Geothermal, Solar and Electric Elements for Domestic Hot Water

Phelps_Clarke · February 2011

I want to know the best way to integrate a Geothermal DeSuper Heater, a Solar Array and a back up Electric Elements for heating Domestic Hot Water.

Right now I am working with one tank with an internal heat exchanger and back up elements that is also acting as the Geothermal Buffer Tank (except that its plumbed wrong so the geothermal has actually been doing nothing but deadheading). The Electric elements are heating the <span style="text-decoration:underline;">entire</span> tank (honestly I've never seen an electric element so low down in a tank) and there is really no room for solar storage other than at temperatures above the electric element thermostat setting of 140 degrees which basically makes the solar almost pointless in the winter and prone to overheating in the summer.

I am thining that I will do three tanks in series, the DeSuper Heater will heat the first tank, the Solar will heat the second tank, and the third tank will be an electric back up tank. I will add a recirculation loop with a pump and a differential controller to move solar heat from the solar tank to the electric tank when the solar is cranking in the summer.

Is this overkill? can the DeSuper heater dump directly into the solar tank? How hot will the DeSuper Heater get the water? Does the DeSuper Heater care what temperature water it is heating? Any thoughts would be appreciated. I will attach a schematic when I finish drawing.

Mark Eatherton · February 2011

You have to look at it from different angles...

First off, what are teh maximum temperature potentials for each source. Solar DHW is around 180 and you might hit that during the summer months.

If memory serves me correctly, desuperheating has an upper limit of around 140 degrees F, so having the desuper heat tank first in the series would be correct. This then brings up the potential of energy availability. Which alternative energy source has teh greatest availability potential? In other words, how often will the AC be running to provide desuper heat, and what is the solar availability?

Preheating the DHW supply to the solar DHW preheater is going to negatively affect the thermal performance of the solar system as a whole, I fear that sending preheated water into the desuperheat tank will cause it to not drop much energy due to the lack of temperature differential.

You have some research to do before you can make the decisions...

ME

Phelps_Clarke · February 2011

My Schematic

This is my plan in drawing form

Mark Eatherton · February 2011

As I said...

If it were me, I'd run the cold into the solar first, GSHP 2nd and Aux. electric last. But that is just my opinion. If you had more air conditioning demand than solar availability, then my opinion would be to go to the GSHP first, then solar, then the Aux.

So, it depends :-)

ME

hot_rod · March 2011

agreed

the solar will be most efficient with the coldest water. I think it depends on the solar fraction compared to how often the desuper heater could run.

Run a solar simulation first to see what solar availability is in your area. RETscreen is one program you can use, or download for free.

Then come up with the number of hours the desuperheater will add to the system.

Depending on how complex you want to plumb and control you could use motorized 3 way valves instead of manual valves to shuffle the DHW draw. Pull from the solar tank first if it is warm enough, then the heat pump supplied tank, and better yet a tankless as the last source to eliminate any standby loss. or the electric tank.

But you will need to maintain the electric tank at a useable temperature 24/7, or install a timer to bring it on early AM before the load hits.

I agree with Mark, not knowing all the design spec that the solar would be first online.

How much DHW do you need to produce on a daily basis? Is this a residential application?

hr

Phelps_Clarke · March 2011

More Information

I talked to the HVAC contractor who put in the geothermal heat pump and he says that his Domestic Hot Water System is more like a first stage water to water heat pump than a DeSuper Heater and that it will only heat Hot Water during the summer. He says that it does not improve the efficiency of the heat pump the way I think a DeSuper Heater does. He says that the max temperature that he can heat water to is about 115.

My thinking now is that since the solar will be cranking in the summer anyway, there is no real need for the extra heat pump heat. It definitely has to be first in line though if we do it, because the solar will almost always be above 115 in the summer, and I would say will probably always be above 115 when there is a serious AC load.

This is just a residential system, with 45 sq ft of east facing collectors (remember, I am just trying to fix up this mess). I think these folks use lots of hot water, and with that assumption, according to retscreen my solar fraction is only going to be somewhere between 60% and 80% even in the summer, so there is at least some room for geothermal pre heat. It is hard to say if its worth it or not. But in the summer I think that the panels could still do some good work even with 115 preheat. Its in the winter that I really want to be able to claim the 50 to 90 territory for the solar.

Anyway, more thoughts to ponder.

hot_rod · March 2011

remember also

that in the summer IF you are pre-heating the solar tank supply water to 115, in addition to lowering the collector efficiency, you need to think about overheating issues with the solar.

So lets say the solar tank reaches setpoint by noon, what is the plan to deal with excessive heat from the array? Without a fairly consistent load you will go into stagnation conditions. From time to time stagnation cannot be avoided, but you don't want to encourage it every summer day.

I cringe when I hear of systems that dump heat all summer long, running an 80W pump to lose free heat, they gathered the first 1/2 of the day!

Do you have a dump zone, assuming it is a closed loop solar? Do you have a control with system cooling function or a way to re-cool in the evening?

Flat plate collectors? Those offer some ability to re-radiate excessive heat in the evening.

As you mentioned you need to get better info on how much DHW they do in fact use. Only then can you accurately design all the components and best solution.

Stick a water meter on the cold feed to the first tank for a few weeks to get actual DHW consumption.

hr

hot_rod · March 2011

remember also

that in the summer IF you are pre-heating the solar tank supply water to 115, in addition to lowering the collector efficiency, you need to think about overheating issues with the solar.

So lets say the solar tank reaches setpoint by noon, what is the plan to deal with excessive heat from the array? Without a fairly consistent load you will go into stagnation conditions. From time to time stagnation cannot be avoided, but you don't want to encourage it every summer day.

I cringe when I hear of systems that dump heat all summer long, running an 80W pump to lose free heat, they gathered the first 1/2 of the day!

Do you have a dump zone, assuming it is a closed loop solar? Do you have a control with system cooling function or a way to re-cool in the evening?

Flat plate collectors? Those offer some ability to re-radiate excessive heat in the evening.

As you mentioned you need to get better info on how much DHW they do in fact use. Only then can you accurately design all the components and best solution.

Stick a water meter on the cold feed to the first tank for a few weeks to get actual DHW consumption.

hr

hot_rod · March 2011

remember also

that in the summer IF you are pre-heating the solar tank supply water to 115, in addition to lowering the collector efficiency, you need to think about overheating issues with the solar.

So lets say the solar tank reaches setpoint by noon, what is the plan to deal with excessive heat from the array? Without a fairly consistent load you will go into stagnation conditions. From time to time stagnation cannot be avoided, but you don't want to encourage it every summer day.

I cringe when I hear of systems that dump heat all summer long, running an 80W pump to lose free heat, they gathered the first 1/2 of the day!

Do you have a dump zone, assuming it is a closed loop solar? Do you have a control with system cooling function or a way to re-cool in the evening?

Flat plate collectors? Those offer some ability to re-radiate excessive heat in the evening.

As you mentioned you need to get better info on how much DHW they do in fact use. Only then can you accurately design all the components and best solution.

Stick a water meter on the cold feed to the first tank for a few weeks to get actual DHW consumption.

hr

Geothermal, Solar and Electric Elements for Domestic Hot Water

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