Residential Solar Relieving Peak Demand

So this would operate effectively in the spring and fall? Here there just isn't enough "cool" at night in the summer. When it is cool at night I open windows. I would like to have a whole house fan, but they are only useful a few months of the year and the rest of the time they leak air and heat like a sieve.

I don't follow. How do you use this temperature range without a heat pump? Even then phase change gives you this more heat than the numbers you used above which only provide 15C for cooling and 25C for heating. The latent heat stored in coconut oil is equivalent to a 26C change in water and without the inefficiency of pushing the heat against the thermal gradient.

No here other than spring and fall. Most days in winter stay below 40F and summer nights often don't go much below 70. In the US southwest where the humidity is low, they use evaporative cooling. Elsewhere it is A/C or they pay a lot for heating.

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Rick C

Thanks for doing the numbers. This is exactly what I suggested to rickman.

The economics of energy are different now, at least in most places.

a bunch of water.

backyard, it's a go, otherwise you lose.

That's already been mostly done here in Australia, since the 1980s.

The main cost of this approach is not the storage or pumping, but the reticulation to the rooms. Only a few luxury houses install hydronic heating.

Sounds great in theory, but my heat pump doesn't cool to 0C. You are talking about an enormous freezer, more expensive and less efficient than a heat pump which directly cools the air.

How about that, I DO have a rather large lake in my back yard. The part I am on is more than twice the size of the part used to cool a pair of 900 MW nuclear reactors.

I'm not allowed to use the water for irrigation, but they might allow me to use it for cooling if I can show it won't do harm to the environment. They were told they couldn't build another reactor without meeting the existing environmental regulations on public waterways, so they might make me prove I won't kill any fish.

Even a moderate sized radiator would carry a *lot* of heat in contact with even slowly moving water. On the other hand, anything in the lake gets quite covered with slime and muck, all manner of life growing on it. I was swimming in the lake the other day and noticed the thermocline is about 5 feet down. I just need to be below that to have nice cool water in the summer. In the winter it may provide a good heat sink for the heat pump, when the air is below freezing anyway. I'd love to find a way to keep the electric coils off.

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Rick C

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common on net zero solar homes. Some are built on small lots in high densit y housing 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 southern side. You build a wind wall to bunch the westerly winds on the wes tern edge of your house to create a low pressure region on your southern s ide. Then air from the cool northern side is forced through your north wind ows, through the house, and out the southern side. This age old technique u sing wind walls has been around forever. In modern times it can be suppleme nted by mechanical ventilation for times there is no wind.

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The water based geothermal uses plastic flexible pipe, a few hundred feet p er ton capacity, they just spiral wind in a trench, turns not touching, and fill it in. That operation with parts and material a few thou maximum, rel atively inexpensive. They do similar for the solar chimney cooling. If you pay 20k for a septic field, you must have 20 toilets in your house.

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n the 1980s), and for residential applications of even modest floor space i t requires a bunch of water. The equipment is large and expensive. Now if h e has a deep lake in his backyard, it's a go, otherwise you lose. He would be better off spending that money on insulation, do away with heat in-/ex-f iltration altogether, and reduce the size of his heatpump.

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you probably don't need to get into the water, plastic hose buried in wet soil should be good enough that's what is used for geothermal here

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That must be why they guy wouldn't even quote a figure for the geothermal system, because it was so cheap. The trench would be many feet down at the top of the coils. I expect they would need to be well spaced. What is the point of laying coils close to each other if there isn't water circulating past them? They would be warming/cooling the same bit of soil. Also, like with your septic system, you are assuming ideal conditions. I am on a piece of rock with trees covering everywhere there isn't currently road or septic or well. I know because I've spent time digging post holes through it.

I may be remembering the figure the septic guy through out there and it was just a guesstimate, but it was in that ballpark.

If geothermal is so cheap to put in, why do so few use it? I believe it also qualifies for incentives.

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Rick C

Lasse Langwadt Christensen wrote on 7/18/2017 7:27 PM:

Do you think that machine will work well here?

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Rick C

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People who live in areas of substandard housing rarely buy SEER 40+ systems .

That isn't geothermal unless you happen to live in Yellowstone or Iceland it is basic ground source heat pump technology - which also works quite well in lakes too if you have a big enough one. (a lot less hassle sinking a pipe in a deep lake too)

Snag is that even if you fit the profile perfectly for it being advantageous (eg. retired so wanting continuous heating) by the time you include all the maintenance costs it is borderline advantage.

I know a few such installations and on paper they should be good but in practice they are a PITA.

The method works by exploiting the ground temperature being more or less constant if you go down a metre or more in the soil. Might not work at all in permafrost territory though. Air source heat pump works OK in a continental climate but quickly ices up in a cool humid UK winter.

That isn't geothermal. Geothermal in most areas involves a deep well down to truly hot rocks and water coming back that is hot springs quality. Bath in the UK has natural hot springs as does Buxton.

Yellowstone is about the only place in the USA where shallow buried pipes would be yielding a decent amount of geothermal energy .

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Regards, 
Martin Brown

The two main options for ground geothermal are slit trenches and wells. Slit trenches work best in new construction with wide open fields that haven't been landscaped yet. The higher the water table the better. Wells work best in retrofits and crowded landscapes like you have. From reading, I believe that 2-300' wells are most common, with multiple wells as needed spaced maybe 5-10' apart. Cap one end of 4 or 6" tubing, extend to the bottom of the well, fill with water to hold it down, inject grout around the exterior to ensure good thermal contact, insert 2 or 3" tubing to the bottom, use a fancy through-tee at the top, and voila, a u-tube. There was a company called Kelix that made inner tubing with exterior fins for turbulence that claimed a factor of 2 or 3 improvement in efficiency (so 1/2 or 1/3 the number of wells), but their web site seems gone.

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Regards, 
Carl Ijames

The two main options for ground geothermal are slit trenches and wells. Slit trenches work best in new construction with wide open fields that haven't been landscaped yet. The higher the water table the better. Wells work best in retrofits and crowded landscapes like you have. From reading, I believe that 2-300' wells are most common, with multiple wells as needed spaced maybe 5-10' apart. Cap one end of 4 or 6" tubing, extend to the bottom of the well, fill with water to hold it down, inject grout around the exterior to ensure good thermal contact, insert 2 or 3" tubing to the bottom, use a fancy through-tee at the top, and voila, a u-tube. There was a company called Kelix that made inner tubing with exterior fins for turbulence that claimed a factor of 2 or 3 improvement in efficiency (so 1/2 or 1/3 the number of wells), but their web site seems gone. ============================================

Oops, Martin is right, I called this geothermal but it's ground source not geothermal. Need more Diet Coke, sigh.

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Regards, 
Carl Ijames

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They call that geothermal in the U.S. It simply means the heat exchange is between the heat transfer fluid and the ground. The other more common techn ology is called air exchange. It does not have to be in water, and actually the most common installation is in soil. And the most common geothermal is a dual exchange system wherein they exchange heat between conventional ref rigerant and an antifreeze solution, and then use a pump to circulate the a ntifreeze through the ground loop of polyethylene piping.

rickman wrote on 7/17/2017 7:46 PM:

I found the usage pattern wasn't right. It was cutting back before 3 PM as I expected as I have the trip point set for 2:45. But it was increasing usage before 6 PM even though I have the set time for 6:15. I finally downloaded the installation manual for my thermostat and found a setting called "Adaptive Intelligent Recovery?" which "starts early so setpoint is reached by start of program period". Once I disabled that the system seems to work the way I intended. Once I can view today's data I'll be able to see for sure.

I just wish they would give you a break on the rate by cutting back during peak hours. They have a rate plan for that, but the hours are longer at 3 to 7 PM and the cost for any usage during that time is $0.43023 or *TEN* times the rate of off peak. The off peak rates drop from $0.07757 to $0.043 or about $0.034. That doesn't seem worth it really. I'd have to unplug the fridge and turn off the well pump!

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Rick C

Why not use a heat pump ?

The smaller the temperature different required, the better the COP (Coefficient of performance) and hence the lower the electric consumption (theoretically COP Thot/Tdif) in which Thot and Tdif are in Kelvins).

It seems hard to find actual COP figures for less than 15 C Tdif, but then more than COP=5 is quoted. While keeping the indoor temperature at 20 C, you can let the water temperature rise to 40 C before COP drops below 5 and 60 C when COP drops below 3.

At night when the outdoor temperature drops, much can be directly dumped out, until the water temperature approaches outdoor temperature. If the water system is open and evaporation is allowed (thus require daily water additions), the water temperature could approach the dew point.

If free or cheap electricity is available in the early morning, use the heat pump to cool the water to 15-20 C below outdoor morning temperatures.

This then would allow a 30 to 80 C water temperature swing from noon to evening, thus reducing the size of the water storage.

Of course, the same water storage can be used for storing warm day temperatures to be used for cold nights.

With phase change material the delta temperature is much, much less, single digit degrees. That makes the heat pump much more efficient or eliminates the need altogether. The idea of using water for thermal storage by cooling at night is absurd. That would be the case where you need 40,000 liters of water for storage since the delta temperature would be a handful of degrees if any at all.

If you want to use a heat pump with water, as others have pointed out you are much better off using geothermal.

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Rick C

I did some calculations on the Tesla Powerwall and based on sucking 12 kWHr from the utility at off peak ($0.04/kWHr) and selling it back at peak summer rates ($0.43/kWHr) and peak winter rates ($0.12/kWHr) it would pay for itself in a bit over 10 years including installation. That's longer than the 10 year warranty and depends on the economics remaining constant for the next 10 years. It also depends on the utility giving me full credit for details like the distribution charge and considering time of use for both usage and generation.

I probably shouldn't bother with worrying about the utility's problem of peak power costs. So far I haven't found much supporting evidence that indicates they have a problem. It would pay me much more to figure out a way to reduce my winter power usage on the very cold nights.

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Rick C

way to reduce

During heating season our thermostat is set at 63 in the daytime and 58 at night. An extra quilt is a lot less hassle than an extra spouse (or so I'm told). ;)

Cheers

Phil Hobbs

Get a fat wife. They were more popular during the middle ages since they were more likely to survive starvation diets in the winter. They were also much warmer.

We did set the thermostat about the same when we lived in VT. On really cold nights I didn't let the night setting kick in (frozen pipes aren't worth a few cents in heating bills) but I often fired up the wood stove (which exacerbates the frozen pipe problem), costing even more (wood is expensive, either in labor, money, or both).

Since I moved South, I don't set back the thermostat more than a couple of degrees and that's not for energy savings, rather comfort. The heat pump doesn't recover from a deep setback, either Winter or Summer.