Thermal storage and water to water heat pump
 

So the house we are closing on in the next week or two has very old hvac system. They are dated 1992. Both the propane furnace and the ac both work well and are in amazing shape for their age. So this got me thinking about geothermal. The house in on 5 mostly flat acres with some trees in a very rural setting. That being said we don't have natural gas, just public water and electric.

The house is brick and was build in 75. The previous edit owners have put pella windows in it and insulated everything pretty well already. The house was build with a typical concrete cistern tank. The owners built a very nice 4 season room on top of it. The pump and bladder tank are still in the basement next to the wall of he cistern. I however do not know the condition on the cistern tank. I estimate it at about 10-14000 gallon tank. Outside dimensions are 12x20 and the basement walls are 9' tall.

I ran across a thermal storage website called Thermal Battery Systems | Site-sourced thermal energy management. This guy also has a YouTube channel from a few years ago https://youtu.be/jfORIfCbxQA

So this got me thinking. Fill the cistern up with water attach a water to water geothermal unit to it and a hydronic air handler. Leaning towards not installing any kind of ground loop. Also we have an above ground pool of about 12k gallon also. I thought about hooking it up to the geothermal in the summer with some kind of temp setting that would switch between the cistern and the pool so the pool doesn't get to hot. The reason why I say water to water with the hydronic air handler is for easier more efficient way to use something like an outdoor wood boiler or in my particular case a wast motor oil boiler.

I've done a very rough heat load at 24k ac and 35k for heating.

Thoughts?

You'll run out of heat storage fairly quickly. At 14k gallons you have 116,836 btu's per degree of water temp. We should assume the starting water temp will be around the average ground temp, so lets be generous and say 65ºF and we can extract heat down to say 35ºF so a 30ºF temperature delta. That gives us about 3,505,080 btu's or 100 hours of heat. Now it will absorb some heat from the surrounding soil so you can get more than that, and the sun room will add a bit more, but the fact is there just isn't all that much heat storage in there.

That said heating the pool during the shoulder seasons with the heat pump, when the house calls for A/C can be a great way to extend the swimming season and increase the efficiency of the system. There are several systems your HVAC guy can install for you. The biggest benefit however is savings on pool heating, so if you don't heat your pool anyway you may not see much savings.

I agree with dEnd.


Steve

While 3.5 million BTU's sounds like a lot of heat, it's really not. The average single family home consumes between 90 and 120 million BTU's a year. So without any heat input, your cistern only has enough (sensible) heat capacity for around 2 weeks of average heating or cooling. We all know that heating and cooling need is not average in nature.

What will happen with a system like this? You will have a week or so of low hanging fruit, when it is very economical to move heat into or out of the cistern. After that, either the heat pump will win or the heat flow will win. You will reach a state of equilibrium where it is cheaper to use outdoor air (unless it is below freezing outdoors and you try to freeze the cistern) to move heat with. Unless you increase the surface exchange area somehow, you have a few million BTU's of window to operate in; after that, heat (or cool) costs extra.

In the video the guy uses what hey calls "hyper loops" to me they just look like those cheap solar pool heaters. That looks much easier to handle and I'm assuming it's much more surface area compared to just a few hundred feet of pipe thrown in the cistern.

I guess if the WMO boiler works like I hope it will, I'll be able to just hear the house directly off off the water from it. Then maybe between the heat pump and the fireplace(with and insert) I will be able to heat the house pretty easily. But we will see how this project comes along this winter. This summer will be full of moving and a diy wedding in our new back yard

http://www.thermcoenergysystems.com/...eothermal.html

Yep, the thermal battery guy is leveraging the system with water being used as a phase change material (PCM) on both ends. The latent heat of fusion required to make water change phases makes a big difference in the energy available in the stored water.

In heating season, the freezing and thawing of ice water holds 150 times more heat than sensibly heating and cooling the water. It also makes the solar collectors WAY more effective. As long as they are producing a source temperature above 0 degC, they are thawing ice. When outdoor temperature is above freezing, the panels can transfer heat even in the dark.

During cooling season, an evaporative cooling tower is employed. With a small "swamp cooler" unit, the heat flowing into the cistern from the heat pump could be dissipated with ease, once the cistern water temperature exceeded outdoor temperature. In your situation, the waste heat could be dissipated into your swimming pool to keep it comfortable. A few floating solar pool fountains could be enough to lose a few tons an hour during the day, and at night the pool would not need help evaporating during peak season.

I have done this before to cool off the outdoor unit of air-source units and it works well any way you do it. I have put drip irrigation misters on a tap water supply with valve, boosted performance over 30 percent. I put a kiddie pool in below a window unit to catch condensate, pumping the stuff with a submersible solar fountain pump and cut energy usage and boosted cooling performance. This all happened in the same climate zone as you.

http://ecorenovator.org/forum/member...inter-time.jpg

http://ecorenovator.org/forum/member...al-storage.jpg


Som really crappy band basic drawings of the proposed system. It certainly will be slightly complicated but I think the controls will be fairly easy. Pretty simple logic behind what would happen

Sounds like you will be making a system like this:
406demo
Here's the product page from Tekmar, who makes the (not cheap) control:
Alternative Energy

I usually post up pics and drawings, but your envisioned rig is unique, and I can't find any that are easy to read. The product literature has plenty of schematics and application drawings included, and (especially the flash demo) should spark your imagination enough to figure out how to plumb what you want.

Happy reading! Let us in on what you decide to do.

EDIT: I found another diagram:
http://www.energyhomes.org/images/co...tedsystem1.jpg
Advanced Technology | Energy Environmental
http://www.energyhomes.org/images/co...tedsystem2.jpg

Thanks Jeff. That is a lot of good info. I ran across a YouTube channel months ago and it turns out the guy is getting ready to put a similar system in. His channel name is engineer775. He is a really great guy and shares lots of good info

So the plan for pumping is to get the ecm pumps with the constant pressure setting. When a zone valve opens it will drop the pressure and the pump with start pumping as hard as it needs to. Each zone with also have a flow meter on it to regulate the flow. Initially I was just planning on buying a water to water unit but they are rather $$$ but I will keep an eye out for a used one possibly.

But I do have a question. What style compressor would make the most sense for this application if I diy. For example a single speed or two stage or even possibly a 3 phase system like randens 5 ton. The only reason I'm even considering the 3 phase option is because I have solar. I have 10kw usable battery storage and a 4400 watt 240volt inverter(magnum 4424). I only have about a 1kW of solar panels tho. So i would need to add some panels. But if I went that route I could in theory run the heat pump all day storing as much hot cold h2o I can and then just run off the storage at night. At least in cooling mode. Heating mode (winter time)would probably not work as well as my new house doesn't have great winter solar but good in summer time. But my main heat source will be the oil boiler anyway.

Edit. If I did the solar and three phase. I would have to hook up some kind of power meter to the solar end of things to know how much power im bringing in and then ramp the vfd up to match the power from the solar panels.

So what is the question here? are you planning on building a custom heat pump? I am here to tell you it is entirely possible. The more planning that can be done before anything is built, the better your custom rig will operate. If you are going to try to run the thing directly off the solar panels and a battery bank with an inverter, you definitely want to go with a variable speed compressor. The single speed compressors need a big jolt of power to start, which limits what you can run that won't kill the inverter. Variable-speed compressors can be soft started, eliminating the startup spike. The decrease in peak demand equates to much more useful capacity.

IMHO, battery banks should only be used for emergency power, or if you are going to have a power source that is completely off-grid. The reason being: charging and discharging battery banks is horribly inefficient. On the way in (and out), parasitic losses are typically 10%. You must also oversize the battery bank as to not have large charge/discharge cycles. For regularly used banks, a 25% cycle is typical. So for 10KWH of storage, you need 40KWH worth of storage capacity. To get more depth of discharge, the AGM or lithium battery types are not economical. All of these factors lead to one conclusion: if you can run a grid-tied system, DO IT!

In your situation, I would definitely look into PAHS and include it in the design. The cistern water temperature could be regulated by the ground surrounding it with a small pump. It could also charge the ground for use a few months later. All the heat or cold that accumulates in the cistern could be spread out or drawn back in using your PV power, so basically free.

I guess the question is what one of the three would be the most effienct way to make this system. Being that I will have a decent size buffer tank does it really make sense to have a variable speed compressor? Obviously with it being coupled to solar it's the best choice in the long run. But will it be worth the extra cost and control setup.? I guess the way I look at it is with the buffer tanks a one speed system will probably do the job just fine. But it wouldn't be nearly as cool as a solar powered three phase system :D

Planning the system is something I really want to do and do well. I will only have bits and pieces of time this summer between moving and planning a DIY wedding in our new back yard. But once winter time comes around I should have more free time. I suppose if I found a 3 phase water to water unit that was gator built for the right price would be nice. So if anyone runs across a 2-3 ton 3 phase unit let me know :D

Good luck on preparing for your wedding. Definitely more important than heating and cooling. In the meantime, keep an eye out on Cincinnati craigslist. Lots of usable units pass through every week. I see 3 of them listed right now for around 200 bucks. Used 3 ton in Hamilton for 200, carrier 2.5 ton ashp in NKY for 200, 2 ton goodman in Erlanger for 300. Probably all single speed single phase units. Some assembly required.

fordguy,

Be careful of what you wish for. Jeff is a master at taking bits and pieces of this and that, and coupled with his superb fabrication skills can make complex things quickly that have dependability. So is randen.

Doing things yourself can be very rewarding, bur start now on some simple stuff. Like a modest air to water boost for your hot water tank. I don't know your welding/brazing and AC skills (guages, suction machine, etc), but this would get you started.

Understand, I am NOT downplaying your skills - in fact, your planning before doing is exactly the right first step.

I started small on several projects and have not regretted making mistakes on those. It prepared me for doing far bigger projects.

The project you are planning is huge and the worst thing is to get 30-40% into it and realize the "oh crap, what have I gotten myself into". A heating/AC system is essential to daily life and doing it right for dependability is essential.

Hope this does not offend!

Steve

Steve no offense taken. I've been a member here for a while but I haven't contributed a whole lot.. So little is known about me.. I've just been trying to take it all in. This is by far the best site for phase change information. Anyway.

I'm one of the jack of all trades type. But I'm a master at none. My day job I work at an R&D shop for a very very large company. I'm a machinist welder fabricator designer engineer and so on. They pretty much come to me with a stack of drawings and say make this.. Although I only have a HS diploma so I'm actually not an engineer or anything of the sorts even tho I do the job from time to time lol

Mechanical things are my favorite. Been working on things with motors my entire life. My dad taught me how to weld when I was 9 or 10 and I've been doing it ever since. I'm also pretty good with electrical and plumbing.

Although I won't lie the refrigeration cycle will be a learning curve and I'm sure it will be a big one. I have two 12k btu units I picked up a long time ago. I also have a 2.5 or 3 ton r22 unit from my dads house. It's old but it functioned when pulled. So I'll have lots of units to mess with whenever I get the time.

But I guess to further this project I need to do some real heat load calculations. I think my locat utility does one of those energy audit programs and I'll look into that after I move in. One of the things I'll probably need the most help with is sizing heat exchangers and tuning sizes. I really like the idea of not having the heat pump in the house. I'd rather have it in the barn. But it's easily 150 feet from the house. The pool is right in the middle of that.


I added a photo to my random album. It's an over view of the new property. The blue line is where the fence will go for the dogs. The blue circle is about where the pond will go. I cannot add the picture to my post because I'm on my phone and it crashes when I try to do it http://ecorenovator.org/forum/member...w-property.jpg

Heat exchangers are no big deal unless you undersize them. If you are using a water-to-refrigerant method, the heat transfer is different from water-to-water or air. With refrigerant in one circuit of the HX, most of the heat transfer is latent. On the other side, all of the transfer is sensible. What happens is the condensing or evaporating temperature balances with your supply water temperature and flow. Sizing and flow control is all about this approach temperature and maximum leverage. Increased surface contact (either more water pressure or larger HX surface area or both) is a game of diminishing returns.

Brazed HXs are more compact than other types, but they must be fed clean liquids or they will foul and clog up. Other types of HX are more tolerant of contamination, but tend to be larger than plate HXs at the same heat transfer rate. SWEP has a cool app called SSP that helps you calculate whatever you need to. Coolpack is another app that is ultra useful for simulation and armchair tinkering, but is somewhat more difficult to use.

If you can weld and fabricate, building a durable enclosure for the rig will be easy. The plumbing side of the fabrication is the most difficult aspect of the build. Using safety-silv solder is much like soldering copper water pipe, but is not as strong as brazing rod. Brazing with sil-fos rod is more demanding due to the high melting temperature, but is self-fluxing and stronger. After doing some pretty gnarly plumbing on these rigs, I now start out with the sil-fos rod. If things go well, I am done. If not, I add a tiny bit of safety-silv to the ugly joint, and presto, everything wets out nicely.

I'm in the same boat as Steve regarding getting some time under the hood on something small before you put together a big rig. A non-reversing heat pump water heater is a really good test bed to tinker with. Load matching, metering device selection, adjusting refrigerant charge, etc. can all be done without a lot of time or labor involved. Actually operating and observing the unit is very informative and rewarding.

Yea I was thinking about a small build first. I found out that my soon to be brother in law has a bunch of equipment I can borrow. I knew he had a farm but I never realized he had so much fun equipment! Skid steers and mini backhoes and so on. So I still have the 2 one ton units. Maybe I'll put together something with one of those in the garage this winter. Could borrow the mini backhoe and dig a trench to put a small loop field in. Or maybe I'll just put in a good sized one for later. We will see with time and $$

If you're going ground source, make sure to plan the whole field out beforehand. Even if you never complete the whole thing, at least you know you didn't put a loop in somewhere it didn't belong. Plan to oversize your field, just in case. That way, if you decide to expand later, you won't be having to make compromising decisions. DIRTFT, because the second time is a lot more difficult.

With the system you cited, a pump-only loop dedicated to evening out the cistern temperature is one of the features that makes the efficiency skyrocket. A little 100 watt circulator pump moving the water through a loop that can realistically transfer thousands of watts of heat is not impossible. It has been done, and the results show up on the electric bill.

Farmers are cool! I recently had to do an engine swap on a car, and had trouble finding an engine hoist. Nobody wanted to loan me one today, now. So I went down the road to a neighbor's farm, and asked him what he had I could borrow for a couple days. I ended up using a hay spear, used to pick up and move those big round hay bales from the field to wherever they end up. He even helped me load it into the bed of my truck! It fit right onto the old Farmall tractor we use to bush hog with.

When I returned the implement, farmer neighbor wouldn't take my money. He said it would have just sat there all weekend anyway. While he wasn't looking, I dumped a can of gas in his truck for payment.

https://youtu.be/vaCSOu4d1po

Long and kind of boring.. But lots of info based on large systems

So this ice bank idea is kind of awesome. Although it seems for most cases it pays back faster if you have lower electric rates at night. I don't believe I have that. But I have almost enough solar to run a 12kbtu unit. So my first test rig might be two tanks of water for hot and cold. Hook up the solar and see if a can freeze a tank of water! Sounds easy :)

Solar thermal is a LOT more efficient than solar PV. In heating season, trying to freeze your cistern with the heat pump and thawing it out with a smallish ground loop and/or solar thermal would be ultimately effective. There are panels being made now, today, that do both.

https://dualsun.fr/wp-content/upload...ecto-verso.png
dualsun wave panel (rated at 250W pv plus 900W thermal)

Solar angel is another brand that is available.

So another question.. Since I have access to the equipment to dig and also the yard to dig. Should I skip the extra pump and just go with a dx type system? I guess the nice thing about the water to water on both sides is in the summer time I can use the heat generated for cooling to hear my domestic hot water. I feel like the dx type system could be much more efficient with less pumps and heat exchangers..

After more thinking.. I think I'm going to stay with the original idea of water buffer tanks on both sides. Maybe I'll make a dedicated heat pump for the water heater that is dx.

I feel the water on both sides gives me so much more freedom to do things with the heat pump..

Yes unless the capacity of the heat pump is small, the DX exchange pipes will use more refrigerant than a plate heat exchanger. The surface area of the piping isn't very large either. A plate heat exchanger is very compact and will use hardly any refrigerant compared to other types. Less refrigerant is pretty much always better.

I am a fan of matching heat exchangers to compressor capacity, plus extra. Once the rig is plumbed, it's done. The water side can be rigged to whatever, however, whenever. Having multiple rigs gives flexibility. The upsize lets you pump more water without losing heat transfer. With variable speed pumps, the exit water temps can be regulated at each unit if desired. This works on both sides of the rig to get you where you want to go quickly.

We have officially closed on the new house. I'm going to take a side panel off of th ac unit as its so old it doesn't have a label on it. I'm going to see if I can figure out how many btu it is. The propane furnace has a sticker in it that says it's a 1992 model. I'm assuming the ac unit is the same age. It still works well but it's just a matter of time I feel. My quick estimation says that I'll need 2 tons for cooling. So it will be interesting to see what size is on the house now.

I haven't decided if I want to put this home made heat pump thing in the basement or in the barn out back. I'm leaning towards the barn just for home owners insurance reasons. The only downfall to this is that I'll have to do a lot of digging and a lot of insulated piping.. But we will worry about that in the next year or so. This winter I'll be working on insulating the barn and the oil burner. If I'm lucky I'll be able to play with the heat pump to.

If the house ends up being a 2 ton unit I'll start looking for heat exchangers in the 3 ton size range. That way I can use them for my 1 ton experiments and then use them when I go to the larger unit. The heat pump will be primarily used for cooling and only heating in the spring and fall when it gets cool at night. My main heat source wil e the oils burner. At least that's the plan for now

FordGuy,

Re putting the heat pump in the barn and piping the hot (or cold) water to the house.

Whoa, whoa . . . .!!!!!!!!

You will loose every advantage of having the high efficiency heat pump. Here is why.

A typical R value for pipe insulation is maybe 3-4. Unless you go 4 feet underground, your winter ground temps will be in the low 30's (in your area) or less. If you pump water at 100 F, then you have a 70 degree differential. Even 4 feet underground, your winter ground temp will likely only be in the 40F range.

Imagine trying to heat a house to 70 F when it is 0 F outside (same 70 degree difference). And imagine just how inneffective an R value in the walls and ceiling of 3-4 R would be! This differential would take about R30 or more to be effective.

Even some 30-40 feet of underground pipe will just result in heating the ground even with R10 insulation.

Put the heat pump in the enclosed house and preferably in the controlled air space.

Lastly, what is the deal with home insurance and the heat pump?


Steve

I was planning spray foam and pex for the run from the barn to the house.. Outdoor wood boiler guys use it very often and usually only have 1-2 degree temp drop.

As far as home insurance goes it's a non ul listed appliance full of propane.. There wasn't an option for that in my policy..

FordGuy,

The amount of heat a wood boiler puts out is HUGE. And the temp is a lot hotter too. The reason for the small temp drop you quote (1-2 F) is because the wood boiler pipes heat up the ground.

You have a much smaller capacity and temperature with the heat pump.

Now you know, do what you think is best. But you need pipes about 4 feet down.

Lastly, the risk of a propane issue is very, very small. Yes, it is a "risk", but one that many on this forum would not worry about. Remember the very small amount of propane that is in a heat pump - tiny.

Caveat emptor!


Steve

Hmm I suppose that's a valid point.. We shall see what happens.. Either way the test rig will be in the garage. Easy to dig by the garage also. Less sidewalks and plants and things

So I guess I really need to get in and so some real heat load calculations. Everything is driven off of that lol who knew? But going off of my hypothetical cooling of 24kbtu I've started to look into sizing buffer tanks. I've come up with 75 gallons on the cooling side and 50 gallons for the heating side. The cooling side was based on a minimum of 45 min run time. That way the compressor has time to run like it needs to. Since I have two 12k btu compressors I'm going to try and do a multi stage system. Being both compressors will be tied into the same heat exchangers. This will give me a really cheap system since I already have them and load control. On the not so hot days just one compressor will run and when it gets really hot and humid both can run.

Is 45 min run time long enough? Should I make the run time longer? From what I understand it takes about 10 min for a refrigeration system to get up to full efficiency?

http://www.ashraebistate.org/sites/d...ge_systems.pdf

So what would be more efficient ? Direct exchange for the refrigerant right into the water tanks? Or brazed plate heat exchangers with pumps?

The ice tank idea is good for a water chiller setup. You would have refrigerant lines running through the tank to freeze the water and a glycol loop running an air cooling rig indoors. With a residential scale system, the biggest concern is insulating the ice tank. The main purpose of the ice tank is to level the cooling load and minimize the compressor capacity required. If the ice tank is not super insulated, heat leaks in and wreaks havoc on your efficiency.

This is a different approach than most ground source units employ. Rather than trying to freeze the cold side, most ground source systems attempt to keep the suction pressure high to lower the compression ratio. The higher the compression ratio, the more work (electrical power) required to move the same mass of refrigerant. So the two strategies are at odds with each other.

The main deciding factor in choosing which method to implement is the peak power required to satisfy the load. Either way, something is going to be massive. Either you have a massive underground loop or a well and a larger compressor that can supply the peak load, or you have a smaller compressor and a massive insulated cold store.

Ok that makes sense. So with the ice tank idea the compressor will work harder do to running a lower pressure on the cold side to get below freezing. Makes sense.

My latest idea. Is to run a smaller compressor say 6-8kbtu unit off of my solar during the day. To freeze/cool the tank down as much as possible. Then have a larger unit of maybe 12kbtu or so to help out in high demand.

We have noticed that the house is very well insulated and does not have a lot of heat gain in the hottest part of the day. We have spent a few nights offer the 4th of July weekend setting the thermostat down to 73 at night. Then when we got up we set it to 84 because we spent most of the day outside. By the time we would go to bed the house would be at 80 or so. The outside temp was in the upper 80s and low 90s all weekend with very high humidity. Night time lows were about 70 and still very humid.

I have a whole house kW meter I'm going to hook up to the ac unit and try to get some average readings in the next week or so. Just trying to get a better idea of what the houses actual cooling need is

Your idea should work out fine for the purpose you intend. If you feed the condenser side ground source water, that would improve efficiency by lowering the discharge pressure. Once you commission the rig, you will get a better idea of the factors at play. Lowering the Delta T just a few degrees can improve your energy savings a substantial amount. Plus, you can dig further into freezing territory without shutting down the heat transfer.

With a custom made system, you get to do all of the testing and measuring, so the sky is the limit as to the potential for savings. It just depends on how much you want to spend up front on the design.

Alright I think I like where this is headed. im not afraid to spend money on this project. Obviously the cheaper the better.

So I had talked about trying to use one pump per side of the system (hi and low side) but I'm not totally sure that will be possible. Seems like I would have to size the heat exchangers pretty well based on x flow rate. That might be difficult but we will see.

I found a good blog article about a system just like you are considering:

https://elkement.blog/2016/01/07/how...ystem-that-is/

Totally custom and self-built by a couple of engineering professional hackers. These 2 have gone down the road that many people avoid and a few have explored briefly. I have no doubt about the theory and synergy of the type of system that they have commissioned and the small amount of energy required to run it. However, reading about time tested operation in reality is quite another matter. This type of information is usually kept under lock and key by those who would seek to exploit it. Happy reading!

That is an amazing blog.. I think I might have to read a few times lol

http://www.spf.ch/fileadmin/user_upl...inalReport.pdf

Found this on one of his posts

Study the photos carefully while you read the captions. It's really quite fascinating. I was just explaining what happens in various months to my wife sitting across the room without benefit of her viewing the photos. By having to explain what was happening in each month verbally, I got a better understanding where the heat comes from. My wife grew up at 45.8°N, where heating is quite necessary. We both understand the concept of geothermal, but had never seen it coupled to solar thermal and air source combined.

So something I find interesting is the ice storage tank sizes.. Most of them are under 300 gallons. My cistern is up around 10000 gallons. Does that hurt or help me? It also appears that the solar gives major gains and I don't have enough room for solar to keep the same ration as they do for gallons to sqft lol