Residential Solar Relieving Peak Demand

My understanding is the peak usage period each day in the summer is about 3 to 6 PM. A few days ago the forecast was for 95F+ for a couple of days. The local electrical utility cooperative sent an email asking members to reduce power consumption between 3 and 6. I complied by cutting off my AC during that period. Before I cut it off I lowered the thermostat a couple degrees so the house would cool down a bit and become a thermal storage tank. It seemed to work pretty well as the temperature inside the house only reached 79 or 80 each day by the time the A/C was turned back on after

  1. I decided to use the programming feature on the thermostat to do this every day.

My utility has an hourly usage view mode and I see the power consumption pattern shift dramatically drawing peak usage earlier in the day when the A/C lowers the set point until 3 PM when the set point is raised to 82F using 2.5 to 3 kW. Then the usage drops to a fraction of a kW until the thermostat is lowered to 77F after 6 PM where it rises to 3 kW for one or two hours until the outside temp drops and power usage drops accordingly.

I don't know yet if this is costing me money or saving me money. There currently is no time factor on the metering currently. The distribution charge by the utility is lower after a base amount of 300 kWHr each month, so the incremental rate is lower than the average rate. Once I have more data I will add up the costs each day and see if I can tell any difference.

But what is notable is the peak demand shift from 3-6 to the periods before and after that period. If the utility is asking us to cut back on demand, it must be important to them. My utility is a coop and so is not motivated by profit. This could be extended by using thermal storage of some sort. I have read about incorporating beads of coconut oil or a particular type of paraffin into wallboard as a phase change material. Coconut oil melts over a range of 74 to 80 degrees (approx) and so would be good for this. There are paraffins which melt in this range as well. During off peak hours the cool air can be passed through a tank of these materials for thermal storage, then when cooling is needed the A/C is left off and the warm air looses heat to the phase change material as it melts. I've not been able to find good info on the latent heat of melting for either of these materials. The coconut oil would be the least expensive, but there is little good data on it. So I can't estimate how large a tank would be needed.

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Rick C
Reply to
rickman
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DoE has been experimenting with this concept for some years now.

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National Gypsum has a drywall product that was used in DoE trials, not sure if it's for sale yet:
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These are mainly for solar homes to moderate the internal temperature swings. But there's no reason it could not be used in a situation like yours.

Reply to
bloggs.fredbloggs.fred

Kinda like streamlining cars. Any car will benefit from using it, but they only seem to apply it optimally to the ones that are otherwise designed for high fuel efficiency or just plain go fast, or in the case of Tesla - BOTH.

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Rick C
Reply to
rickman

I often thought about 2 rainwater tanks, one inside the other, with some good insulator between the 2. Just pump water in from a radiator ( cooler ) at night, and out to the house in the day. In winter, reverse the process.

Havent got around to sizing yet. One of those sometime projects.

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Regards, 

Adrian Jansen
Reply to
Adrian Jansen

If you already have hydronic heating, why not? But it's expensive to fit during a build, and more expensive to retrofit.

Clifford Heath

Reply to
Clifford Heath

What does the 2nd tank do?

NT

Reply to
tabbypurr

Water does not have a terrific heat capacity, you will require tons of it. Phase change is the answer for high density storage, the big problem is all of the candidate materials are hazmat.

Reply to
bloggs.fredbloggs.fred

That very much depends on where you are.

For the UK, see

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If you want to look at days in the winter, you can download .xls files with the raw data used for those graphs.

Reply to
Tom Gardner

You can shave quite a bit off your mid afternoon cooling requirement by modifying the house to slow ingress of heat. How you do that is up to you but I found the relatively thin insulator with foil on the inside of sunward surfaces to be very affective at reducing thermal load. My own house in the UK has so much thermal inertia that it stays cool during the day simply by having left upper windows open at night. Aircon load in the UK is virtually non-existent our peak summer load is very flat:

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It is a bit spikier in winter when heating is necessary.

You would probably use less electricity by tolerating a slightly warmer set point all through the day than by super cooling and switching off for peak. Shading the sunward side of the building with vegetation would make a huge difference (eg ivy or virginia creeper).

A smart meter that will show you consumption in realtime and daily usage is quite handy for this sort of thing.

Latent heat stores are a bit of a mugs game. Anything that works well enough eventually degrades or corrodes its container in such a way as to either stop working or leak out. Big rocks or scrap iron in a huge insulated pit in the ground is probably as good as it gets.

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Regards, 
Martin Brown
Reply to
Martin Brown

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That has been found to do a number on the exterior cladding/roofing so as t o accelerate their deterioration.

Mass of any kind in new construction is prohibitively expensive unless you do something like use dirt. Even most new brick homes in U.S. only use bric k on the front wall, use wainscot, or thin brick cladding to give the appea rance of brick. Heavier materials require heavier support framing, so they' re avoided.

Reply to
bloggs.fredbloggs.fred

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A solar chimney is a distinct and economical possibility that works well fo r solar cooling (oxymoronic I know) when designed properly. You draw outsid e air in through an underground cooling network and ventilate it out the to p.

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Reply to
bloggs.fredbloggs.fred

Water has a very high specific heat of over 4 kJ/kg/K, thus with a 30 C temperature change 120 kJ/kg can be stored.

For heating applications in cold climate with cheap night time electricity about 200 kJ/kg with 50 C temperature change can be stored for the day. A typical well insulated storage tank would be 1000 liters, so 200 MJ could be stored or about 60 kWh. With 16 hours of expensive electricity, about 4 kW could be extracted every hour. This is sufficient in well isolated houses.

For cooling applications, the heat pump must have a cold sink into which dump the accumulated heat. If the cold sink is the ambient air at +30 or even +45 C, the heat pump must work very hard to get rid of the heat, consuming a lot of electricity. In addition the heat transfer from internal plumbing to ambient air is not very efficient, so the tube temperature must be well above ambient air temperature.

However, if the heat can be dumped to cold water, the electric consumption drops significantly. Also the heat transfer from internal tubing to water is more efficient and the pipes can be at nearly water temperature.

If you can cool the water to 0 C during the morning, you could dump 30 kWh heat into a 1000 liter tank, before it reaches 25 C.

The main problem with water is the limited temperature range, at least in an unpressurized system.

However, if you make snow or ice cubes in the morning, melting it requires 333 kJ/kg, which is equivalent of heating water from 0 C to

+80 C. 200-300 kg of ice in the morning would significantly help the air conditioning during the day.
Reply to
upsidedown

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all of the candidate materials are hazmat.

Just about everything you mention is in use or has been tried (mostly in th e 1980s), and for residential applications of even modest floor space it re quires a bunch of water. The equipment is large and expensive. Now if he ha s a deep lake in his backyard, it's a go, otherwise you lose. He would be b etter off spending that money on insulation, do away with heat in-/ex-filtr ation altogether, and reduce the size of his heatpump.

Reply to
bloggs.fredbloggs.fred

"Underground cooling network"? Sounds expensive unless you live next to a cavern. Last time I replaced a heat pump I talked to a geothermal vendor. They were talking about some 4 or 5x cost factor and didn't quote prices because the risk of what they find underground is all yours.

I have thought about using water cooling, but there are a lot of issues there. My well is likely supplied by water moving through the soil from the lake or if not that, the supply rate is huge. So I think I could use that for a heat sink. It would use a lot less water if done evaporatively, but that creates other problems.

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Rick C
Reply to
rickman

It can help offset the load, but here at least, in the summer outside temps don't get all that low at night.

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Rick C
Reply to
rickman

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This is not a mechanical geothermal. The solar chimney cooling is very comm on on net zero solar homes. Some are built on small lots in high density ho using neighborhoods (LosAngeles). I think they just bury a long coil of 4-6 " poly pipe about 6' down. You can also do the same if the location of your house is favorable. Say a cool wooded northern side and fully exposed sout hern side. You build a wind wall to bunch the westerly winds on the western edge of your house to create a low pressure region on your southern side. Then air from the cool northern side is forced through your north windows, through the house, and out the southern side. This age old technique using wind walls has been around forever. In modern times it can be supplemented by mechanical ventilation for times there is no wind.

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Reply to
bloggs.fredbloggs.fred

Yes, a hundred years ago when there was no refrigeration, people did all manner of things to feel even the slightest bit cooler. One of the things they would do was travel. They would move out of the swamp called Washington, D.C. and spend the summer in Braddock Heights, MD among other places. Those who had real money would travel much further to Canada and other points.

Pipes in the ground don't sound inexpensive to me and wind walls don't sound effective on days with max temperatures of 95F. Actually, I have a set of pipes some feet down in the ground. That's my septic system and replacing it would cost some $20,000 today.

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Rick C
Reply to
rickman

Just a container to hold insulating material around the storage tank. Any suitable structure would do.

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Regards, 

Adrian Jansen
Reply to
Adrian Jansen

I was thinking more like 5000 or 10,000 litre tank, and using the cool air at night to cool the water. No heat pump needed, just two circulating pumps, one tank > outside radiator, the other house > tank.

Temp range for water is of course 0 - 100 C, but even 10 - 50 C gives a lot of Kj to play with. Thats more than most of the phase change systems can use.

Of course you need day/nightime temperatures in a reasonable range compared to your house operating range, but certainly for here ( Queensland, Australia ), we have that.

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Regards, 

Adrian Jansen
Reply to
Adrian Jansen

Build a shed around the tank and when you receive a shipment in styrofoam dump the beads or blocks inside. I wonder how long it would take to fill the building around the tank.

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Rick C
Reply to
rickman

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