|
|
You are not authorized to post a reply.
|
 Prev Next
|
| Author |
Messages |
|
billdoors@shaw.ca
Posts:0
 |
| 06/02/2008 4:56 AM |
|
I am having a new house built in the Vancouver, Canada area (Pacific Northwest), about 4000 sq ft conditioned space, all SIP (walls and roof). The design of the building looks like 2 smaller houses, one with a 300 sq ft footprint, the other about 1200 sq ft, connected by a suspended bridge of sorts which contains a living/dining/etc area.
It has been suggested that 400-500 sq ft of collector on my 7.5:12 pitch roof would be desirable if I want to do DHW and hydronic heating. The type of system I am considering is flat plate drainback, based in part on the recommendations by Ramlow and Nusz. There should be no problem getting at least that much area on unobstructed south-facing roof.
One of the options presented to me is to use the foundation area under part of the building (larger part) to house a large concrete primary tank. At 288 sq ft area inside the foundation walls, I get about 2150 gallons per foot of depth of tank. It has also been suggested that I might not want to insulate around this tank, in order to heat up the ground around (and under) the house, in order to provide extra heatsink mass as well as to reduce the temperature difference between the environment under the building and the building itself, thus reducing the flow of heat out (regardless of how much insulation I put in place in the floor of the building).
My questions for the members of the forum are:
1) What are the considerations to figure out optimum size of the tank? (apart from cost to build, natch) ie, heat flow in via the collectors, heat flow out through the house & sides of tank. As the tank acts as capacitor/reservoir, is it the case that I can make the tank as big as I can afford, or should the size of the collectors not figure into this?
2) Does the idea of having the tank be uninsulated, in order to deliberately leak heat out into the dirt under the rest of the ground make sense?
Thank you for your help, everyone. |
|
|
|
|
|
|
PanelCrafters
Registered Users
Posts:1330
|
| 06/02/2008 7:02 AM |
|
Posted By billdoors@shaw.ca on 06/02/2008 4:56 AM
1) What are the considerations to figure out optimum size of the tank? (apart from cost to build, natch) ie, heat flow in via the collectors, heat flow out through the house & sides of tank. As the tank acts as capacitor/reservoir, is it the case that I can make the tank as big as I can afford, or should the size of the collectors not figure into this? This Link should help.
2) Does the idea of having the tank be uninsulated, in order to deliberately leak heat out into the dirt under the rest of the ground make sense?
No. I would insulate the tank!
You might also look up Solar Harvest. |
|
....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
|
|
billdoors@shaw.ca
Posts:0
|
| 06/02/2008 7:38 PM |
|
Thanks, jc. The Solar Harvest home in Boulder looks to use a 6000 gallon tank. This seems to be fairly straightforward to accomplish in my own house with a 3-foot deep tank.
The whole thing with the uninsulated tank made no sense to me, either. That having been said, the other aspects of the proposed system seem somewhat attractive. The chief feature is a complete roof-integrated solar collector, all done as one unit and with the roof membrane integral to the collectors. I've attached a link to the company website here: http://www.selectsolarsystems.com/
The system itself is otherwise typical flat plate drainback, with a heat pump to get heat from the large primary to a smaller secondary (300 gallon) tank, from which heat is then exchanged directly out to the house via regular hydronic lines, and an exchanger used to preheat water in a DHW boiler. I think the heat pump is why there was not so much concern on the part of the manufacturer regarding insulation of the large primary tank, as there would be with a simpler heat exchange scheme.
Has anyone had any experience with this style of system, or know of any other similar systems (essentially building-integrated liquid solar heating, rather than BIPV) elsewhere?
Bill
|
|
|
|
|
PanelCrafters
Registered Users
Posts:1330
|
| 06/03/2008 3:58 PM |
|
Posted By billdoors@shaw.ca on 06/02/2008 7:38 PM
The Solar Harvest home in Boulder looks to use a 6000 gallon tank. This seems to be fairly straightforward to accomplish in my own house with a 3-foot deep tank.
It seems to work for him!
The system itself is otherwise typical flat plate drainback, with a heat pump to get heat from the large primary to a smaller secondary (300 gallon) tank, from which heat is then exchanged directly out to the house via regular hydronic lines, and an exchanger used to preheat water in a DHW boiler. I think the heat pump is why there was not so much concern on the part of the manufacturer regarding insulation of the large primary tank, as there would be with a simpler heat exchange scheme.
I don't like the idea of a Heat Pump at all! They consume copious amounts of electricity, which is getting more expensive every day)[And it shouldn't be, but don't get me started!] I will be building in an area with high solar isolation(not too far from Solar Harvest), so what he's doing is very relevant to me.
For a backup, I was thinking along the lines of a direct vent(sealed combustion) pellet stove. With one important feature: It could also heat water flowing thru a pipe inside of it, to recharge my main tank. Cost? The pellets and the electricity for the pump. That way 100% of my heat(don't need A/C at this elevation, in the mountains) comes from my main tank. That reduces the costs of having a seperate backup system(like an additional W/H or boiler), and gets me 100% off of natural gas and/or propane. I could even use a PV panel to power my pumps!
Of course, I would also Super Insulate(just like Solar Harvest - but using SIPS) to reduce my Heat Loss and make solar even more feasible. |
|
....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
|
|
billdoors@shaw.ca
Posts:0
|
| 06/11/2008 2:19 PM |
|
I've done some more plugging away at the design, and I think I'm beginning to understand the rationale behind the heat pump, though I think I'll need to run the numbers a bit more carefully yet.
One of the differences between here in Vancouver vs. the Solar Harvest site in Boulder is that Boulder gets about 1.5x as much insolation as Vancouver. This is according to the averaged data via FirstLook. In addition, I think that we get more consecutive cloudy days in Vancouver, which cuts the effective insolation by half or more. This means that I'd need to have more days' heat storage in order to ride out the periods between sunny days, a bigger collector to stuff energy into storage when it is sunny, and a greater likelihood that the temperature in the primary storage tank itself is going to be lower due to energy leakage as well as draw by the main heating/DHW system. (vs Boulder)
The lower temperature in the primary tank makes it seem to me like you might want to use a heat pump to get the energy out. Certainly, the Select Solar system specs a fairly small heat pump, as presumably you wouldn't need something as big as you'd use for pumping out heat from much cooler groundwater. Most of the time, there should be a higher temperature on the primary storage tank side, so the heat pump shouldn't need to have to do so much work.
In any case, after having done the preliminary heat analysis, I think I'm going to have to see where we can reduce the window area as well as add to the insulation. The model is predicting between 15kBtu/hr to 30 kBtu/hr, depending on outside temperature and assuming 0.2 air exchanges per hour, and of course, walls and windows are the only other variables I can affect in the design at this point. |
|
|
|
|
billdoors
Registered Users
Posts:3
|
| 08/05/2008 2:16 AM |
|
| One other thing about heat pumps is that, in this configuration, the lift would be relatively low as compared with using a heat pump in either an air- or ground-sourced configuration. By having the source end of the heat pump in a reservoir which has gotten some additional energy input from the sun, the COP of the heat pump ought to be very high. I just recently realized that at the extreme end, where the sun is able to get the water tank up to working temperature, the heat pump ought to consume almost no energy as the lift would be essentially zero. (almost as opposed to exactly, as there will likely be some mechanical, etc. losses) |
|
|
|
|
|
| You are not authorized to post a reply. |
|
|
|
ActiveForums 3.6
|
|
|
|