Running a fridge from PV is not easy.
But there is one very interesting point: generally, the more sun you have, the more your fridge will need to work. So PV production is quite related to fridge energy needs. This is why I think that cooling is one of the best PV uses.
need storage
You don't have sun at night, so you need some sort of storage which could provide energy for this time, or more. Battery storage is the first one you can imagine. But it is expensive. Much simpler is ice storage: it's very cheap, quite effective, and never dies like a battery. Instead of storing electricity with chemical/electric losses from the battery itself, you just store cold in form of ice. If your fridge compressor is well dimensioned, it doesn't run all the time (perhaps 30%-50%, depends on many variables), so there is plenty of unused time left to freeze water. Just to show how effective it is: melting 1kg of ice gives 90Wh of cooling energy, which you could obtain with 60wh feed into the compressor (assuming Cop = 1.5 to achieve such temperature). Using a lead battery (45Wh/kg) with a conservative 30% discharge, you would need a 4kg battery!. So storing in form of ice is a must to go. You will for sure need a bit more inside volume to put your ice, which should be be near the cooling plate (evaporator).
efficiency always recommended
Then the fridge itself should be efficient, this means well insulated, because losses are continuous through the fridge walls. Insulation is what will give you more autonomy. Freezers with thick walls can be used. Best are chest fridges: you don't loose all the cold air inside every time you open them.
Big problem: driving the compressor
1-You can go for a DC compressor which can be run direct from battery (tried directly from PV: not recommended if PV is limited). There are some models of these ones: Steca, Sundanzer, and others. It is the best solution, somehow expensive. I bought such a fridge when I began to go offgrid: 160l inside volume, 20l of ice storage, it is still working well, and gives me 4 days of autonomy. Instead of buying a whole fridge, you could also replace the compressor with a DC one. You can find some kits with evaporator which supposedly can be mounted easily (for boats, vans,..), I don't know anyone who has done this.
2-Usual fridges work with the AC grid, and have a strong inrush current at start. If you want them to be run from an inverter, the inverter power will need to be oversized, could be by a 10 factor if not more. A bigger inverter means also bigger no load current: more energy just to run it for nothing . So if you want to limit these losses (and don't need the inverter for something else), you have to shut it down when you don't want the compressor to run.
An usual compressor is just a motor, so it can be run from a cheap modified sine inverter, as long as there are no electronics to feed (thermostat for example).
Putting all together
If you have your PV system: panels, charge controller, inverter, and the fridge with some ice storage, you still need a control system to: shutdown the fridge at night, start it when we want to, and stop it when all the ice has been build, or to protect the battery. This needs connection to :
- the charge controller (is the PV giving energy?, what is the battery charge status?)
- the inverter you want to control
- the fridge to get the temperature or thermostat status
and, of course a bit of logic which can be done for example with a little microcontroller arduino style.
Next post about my last (test) build, a bit extreme, but already working.
But there is one very interesting point: generally, the more sun you have, the more your fridge will need to work. So PV production is quite related to fridge energy needs. This is why I think that cooling is one of the best PV uses.
need storage
You don't have sun at night, so you need some sort of storage which could provide energy for this time, or more. Battery storage is the first one you can imagine. But it is expensive. Much simpler is ice storage: it's very cheap, quite effective, and never dies like a battery. Instead of storing electricity with chemical/electric losses from the battery itself, you just store cold in form of ice. If your fridge compressor is well dimensioned, it doesn't run all the time (perhaps 30%-50%, depends on many variables), so there is plenty of unused time left to freeze water. Just to show how effective it is: melting 1kg of ice gives 90Wh of cooling energy, which you could obtain with 60wh feed into the compressor (assuming Cop = 1.5 to achieve such temperature). Using a lead battery (45Wh/kg) with a conservative 30% discharge, you would need a 4kg battery!. So storing in form of ice is a must to go. You will for sure need a bit more inside volume to put your ice, which should be be near the cooling plate (evaporator).
efficiency always recommended
Then the fridge itself should be efficient, this means well insulated, because losses are continuous through the fridge walls. Insulation is what will give you more autonomy. Freezers with thick walls can be used. Best are chest fridges: you don't loose all the cold air inside every time you open them.
Big problem: driving the compressor
1-You can go for a DC compressor which can be run direct from battery (tried directly from PV: not recommended if PV is limited). There are some models of these ones: Steca, Sundanzer, and others. It is the best solution, somehow expensive. I bought such a fridge when I began to go offgrid: 160l inside volume, 20l of ice storage, it is still working well, and gives me 4 days of autonomy. Instead of buying a whole fridge, you could also replace the compressor with a DC one. You can find some kits with evaporator which supposedly can be mounted easily (for boats, vans,..), I don't know anyone who has done this.
2-Usual fridges work with the AC grid, and have a strong inrush current at start. If you want them to be run from an inverter, the inverter power will need to be oversized, could be by a 10 factor if not more. A bigger inverter means also bigger no load current: more energy just to run it for nothing . So if you want to limit these losses (and don't need the inverter for something else), you have to shut it down when you don't want the compressor to run.
An usual compressor is just a motor, so it can be run from a cheap modified sine inverter, as long as there are no electronics to feed (thermostat for example).
Putting all together
If you have your PV system: panels, charge controller, inverter, and the fridge with some ice storage, you still need a control system to: shutdown the fridge at night, start it when we want to, and stop it when all the ice has been build, or to protect the battery. This needs connection to :
- the charge controller (is the PV giving energy?, what is the battery charge status?)
- the inverter you want to control
- the fridge to get the temperature or thermostat status
and, of course a bit of logic which can be done for example with a little microcontroller arduino style.
Next post about my last (test) build, a bit extreme, but already working.
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