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LadyN
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Joined: 26/01/2019
Location: United StatesPosts: 408 |
| Posted: 10:41pm 18 Mar 2019 |
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Is there value in using boost convertors in this situation?
Short version:
I have 6x 300Wmpp (Voc = 40V, Isc = 8A) PV Panels.
The shed where I work is 100 ft away from these panels
The power output from these panels will be used to offset power used from the grid using as hassle free and seamless a system as possible
To minimize cabling costs and conductance losses, I have been considering boosting the power output from these panels the following way:
i. Boost the 90 - 100V (3x panels in series) from panels to 117 - 120V. This reduces the current to ~7A and allows me to use 16AWG landscaping cable that can be purchased wholesale instead of more rare (and expensive 12AWG "solar" cable)
ii. By having the panels output power at 120V and being able to closely control this output voltage (by controlling the boost), I can use a diode OR configuration to blend rectified grid power with PV power
Thus, the output power from the diode OR configuration blends PV and grid power in a hassle free and seamless way.
Is this all crazy talk?
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LadyN
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Joined: 26/01/2019
Location: United StatesPosts: 408 |
| Posted: 10:50pm 18 Mar 2019 |
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Detailed version:
I have, for now until I get more, 6x 300Wmpp (Voc = 40V, Isc = 8A) PV Panels.
The shed where I work is 100 ft away from these panels because I have to stay in a HEPA environment. There is no covered area between these panels and the shed so my options to keep panel controllers is either locally at the panels or 100 ft away from these panels at the shed. Dad prefers them near the panels as otherwise he will have to do more work to feed cables and insulation into the shed to continue to keep it HEPA compliant.
The power output from these panels will be used to offset power used by DC tools at my brothers shop and to heat the house electric water heater, both adjacent to the shed.
Of course, the MPP 1.8kWh power output from these panels will not completely offset the power needs of these two loads but the idea is to:
1. Offset the grid power use with what these panels can optimally produce
2. Make this as hassle free and seamless as possible
With the pocket money I have available currently and with help from family, I can purchase enough hardware to mount 3x of these panels for now, so the other 3x panels will have to wait their turn until we can purchase more mounting hardware unfortunately.
To minimize cabling costs and conductance losses, I have been considering boosting the power output from these panels the following way:
i. Boost the 90 - 100V (3x panels in series) from panels to 117 - 120V. This reduces the current to ~8A and allows me to use 16AWG landscaping cable that can be purchased wholesale instead of more rare (and expensive 12AWG "solar" cable)
ii. By having the panels output power at 120V and being able to closely control this output voltage (by controlling the boost), I can use a diode OR configuration to blend rectified grid power with PV power
Thus, the output power from the diode OR configuration blends PV and grid power in a hassle free and seamless way.
This way I don't have to worry whether the sky is cloudy for a day or for a moment and the power tools/water heater source power without additional wiring.
PS: Let's for a minute assume the water heater thermostats are 8A DC safe as this is a complete new discussion |
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ltopower
Regular Member
Joined: 08/03/2019
Location: United KingdomPosts: 64 |
| Posted: 11:33pm 18 Mar 2019 |
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If your only boosting to 120V you may not get as much power through so adding some DC caps inline would help.
110V AC peak is 156V so a portion of the AC peak waveform will end up from the grid, buffering the solar into capacitors at this time would help get more energy out overall.
The boost units $15 each. have an input limit of 30A (or 40A for the 1500W)and output of 20A with maximum voltage around 90V. One option would be to boost each pannel to say 55V and put this in series, so solar is then above grid. The output voltage level is then maintained at this stable point....
If not enough solar is available the output current from the boost units drops first, but it would maintain the voltage level. Until the load pulls the voltage level down to the grid level.
Putting more solar in series and connecting rectified to the grid has the dissagvantage of when there is not enough load the voltage will rise to open circuit level.
If you had say 5 panels in series (30x5 = 150Vmp) the maximum power point votage is close but when the load drops the DC voltage level will then rise closer to 40x5 = 200V. Might not be an issue.
Once you have more panels or a different solution you also then have the benefit of 3 boost / variable output power sources to experiment with... |
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Warpspeed
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Joined: 09/08/2007
Location: AustraliaPosts: 4406 |
| Posted: 05:14am 19 Mar 2019 |
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There will be a lot of times when there is no solar at all (night) or very little solar (total cloud) so running totally off the grid is unavoidable some of the time.
The best way to combine power, or power share is with dc, and preferably higher voltage dc because with a lower current, its going to be lot more efficient, especially where there are long cable runs.
I think the very first step would be to build an inverter, even if its at low power to begin with, that can work over a fairly wide input voltage range.
Run that off a grid powered rectifier, and as you are in the US, you will probably have two 110v phases with respect to neutral (ground). With only two large diodes that will generate a very healthy +155 volts of dc.
Exactly as Itopower suggests, you can then feed a series string of solar panels into that. The only requirement is that the maximum power voltage of the series string is just above +155v coming from the rectifier.
Clearly five 24v 300 watt panels are going to be marginal in voltage, but you might be able to score a 12v 150 watt panel from the same solar panel "family" that will give you an extra 15 volts or so, which would be ideal.
You can then power loads up to the capacity of your inverter, and get as much solar contribution as is available. A system like that here where I am provides 80% solar in summer, and about 55% solar in mid winter.
From there its the upgrade path as funds permit.
More solar panels.
Larger transformers and mosfets for your inverter, and that can be done stage by stage too.
That is pretty much what I have done, although I did finally fit a battery to go completely off grid. But that is just not a cost effective thing to do where grid power is available.
Perhaps a small battery that can just supply minimal night time power might be a nice thing to have if the grid goes down. But a battery large enough to supply huge day time peak loads in really cloudy weather is jut not going to be cost effective.
Cheers, Tony. |
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zaphod
Regular Member
Joined: 03/06/2018
Location: United KingdomPosts: 93 |
| Posted: 09:31am 19 Mar 2019 |
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Hmm personally I don't think off the shelf boost converters as sold on Ebay and the likes are suitable for PV. IMOP your best bet is an grid tie inverter at the panels location that overcomes your wiring size problem except they cannot be wired with wet string as they do require a reasonably low impedance connection. Then you can share anything anywhere at your local grid voltage, DC tools remaining powered by rectifiers.
Most grid ties are happy with a high voltage input so if it is not possible to co-locate the GTI & panels you can have a series string of 6.
Your point about thermostat contacts is very valid, they are not designed for high voltage high current DC and will simply cause a fire if used in that way. The best way to solve that issue if you want to DC feed the heater is to use the thermostat contacts to switch the gate of a mosfet (low voltage & low current) and have the mosfet switch the main power circuit.
There are grid heater diverter products designed to heat water in favor of exporting power to the grid however they cannot give priority to heating over your household loads if that's what you want, that requires a more sophisticated system level design.
Cheers Roger
1Kwp DIY PV + Woodburner + Rainwater scavanger :) |
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petect
Newbie
Joined: 03/02/2018
Location: United StatesPosts: 15 |
| Posted: 01:13pm 19 Mar 2019 |
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LadyN
For what they are worth, here are a few thoughts about your project.
Most commercial pv inverters will accept a 400v – 600v DV input. If the inverter is to be located in the shed, in order to minimize losses and keep wiring costs down, I would series / parallel the modules to get as close as possible to the max input of the inverter.
If the inverter is located at the modules, you will be working with 240v AC.
Either way the voltage will determine the current needed for the power output, and that will determine the wire size.
I would seriously consider over-sizing the wire to minimize losses – I don’t think 10 awg is excessive for 1800 Watts, especially if you are thinking about adding more modules in the future.
THHW and THWN can be found on ebay for reasonable prices. They can be used in wet environments, and are rated for 600v. Landscape wire is LOW voltage. It is NOT rated for even the 120v you mentioned.
DC is nothing to play with. Even at voltages below 120v, it can be deadly. Whether you go dc or ac you should NOT use landscape cable.
I don’t know how you would get the power from the modules to the shed other than to bury them. NEC requires a minimum depth of 18” and to be run in sch 40 pvc conduit (this can vary with jurisdictions).
Running extra wires, or a pull cord in the conduit can save a lot of grief if you decide to make changes in the future.
“Solar” cables are usually only needed from the mc4 connectors at the modules to the combiner box. THHW can be used after that, within conduit. “Solar” combiner boxes aren’t necessary either. Waterproof electrical boxes are perfectly acceptable at a much lower cost.
I hope this helps.
Pete |
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