Geo and Solar

scott markle_2

What I understand about geo is that by sending chilled water through a ground loop we can "extract" heat from a 55 deg. source. Heat moves from one place to another based on differential of source and sink.

Seems to me that this same principal could be used to improve the efficiency of a solar collector. What if we could send 45 deg. to a collector instead of the 80-90 return temperatures possible from a radiant system?

What I don't understand is the operating permitters of the heat-pump. If you could boost the temperature of the water from the field by another 10 degrees (with solar) would this boost be efficiently utilized by the heat pump?

Has anybody looked at this approach? What are obstacles?

Instead of boosting the temperature going into (or out of) the field why not bypass the field and send the chilled water directly to the collectors when solar resources are present. This could give the field some down time and keep it's capacity (efficiency) up.

Derrick Ellefson

Solar and Geo

What is the best way to intagrate solar and geothermal? I heard a couple of ways to do it.

1) Have the solar heat the ground where the geo loops are
2) Have the solar supplement a double coil tank.

any suggjestions? thanks

MPF

3.

Install hyrdronic radiant floor heat and have the solar heat the floor loop(s)directly.

Derrick Ellefson

The entire house will be radiant. 5300 ft with supply temps around 110F

hot_rod

Cover the loads

DHW, heating, then storage in tanks.

First a load calc for the building

Next a solar design calc via one of the solar software to access your areas solar potential and output.

Determine what % of the load you want to cover with the solar.

Run an economics calc on the proposed system, also. Most software will include this program.

Design the heating emitters to run at the lowest possible temperature.

hr

klaus

Option #2

Have the solar supplement a double coil tank.

Then you can use this tank to pre heat your domestic as well as add heat to your buffer tank off your GSHP.

MPF

I'd flip it,,,

I'd make the house the primary load and heating the DHW secondary in this case. With the radiant floors requiring only 110* supply it seems more practical/ economical to me to use the solar to heat them directly and if there's any heat left over send it to the DHW. I'm also a big fan of creating some thermal mass to "store" heat during the day when the solar is producing and then allow it to release into the house at night, either actively or passively. Usually I shoot for heating the basement slab as a means to achieve this. The biggest deal (I think)is how to prevent the solar rack from overheating in the summer. Maybe it's me but using the solar to heat DHW instead of heating the house with it first just don't seem right.

hot_rod

the design number

will guide you.

Large heating arrays with no work to do 4, maybe 6 months of the year need to be looked at carefully with the economic software.

Unless money is no object and "being green" is the driving force

I don't know the location, loads, budget, etc so it's hard to guesstimate the best design or application.

hr

Derrick Ellefson

Well what I was thinking was use the solar to raise the ground temp around the geo loops. I has always thought the greater the soil temp the greater the capacity for geo. There is no cooing going in this house so I also could use that as heat dump in the summer. Does this make any sense or am I thinking out loud again.

MPF

Just an opinion now,,,

but if I was going to go that way I'd be more inclined to use the solar to raise the temp of the water coming out of the ground before it reaches the geo unit. Using it to heat the ground seems like maybe a portion of it will be lost? With the correct piping arrangement you could still use it as a dump in the summer.

Derrick Ellefson

What way do you feel is more efficent. Use both systems to heat the same source, or use one to help the other?

MPF

Agreed,,,

To keep the $ somewhat reasonable it may be better to consider solar as only providing a portion of the load. I believe the cost of an install where it is only used for DHW, as some mfg suggest, is cost prohibitive, especially if you start figuring in the payback time. My rough calcs on a job I looked at gave me 16+ years before the initial extra outlay was covered. Kinda hard to sell This is why I lean toward heating the low supply temp radiant house with it first, before DHW, I'm thinking you'd realize a much faster pace on the payback.

MPF

I just think,,,

you'll experience some energy loss by sending it to the ground, some of it will bleed away. If you feed the geo side directly with it there will be minimal loss and the warmer the water entering the geo side the more efficiently it will operate, providing you don't exceed any maximum temp parameters that may exist. If you have a desuperheater circuit on the Geo unit you can also make your DHW that way which will incorporate the solar into providing some of the energy required. Now you're using solar to accomplish both tasks.

MPF

But I still think,,,

heating the floor loops directly with the solar is the best way. Now, if the solar is keeping up, there is no need to run the geo compressor. Your way requires that you always run the compressor and ground loop pump(s)to heat.

hot_rod

Water is a better conductor than dirt

And easier to insulate to manage the BTU storage. I think it would be better to store some, if any excess, in tanks inside the building. If you chose indirect or solar storage tanks, maybe dual coil, you could get DHW, storage, solar input, and radiant off of one tank.

hot_rod

This is why I'm leery of rule of thumb

and guestimate solar design. Computers and solar design programs take most all of the guess work out. They allow for easy "what if" models to be calculated. Economics including interest rates, ROI, etc can all be spelled out.

www.retscreen.com is a free downloadable load calc program. F-Chart, Maui Solar, T-sol, PolySun are all easy to use design programs.

The stars, and the sun, may lie but the numbers never do.

hr

Derrick Ellefson

Thanks for the great answers

MPF

Thanks HR...

for the list of programs!

[Deleted User]

I've looked at it...

and yes, any time you can increase the supply temps from the loop to the GSHP, the C.O.P. does increase. If the loop field is also being used to reject heat, dumping solar heat could be problematic by causing a drop in cooling C.O.P's, but if the loop is used for heating only, why not send EXCESS solar btu's to the field for (hopeful) storage. If you have a shallow aquifer that is dynamic and not static, it would probably wick away any heat you send to the field, but from the stand point of avoiding collector stagnation, it makes a LOT of sense.

This is exactly what I will be doing with my intentionally oversized solar DHW system. Once I am done filling my in house storage tank with energy, if there is still significant energy available, I will dump it into the well I drilled in my back yard for that sepecific purpose, and keep my collectors from stagnating, and causing consequential thermal degradation of the glycnoids.

My "dry well" is around 12' deep with a 2" copper pipe with a 3/4" dip tube going all the way to the bottom of the 2" pipe, and the 2" pipe is grouted into the hole for good thermal contact. It works like a champ for dumping heat.

Unfortunately, the data logging I was doing failed, so all data was lost, and I couldn't determine if I was building a "heat bubble" during the summer months.

ME

ME

Derrick Ellefson

Thats pretty much what I was thinking. This will be heat only geo. How do you determine how much tube to use for the solar and how does that get installed with the geo loops. Is there any kind of design data for this?

Derrick Ellefson

How about use cogen technology? Using the heat generated by the motor the heat the ground around the geo. Generating power for the home and geo compressor?

Metro Man

Gee-O-Solar

Personally I would keep systems separate. Use one large solar tank that is sized correctly. Maybe a sidearm off of it for higher temp sdhw production.

Thinks its a waste of energy to "dump" into ground. Insulated water storage is best thing going.

If you can get your 500 gallon + tank to 150*F or more using 750 watts (approx. pump power for 5hrs)..... doesn't get too much more efficient than that.

my 2c

Metro Man

scott markle_2

All theory I concede, but consider this.

A collectors efficiency is determined by the delta of ambient air and return from attached radiation.

Since we like to condition to 68 this puts our lowest possible conventional solar heating return temperatures somewhere above that temperature. WW heat pumps have ground loop supply temps 35 degrees lower than that.

Probably over estimating the gains (no math) but it seems intuitive to me that chilled water could do a lot to elevate the number of btu's we could pull from a winter sky. If we could double the collectors winter efficiency we could halve the required collector area. When solar is not available (night, cloudy etc.) we could use the geo field, which also could be sized smaller because of the lighter load. This ability to switch sources would also eliminate the need for the storage generally required of high fraction solar systems. In the summer the smaller collector area would be better matched to DHW, it might not need a heat dump.

Why heat the field? just bypass it or solar supplement it on the way out.

Even if we supplemented on the way into the field a 20 degree rise from the solar would be very impressive. This means that we would still be going into the field below 55 deg. ground temperature. So regardless of whether the aquifer was static or not we would not be loosing heat, only reducing what we can pull, or warming what we have previously chilled. "Loosing" heat to soil we have already chilled is different from loosing it to the ambient mass of the earth. I'm not sure this would be a net loss, seems to me to be a legitimate way of "losslessly" banking some energy. It's like a storage tank in a room that is hotter than the contents of the tank, how can we loose?

High solar fraction heating is hard to justify economically. An array sitting largely idle (or worse yet dumping) is inefficient. Inefficient in the sense that when the most solar insolation is available we aren't' using it. All the low entropy gear involved (copper and glass, pumps, valves, etc.) has a carbon dept. - It takes lots of old fashion fossil energy to pull ore from the earth and turn it into plumbing. Before any alternative energy system produces a net positive alternative btu. it must first pay back it's "cost" of manufacture. That is why the efficiency of alternative systems is every bit as important as conventional sources of energy. Sunlight is free- collecting it not.

Notwithstanding The KISS principle, there's Probably some obvious mathematical reasons why direct use of heat pumps and solar doesn't make sense.

Metro Man

E-Fish-Aunt

I would look more at "system efficiency" rather than just collector eff.

The amount of power needed to drive a solar heating system is very low in comparison to pretty much anything else out there other than passive or NU-CLAR. With variable speed pumping technology these numbers continue to drop.

What is power consumption of that ever-present compressor compared to solar or conventional?

Pay back and rate of return.... I still think (at least in Colorado) a properly designed solar thermal system will out shine geo.


Metro Man

klaus

Solar storage tank

Where do you find tanks like that MM?

[Deleted User]

he BUILDS Them....

Hence, his better idea :-)

ME