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Hi,
in the coming summer (first summer with solar for me) I would like to make "cold" during the day and "release" it at night so not an air conditioner that would work directly on the room whilst running, more of an ice-maker that stores up coldness to re-melt later.
Does this kind of thing exist or would I have to invent something?
Apologies if this is not the place for this sort of question.
You might get somewhere by using the guts of a deep freeze to freeze phase changing materials during the day and then blowing the room air through it at night.
I'm moving this to modifications. Better there, even if it's completely out of scope for the community.
Temperature and energy are only proportional when the phase state is the same.
Your energy storage answer lies in storing the latent energy involved with the phase transition from a solid to a liquid.
If I recall correctly, changing a litre of ice at 0 deg C to a litre of water a 0 deg C takes about as much energy as heating a litre of water from 0 deg C to 80 deg C. ( I might be a bit off here).
Anyway, the point is that a massive amount of energy can be stored and redeemed not by making something colder but by supplying and removing the energy needed to change its state.
Energy transfer between components is more effective the more extreme the temperature difference. This will affect the design because the "cold" must be insulated from ambient energy transfer and yet have enough of a temperature differential to cool the intended medium.
Think this is an excellent question - finding new ways to maximise self-consumption seems very relevant to developing an ESS.
First summer with solar for me too and today's yield of 14kWh on a not especially sunny day is surprising (currently 3.4kWp of arrays and more planned). Our solar noon is 39 deg altitude today, 60 deg in June (UK Midlands). So, finding ways of making use of excess solar (given no export incentive) is going to be more pressing.
My first thought is get your house up to Passivhaus standard of insulation, air tightness, cold-bridging and - the key part - lots and lots of thermal mass. The house will then have a very slow temperature-change characteristic and thus running aircon in the day is appropriate - the house will not heat up appreciably at night. Obvs, not a short term solution.
What makes this even more attractive is using a 48V DC aircon unit (widely available in the US) - thus no inverter losses. Anyone have any experience of using these in a Victron ESS system, i.e. running the unit directly off the DC side of the system?
This way you would have the best kind of control... It wouldn't be fun to have a sudden temperature drop and no stopping that "stored" cold you are talking about.
I have lots of thermal mass though miles away from passivhaus standards but it's the nighttimes that can be unpleasant. I suppose an aircon unit would cool the house in the day so there would be something to absorb the heat during the late evening.
In my ideal world the stored cold could be saved for tomorrow. It would be like a freezer that freezes a quantity of ice for example. If at night it was unpleasantly hot you would just open the freezer door. If not hot, leave it closed. Obviously I don't want freezers in bedrooms and puddles all over the floor every morning!
I’ve heard about or seen a commercial solution for this a long time ago. But can’t remember what or where.
Don’t have the time now but first thing I’d do is spend some serious time on google for this.
Few results:
https://www.thuleenergystorage.com/">https://www.thuleenergystorage.com/
https://www.greentechmedia.com/amp/article/how-sun-power-and-ice-energy-can-play-together">https://www.greentechmedia.com/amp/article/how-sun-power-and-ice-energy-can-play-together
https://www.sciencedirect.com/science/article/pii/S2590174522001428">https://www.sciencedirect.com/science/article/pii/S2590174522001428
A possible alternative?
An efficient way to store cold is to use water with antifreeze, cooled down with a refrigeration compressor when there is a surplus of solar energy.
If you cool the water down to -35C during the day and 'take out the cold' at night so the temperature rises to 0C, it can remove approx. 40 kwh of heat per m2 of water/coolant.
This is not an exact calculation, just something I remember on the fly as use on board fishing vessels for cooling fish in water tank.
There are heatpumps (not aircons, although inverters are technically the same technology) that are able to operate also in reverse, so you could use it for (floor / wall) heating and cooling.
Regarding a cold store, I remember that some heatpump architectures use an ice storage as environmental energy source, to draw/and store energy.
I'm using a solution for a similar use case: Cooling down a fridge/freezer when there is plenty of solar power and using the stored cold otherwise. To facilitate that I use standard freezer packs which are simple, easy to handle and available in various sizes. Alas some of them are inappropriately plugged. My first attempt was a standard fridge with a freezer compartment but too inefficient. Next step was a freezer half loaded with freezer packs which was able to keep the temperature just above 0°C overnight.
Finally I moved to a current portable freezer where I lined the walls with freezer packs. This implementation can keep it's content in the lower part frozen for up to three days without any power and it can be powered from 12 V DC directly, reducing it's energy consumption in a PV environment.
If you just want cool air at night you could fully pack the box with freezer packs - or use appropriate canisters filled with water and an antifreeze agent[1] to lower the melting point - and open the lid at night, eventually adding a ventilator.
[1] When making DIY freeze packs be sure not to add to much antifreeze agent so that the water still freezes. The phase change from fluid to solid is what makes freezer packs efficient.
All interesting ideas for me to explore.
I'm surprised though, that the freezer could only keep just above 0°c over night. When there is a power cut "they" tell you your freezer will be fine for 24 hours.
check its seal (if its clean and not defective, still soft enough to close firmly over the body) and if its standing firm and flat/level on all its stands/feet. Also a trunk style freezer (with the door on top) is less prone to the effect, than a normal, fridge style.
Place/operating it in a cooler place helps as well. If in your normal kitchen/home area, make sure you do not position it over a spot where heating from below, like from regular floor heating or from where hot water supply pipes to your sink or bathtub are running under it.
The point here is the freezer was only powered when there was enough solar power determined by the battery voltage had risen above 13.6 V and not dropped below about 13 V. Thus it was on for a relatively short time only. If powered from mains and there is a power cut you can assume it had been on for more than a day and the contents properly frozen at -18 °C. That's a difference.
It's similar with my current mobile cooling/frreezer box. It will be able to keep its contents frozen for three days only if there was enough solar power to cool it down to about -16 °C over a couple of days before. Yet the mobile box is isolated much better and the design with a lid on top much better suited to keep the cold inside than a standard freezer with a vertical front door. Especially the mobile box is cooled on all sides and I could add freezer packs to all walls.
