Does it make sense to use PV to preheat DHW? I don't have room on the ground for solar and don't want to do a drainback/pump system on the roof. I'd also love to head into the direction of PV for the house long term. Can you just go direct with the juice to a electric HW tanks without a transformer + batteries contributing to loss?
I think you probably could.
For a Solar Electric Panel water heating system:
Here is an example
DMSolar.com sells PV panels cheap -- just as an example, pick the DMSolar 250 Watt panel:
http://www.dmsolar.com/somo25.html
250 watts at STC conditions (this is full sun (1000 watt/sm) and with a panel surface temp of 20C (very optimisitic))
Cost $320, or $1.28 per watt, but more $1.50 per watt with shipping.
Claimed efic 17.8%
Size: 65.6 inches by 39.1 inches or 17.4 sqft.
Voltage at max power with full sun is 30.6 volts.
If you hook 6 of these together (see below for why 6), then you have an array that:
- Output is 6*250 = 1500 watts under ideal conditions
- Array size is 235 inches wide by 39 inches tall, which is 104 sqft
- total collector cost $1920 (plus shipping plus mounting rails for the 6 panels)
- If you series connect all 6, you get 184 volts at 8.2 amps under full sun, which is the expected 1500 watts
I guess you could connect this to a resistance heating element directly in the tank -- at full (1000 watt/sm) sun, the panel produces max power at 184 volts and 8.2 amps -- this matches a resistor of (184/8.2) = 22 oms.
This would certainly be simple, and maybe with some creative panel selection you could match an off the shelf heating element.
At this one sun condition, you can pick a resistance that allows the panel to be at its maximum power point, but as the sun drops down in intensity, the panel current drops down. At lower sun levels if the resistance does not change, the panel will no longer be at its max power point, but it will find some operational point -- not sure how much efficiency this costs you. Maybe someone knows how to figure this?
The I-V curve for the panel is provided here:
(broken link removed)
Compare this to the Solar Thermal Collector:
Pick a 4 by 8 ft Heliodyne Gobi collector.
Under the same sort of sun conditions, but using a 50F ambient temperature, this calculator:
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm gives:
- efficiency 55.9%
- Output = 177 BTU/sf, or 5664 BTU for the 32 sf collector, which is 1660 watts
- Array size is 48 inches by 96 inches, or 32 sqft
- total collector cost about $800 (plus shipping and some mounting hardware)
So, for about the same power output
- The PV system takes up (104/32) = 3 times as much roof space
- The PV system collectors costs somewhat more than twice as much as the thermal collector
If you are into simple DIY projects, the same 4 by 8 panel can be built for about $5 /sf, or $160 plus zero shipping.
I picked these sizes because both of these configurations would be just large enough in a descent sun area for a family of 2 that was careful about hot water usage -- kind of a minimum size rig for a family of 2.
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For about another $200 per PV panel, you could add Enphase micro-inverters to each panel, and with just a little bit more wiring the the whole thing becomes a grid-tied PV system. This has the advantage that the title of this thread gets into, but you are now up over $3000. In spite of the arguments Martin makes, I doubt that it comes up a net positive, but I've not worked through all the numbers.
Gary
