1200W grid tie suggestions?
maccluer Registered Users Posts: 14 ✭
I've got 6 Kyocera 200W PV panels yielding a nominal 157 volts at 8 amps. (They actually do better than that by 15% on cold clear days.) They are presently doing a bang up job heating the water sent to my shop's radiant floor. During the summer tho I'll have power to sell.
Are there grid tie inverters for such small systems?
Are there grid tie inverters for such small systems?
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i am confused as to why you would use pv to heat water with as solar thermal is more efficient than using pvs. i am also not too sure what you have in mind in trying to put the pvs to the grid when you are using it to produce heat for your water adds another step to it would lessen that efficiency even further if still used for heating.
as to the inverter, you are in a bad voltage range at present for inverters as straight gt like voltages much higher than that and battery based systems do better with lower voltages. 157v is above the ability of most charge controllers at present and would not be very efficient in downconverting to the lower voltages needed for battery based systems.
Why on earth would you use PV to heat water to heat your shop, when for ~1/3 the money you could collect hot water directly with better than 3 times the efficiency, resulting in about 9 times the bang for the buck?
Unless you heat the water only as a diversion load for the PV when every other load is taken care of and/or batteries are full charged.
I've been there before with solar collectors, even built refrigerant-charged ones.
Too much maintenance and the controls/sensors are always malfing. I have designed a fool-proof worry-free solid-state analog maximum power point tracker for the PVs---it's trivial if your "battery" is your dump load. A radiant floor is the ultimate battery, all energy captured is stored and used. All the PV energy I take in is used to replace the commercial 4X electric rate I must pay to heat the slab.
But my question to you is how to sell summer PV power.
You do have to admit you are using a collector with maybe 15% efficiency rather than a collector with maybe 85% efficiency though.
Buy a couple more panels and go with a grid tie inverter if you wish to sell to the grid.
As for your current method of heating your radiant floor, if it works for you, great. I personally would put up with a bit of maintenance headache for such huge efficiency gains, and use my PV for a better result.
You did however point out the flaw in your own logic by saying "A radiant floor is the ultimate battery, all energy captured is stored and used" This is true, only to the extent that you can use the heat. Just like a battery, once the battery is full, you are in essence throwing production capacity away. Not only that, but for the X months that you don't need heat in the floor, you are throwing huge potential capacity away.
By my quick, imperfect calculations, your 1200 watts of PV running at 80% of name plate average would yield ~960 watts of power available. (Call it 1kw just to keep the math simple) 1kwh=~3,400BTU/hour * 5 hours=17,000BTUs into the water.
The cost of 1.2kw of panels in todays market (not counting any connections/controllers pumps etc) might be ~$2.50/watt, making the panel cost ~$3,750.
I am betting that for <$3,750 I can build a simple collector (flat plate/drainback/glycol loop etc) that will deliver way more than 17k BTUs/day into the water, including the pumps and controls.
The last simple flat plate collector I built, cost ~$400, and produces ~3000 BTU/hr, (sunny December at LAT 50' summer probably double that) and is much more forgiving of clouds/shadows/haze etc.
Just my opinion,
What you say is correct, but with flat panels I will not be able to sell summer power. Also, life-cycle costing of these investments is tricky. Many people believe that discounted cash flow analysis should not be applied to many alternative energy investments. How can you discount avoidance of future permanent harm to a present value?
In one sense, using PVs is like giving to the Nature Conservancy, in that it encourages stewardship (and is a lot more technically interesting than writing a check). Look at the flat panel option: I would need a heat exchanger with the floor water since one does not want to risk circulating a gycol mix that degrades over time and has to be periodically dumped into the environment. I would need a second pump, controls, sensors. With my PVs, Kyrocera has done all the tough engineering design with a maintenance-free collector. Instead of plumbing, I run 2 wires thru a 0.5 inch hole and connect them across parallel Darlingtons in series with the bottom element of the water heater (top element is the 220VAC backup) and some additional minor electronics. Elegant and simple.
What minimum voltage is required before I can use an off-the-shelf grid tie?
I understand what you are saying, but it just seems that you could "be writing a bigger check" to the nature conservancy.
While you might not be able to sell in the summer, the cost is much smaller. Why for example couldn't you do a simple drain back system without a glycol loop? I can't remember where you are, but a couple of simple solenoid valves and a potable water drain back is pretty simple to do. Baring that most simple controllers have an anti-freeze setting that circs a bit of heated water through the collector to keep it from freezing. The last one I built worked great as low as ~-15c.
Like I said earlier, if it works for you great, I would do something different however.
If you have a hybrid inverter (GT/Off-Grid inverter with battery bank)... Xantrex sells both in 24 VDC and 48 VDC battery bank (~>32 VDC Vmp or >62 VDC Vmp) to Voc of ~140-150 VDC maximum.
If you have straight GT (no battery bank); generally they run (useful) Vmp hot weather from >200 VDC to >400 VDC (up to 550-600 VDC Voc cold weather maximum). I think I have seen a few that may run ~150 VDC Vmp minimum for true battery-less Grid Tie. Might find a bit lower if you look around (or somebody else has some suggestions).
Thanks people for your help. It appears as if the SMA Sunny Boy 1700 U is perfect for my system and allows expansion to two more panels.
How does everyone feel about the Sunny Boys?
The most amazing development: When I began the application process for interconnection, the contact at Detroit Edison (DTE) informed me about their "Solar Currents" program: They will
1) send me a check for $2.40 per installed Watt ($3680),
2) allow true net metering, and also
3) pay me 11 cents per kWh generated for the next 20 years.
In exchange, I must sign over my renewable energy credit (REC), a sort of
chit on some national exchange. This does not mean that I lose the 30% Federal tax credit. Since my Kyrocera 205's are $2.50/Watt and I've done the installation at my costs, the system will be nearly free, plus yield future income. If I'd had known about this program I would have considered a larger system.
Moral: Investigate what's available.
The key is you did your own installation and I assume was a permitted, approved and inspected. This is not something the average homeowner would tackle.
I did a 5kw system two years ago an even with the higher panel prices actually made some $$ after all the rebates.