Planning a large scale hydronic system with Seasonal Thermal Energy Storage

[skyking1]

We are building a new home in the Olympic rain shadow, where the sun shines more and it rains less than anywhere around it.
It has a less than ideal roof pitch for flat collectors at 3:12, but this is a better pitch for my personal safety
I am using standing seam metal so I can employ the clips ffrom S5! or similar and avoid roof penetrations.
The current idea is to have a mixed system of a modest PV array of ~5Kw, and a large vac tube array that will feed an insulated soil/sand thermal mass through the hot season.
The hydronic system works with a maximum of 40C input temps, and I am working to size the array and storage to drive it up to 65C for a delta T of 25C.
Initially I had thought of a ~140,000 Kg mass, but now I am leaning to a smaller mass to ensure I get to the higher temperatures I seek. I will put in the infrastructure for a second mass so I can look at first year data and make adustments.
Since this is a new home build, I will have chases built in to the ridge cap and conduit installed and pre-inspected for solar panel installations on both south facing roofs.
I will have about 5 tons of GSHP loops in the ground, and was going to do a water to water Ground Source Heat Pump(GSHP) for the planned hydronics, as well as a 3 ton water to air GSHP for our brief air conditioning season and to provide for quick reaction heating. That heat pump is large enough to heat the whole house all year, but we want warm floors.
With this heat mass idea and collectors, I would omit the water to water 3 ton heat pump entirely. If it was all for naught, I can install it and take that credit next season.

With the relatively low costs of PV panels, I may be barking up the wrong tree, but I want to do the mental exercise before I dismiss it entirely.
Has anyone looked into this idea lately?

 

[skyking1]

I got to looking at the data sheets and SRCC cerifications for the Duda solar panels.
https://www.dudadiesel.com/files/SRCC-DS5825.pdf

My locale has an average of 4.08 kWh/m².day for the insolation.
That puts it below the medium category in the data sheet above, for an average of ~19,000 BTU per day per 25 tube panel.
5 panels = 95K Btu/day.
I am definitely splitting the storage mass in half now.
It is easy to put an insulation partition across the rectangualr area. I would plan on the near mass to be the higher heat storage. The far mass piping would run through the bottom of the near mass.
Everything about heat collection hinges on delta T's. The panels make more BTU when the delta is lower between fluid temp and air temp.
In the summer months the air around the panels on the roof will be the hottest, so they will make far more than the average. I will try to operate the panels to produce a delta of around 20 degrees between the fluid and storage mass. This will keep the panels in a more efficient range.
Hot summer will jack up the hotter storage mass, and the cooler shoulder seasons will heat up the cooler mass.

I will have enough PV and battery capacity for these things:
1) the lights to work.
2) the food does not spoil.
3) the sewage ejection system pump works. That one may not happen as we might get a gravity side sewer but I have to plan for it.
4) the circulation pumps for the collectors and the hydronic heat keeps working.

That's (4) 15 amp circuits that get backed up.
It keeps the inverter and battery bank small and gets done what we want.

 

[gotbeans]

Those are the glass vacuumed tubes like these? https://www.aliexpress.us/item/3256804289665016.html
The pdf has no pictures, come on guys!

I've seen some people water cool their solar panels to produce hot water / more cooling on panels so more power.. but it seems like way too much work. Surprised no closed loop ones are around that just plug in

 

[skyking1]

Here is link to the panel, but that PDF has the science.
https://www.dudadiesel.com/choose_item.php?id=DS-SC5814-25T

 

[gotbeans]

yea same kind/one, good luck with it!
only downside I've seen with them is they don't hand hail as well as solar panels do

 

[skyking1]

the trouble with combo panels is when you get effectively heated water from them, that is too hot to help the Photovoltaics.

 

[skyking1]

yea same kind/one, good luck with it!
only downside I've seen with them is they don't hand hail as well as solar panels do

They don't get a whole lot of convective weather where I am buidling.

 

[Rusty959]

@skyking1 are you planning on heating the house directly from your thermal energy storage or running a heat pump between?

I have put a massive amount of thought into such a system over the last year. My current observation is:

1: If you don't heat your home directly from the thermal energy storage, meaning without a heat pump between, it is going to be very hard to have a ROI for a large system. The efficiency gain for a heat pump heating your house with say 100°F instead of 55°F temps isn't worth it.

2: It is hard to efficiently get enough energy from a solar collector into water in the ground at enough delta t to make it worth it. As you point out, that delta T is about all that matters.

If I had infinite space, no net metering, and a goal of being as self reliant as possible, I'm leaning toward resistive heating sand being the way to go. You lose the gain of going direct from sun to thermal, but you remove so many of the issues that occur by trying to heat water in ambient locations.
I'd probably double my PV array and go with a system almost exactly like Batsand, wether I designed it myself or went with their solution.

 

[skyking1]

it will be direct, the fluid for the collectors and heat storage mass is common with the hydronic system. No heat exchangers, heat pumps, change of state.
In terms of F, the hydronics have a maximum operating temperature of 105F. That is taking into account the limits of the LVP flooring.
the earth around and below the mass is 52F.
I'm looking for two storage temeratures, one low heat at about 110 and the high heat closer to 150 by the end of the summer production months.
I have *free* insulation, and that is the economic difference right now. I can insulate the mass to R80 or greater. This will greatly inhibt loss at the storage.
So look at the Delta T on the shoulder months. 120 - 50 is 70F for the delta T when using the low mass. That's cruising 10F over the temperature of that mass.
In the shoulder months there is very little heating getting siphoned off to the house.
I do have a 3rd storage mass in the concrete basement slab. I can run the basement a little on the warm side and that will help carry the house overnight.
i'm using Heat Sheet R10 Heavy under the floor for ease of hydronic installation.
https://heat-sheet.com/what-makes-h...dxYFL8OoHrESs_1SCQoLaAHKWoTwPrdoaAgUMEALw_wcB

That is the same vendor for the ICF basement walls. Those are R26 with 8" of concrete in between.

 

[Rusty959]

it will be direct, the fluid for the collectors and heat storage mass is common with the hydronic system. No heat exchangers, heat pumps, change of state.
In terms of F, the hydronics have a maximum operating temperature of 105F. That is taking into account the limits of the LVP flooring.
the earth around and below the mass is 52F.
I'm looking for two storage temeratures, one low heat at about 110 and the high heat closer to 150 by the end of the summer production months.
I have *free* insulation, and that is the economic difference right now. I can insulate the mass to R80 or greater. This will greatly inhibt loss at the storage.
So look at the Delta T on the shoulder months. 120 - 50 is 70F for the delta T when using the low mass. That's cruising 10F over the temperature of that mass.
In the shoulder months there is very little heating getting siphoned off to the house.
I do have a 3rd storage mass in the concrete basement slab. I can run the basement a little on the warm side and that will help carry the house overnight.
i'm using Heat Sheet R10 Heavy under the floor for ease of hydronic installation.
https://heat-sheet.com/what-makes-h...dxYFL8OoHrESs_1SCQoLaAHKWoTwPrdoaAgUMEALw_wcB

That is the same vendor for the ICF basement walls. Those are R26 with 8" of concrete in between.

What is the free insulation you have access to? One of the annoying things I found in my planning is a lot of the material I'd traditionally want to insulate with under ground starts to break down around these temperature over the years.

And to be clear what I mean with direct, I get you are going direct from your collectors to storage without phase change, but it sounds like you're going from storage to heating the house with a heat pump. Else you can't make great use of any temperature under maybe 80°F or so depending on the goal. The difference of a ground source heat pump heating with that same 80°F water vs ~52°F strait from the ground, at least in my case, is pretty insignificant to be worth it from an ROI perspective.

That's why I'm personally leaning toward straight sand storage at several hundred °F so I can go direct to a radiator vs requiring a heat pump to utilize the stored heat. I just hate that means I have to start with electricity and can't make use of the better efficiency of direct solar collectors.

 

[skyking1]

No, the hydronics does not go through a heat pump. The heat pump is water to air, an HVAC unit for the very short air conditioning season.
It is sized for the Manual J cooling load, and also for the manual J heating load.
The hydronics has nothing to do with the heat pump.
So let's say I get 10 months of complete heating out of my solar system, and partial heat on the other two months and GASP!
I have to turn on the HVAC.
No big deal

I'll have ~8Kw of PV panels up there doing net metering,

 

[skyking1]

I just hate that means I have to start with electricity and can't make use of the better efficiency of direct solar collectors.

If you wait 18 months or so I can share my data with you.

What is the free insulation you have access to? One of the annoying things I found in my planning is a lot of the material I'd traditionally want to insulate with under ground starts to break down around these temperature over the years.

It is scrap SIP panels from a manufacturer not too far away. They have to dispose of it, because the labor to disbond the OSB from the foam is prohibitive.
I will pay the shipping for simplicity's sake. Probably $1500 for one of these full.

That's a far cry from the $8500 plus shipping for new Geofoam.



The EPS foam has these properties for temperature limitations:
Maximum Service Temperature, Air 70.0 - 118 °C 158 - 244 °F
Vicat Softening Point 99.0 - 104 °C 210 - 219 °F
Softening Point 70.0 °C 158 °F
Decomposition Temperature 220 °C 428 °F

My planned storage will be > 150F The foam will hold up just fine.
That dovetails with keeping things close to the required temperatures for the heating at 105 F

So here's the math on storing in soils and sand.
Moisture matters!
"Dry sand soil has the unit specific heat capacity is about 800 J/kg K and saturated sand soil is about 1632 J/kg K "
So loading it up to ideal moisture doulbes the heat capacity.
I can manage that with a 6 mil liner or two. I'll build it wet and keep it wet.
1600 J = ~1.5 BTU
My mass will be 120 cubic yards of sandy clayey soil, at optimum moisture content. It will weigh about 2750 pounds per yard.
330000 pounds = 150000 Kg
So, for every degree of change that mass will yield 225,000 BTU
My home's manual J number for heat is 25013 BTUH
That's the worst case heating requirement.
So that's 9 hours of heat per degree of change. 9 hours of the coldest days of the year.
Those are degrees Kelvin or Centigrade, so it helps to look at in with those units.
I expect to get useful heat down to 37 C
I can have storage capacity of 65 - 37 = 28 degrees C
That's 10.5 days of 2+ tons of heat for 24 hours per day, and assumes the sun never shined. Those panels produce heat in any temperature when the sun shines on them it is just reduced by losses due to the high delta T.

So for the first go around I will build that in two masses as laid out, and start with 4 collectors. I'll bring out another loop of conduit sleeves so I can add another mass. The plumbing on the roof will be sized so that adding panels would be no drama.

 

[Rusty959]

Great. Sounds like a solid plan. I wish you the best and will aim to keep an eye on your progress. Amazing find on the insulation it seems.

May or may not be helpful, but the idea I've been tossing around in my head is an insulated 2 chambered cistern effectively. Using water reduce excavated area and simplify plumbing, but can go up in complexity from there. There are downsides to water only as well.
One chamber much smaller than the other, acting as the primary and first to heat. Thermostatic mixing valve that will select the source before being pumped to the collectors, and then the return side just returns near an overflow in the primary chamber so once the primary hits the mixing valve's temp the secondary will begin to heat.
Would be easy to adjust thermal capacity since the large secondary side water level would define it.

 

[skyking1]

I will call my supplier about high temperature PE pipe. Here is an example.
https://www.globalindustrial.com/p/...UaieED9qjgw2gkwW_VsRnKTYTm7fjcoBoCzSsQAvD_BwE

I need about 3000' of 3/4" for the storage masses. By using that high temp pipe, now the upper limits of the heated fluid is 82C
With some safety overhead it will operate at 75C.
I won't know how hot I can get the mass until I get going, but raising the upper limit improves operational flexibility.
These vacuum tube collectors must be plumbed with expansion tanks, because any pump failure can send the fluid over boiling rather quickly.
I'll have those tanks but the goal would always be dumping heat into the mass for that regulation.