btu's on a really good day that happens once every four days. And we haven't even started thinking about DHW yet, I give it a rough 60K needed there. This system would knock the heck out of your fuel bill but don't over rate it. I think the heat pump would do the trick with just the DHW in mind since your heat load for that is about a third of HWH load. Talk with your solar guy, he'll know better than me if he's on his game. This why geothermal works, you have tons of water (or dirt) with btu's in it to be harvested, your system has 300-500 gals. and a weak winter sun. Your parts seem to be priced right for a vendor to make a living and you have someone to stand behind the system.

bartman99

New Member

Posts:57

04 Apr 2008 04:45 PM
Posted By Dayton2 on 04/02/2008 3:56 PM I have some questions. What is the maximum entering water temperature that the heat pump can handle? At what outside air temp does the heat pump have to shut down before freezing the collector water? Or will it have that problem in your climate? Do they give a table showing the heat pump COP vs collector water temp? Thanks, I have been thinking of doing this myself. The heat pump is from here FHP It is a 1.5 ton WW unit (they don't make the 1.5 ton anymore, just 2 ton and up). The Spec sheet is here. COPs run about 4-5 for 70F entering water (I think I misunderstood the original COP statement from my prior message). I've been told that the maximum entering water temp is 100F. The solar water will be tempered so that it does not exceed the max temp. My contractor/designer also told me with water of that temp, the BTU/H approached 22k output. The unit will be inside, but will be controlled so that when the water temp in the storage tank reaches 40F, the HP will shut down. Hope this helps. B

gregj

Basic Member

Posts:326

07 Apr 2008 01:11 PM
Bartman, It occurs to me that if you added a ground loop to that system so that the heatpump could use your 300 gallon tank down to 40, then switch to ground loop then you might never need that propane.

bartman99

New Member

Posts:57

07 Apr 2008 01:29 PM
I think that is probably true, but would add significantly to the cost (I think). I might be something for the future, however. BM99

gregj

Basic Member

Posts:326

08 Apr 2008 03:02 PM
Have you run the numbers to compare? My concern is that the solar system might not generate the btus you are looking for - especially in the winter when you want them. I'm not an HVAC expert by any means so will readily admit that I might be way off base but I'm guessing the ground loops will be less expensive to install than the solar setup and that the ground loops will also be cheaper to run on an annual basis because they will generate the btus when you need them. I would think the ground loops as a primary heat source with the solar setup as an opportunistic alternate heat source to take advantage of sunny winter days and provide a great deal of spring and fall heating would be a nice setup. I'm not knowledgeable enough to run the numbers using actual historically observed sun conditions for an area and the known system efficienies in order to generate a comparison but I bet someone could be found. Topgas seems to have some experience with the actual productivity of similar systems. I really like your setup so I'm not trying to discourage you, I'm just trying to encourage you to get as much data as possible before making a decision.

Topgas

New Member

Posts:63

08 Apr 2008 08:21 PM
Some of the reasons I'd consider geo over solar: With geo you don't have to mount panels to your house, people call me a hippie....??? Your house is more marketable (resale) since you don't have those unsightly things hanging off the house. For heating, it looks like geo can actually do the job. You can actually hire people that know what they're doing to work on the system, solar you have to be careful from what I've seen. You can do your DHW and HWH with geo. The only thing I wonder about is the electrical bill but from what I'm told they're not that much.