Heat pump idea

moan 77

Good evening all,

The following is an idea I have maybe it totally off the wall or it has some merits, all help will be great appreciated.
Here goes. I have 2 solar panels facing south each is 3 sq meters of flask type tubes. These will be connected to a 300 lt tank for hot water by summer I'm not sure as to winter temperature that can be achieved, I've heard of +25 / 35 c. In winter I'm hoping to shut the solar coil going to my 300 lt tank and instead direct the flow to another 300 lt tank, the water in this tank will be used as follows.

I'll have an ice bank which I'll be using as my evaporator ( instead of outside ground or lake/ well) this ice bank will also have capacity of 300 lt. My compressor will remove heat from water in ice bank and give this heat up in the heat exchanger which will heat the summer 300 lt tank, the solar coil will now replenish the heat into the 300 lt tank which in turn will be pumped into
the ice bank and on and on it goes till summer time where heat pump will be shut down and solar will be opened up to the summer 300 lt tank.

I hope I haven't lost everyone as I know it not easy to follow and as it's in me head it hard to get out correctly sometime. If anyone can fill in any blacks or correct me where I'm wrong that would be a great help.
Thanks again.

Mike2006

From how I read it, you want to use your 'ice bank' as your 'well' or ground loop' which would be in a traditional heat pump installation. This would save you the costs of drilling a well or excavating.

I am not sure how well this would work.

One consideration would be, if your solar coil could not heat the ice back efficiently, then your heat pump would have to work a lot harder and would be very inefficient.

I would be asking myself, why are installers not already doing this if it is feasible.

It is a very interesting concept though and if you follow through with it, I would be interested to hear on how you get on.

It could have the potential to save a lot of cash on the installation costs.

Mike.

Condenser

moan 77 wrote: »

Good evening all,

The following is an idea I have maybe it totally off the wall or it has some merits, all help will be great appreciated.
Here goes. I have 2 solar panels facing south each is 3 sq meters of flask type tubes. These will be connected to a 300 lt tank for hot water by summer I'm not sure as to winter temperature that can be achieved, I've heard of +25 / 35 c. In winter I'm hoping to shut the solar coil going to my 300 lt tank and instead direct the flow to another 300 lt tank, the water in this tank will be used as follows.

I'll have an ice bank which I'll be using as my evaporator ( instead of outside ground or lake/ well) this ice bank will also have capacity of 300 lt. My compressor will remove heat from water in ice bank and give this heat up in the heat exchanger which will heat the summer 300 lt tank, the solar coil will now replenish the heat into the 300 lt tank which in turn will be pumped into
the ice bank and on and on it goes till summer time where heat pump will be shut down and solar will be opened up to the summer 300 lt tank.

I hope I haven't lost everyone as I know it not easy to follow and as it's in me head it hard to get out correctly sometime. If anyone can fill in any blacks or correct me where I'm wrong that would be a great help.
Thanks again.

Can you do a drawing of what you describe as its pretty unclear what you mean.

Even configurating this in every way I can think of it does not stack up and won't work. But if you give me a drawing of exactly what you mean I'll be able to explain exactly why.

moan 77

I'll try a post a drawing of what I mean, I hope it will be a bit clearer.

Condenser

moan 77 wrote: »

I'll try a post a drawing of what I mean, I hope it will be a bit clearer.

If you can label the tanks as Tank A and Tank B and explain the process you had in mind using both those to describe your water flow I'll then try to explain the process in the same terms

moan 77

I think I've attached a drawing, let me know what you think of my lay-out.

Condenser

Ok, heres your problem, and we have to go through the basics of refrigeration and heatpumps to explain why it won't work.

A heat pump or any piece of refrigeration equipments heat output is made up of two basic constituents 1. The extracted energy on the cooling side, plus 2. the Electrical energy used to drive the compressor. So for example a 12kw heat pump with a COP of 4 will break down as 9kw of energy extracted on the cooling side plus 3kw electrical load.

In your idea you would be extracting that 9kw from the ice bank, which would have to be constantly replenished at a rate of 9kw per hour so as not to drop the tank temp and drop COP (3% each degree). I very much doubt the solar panels could do that at full throttle and certainly not at dull days or at night.

Drawing back from the buffer is not an option either as you will be using the energy you just created to replenish your ice bank, so you'll either have no heat for the ufh or you won't be replenishing your ice bank. The only additional energy would be the electrical energy added by the compressor and at that stage it would make more sense to run an immersion as you'd put 3kw in and get 3kw out.

Alot of people assume that a 300L buffer or a 600L buffer are quite substantial, they're actually quite tiny in heat storage terms. The HP would reduce the temp of the ice bank in just a few mins of running, so unfortunately for you the idea isn't a runner.

Carlow52

Condenser,
In fairness to Moan 69's ideas I think it behooves you to explain a bit further why the system wont work.


I don't doubt for a moment that you 'know your stuff' here but you need to consider the audience.

Given that the solar heat from a revenue cost perspective [as opposed to a Capital cost] is essentially free heat, and collecting it from the panels seems more efficient than letting the sun heat the soil in a ground source system, the presence of 'free' energy as defined must be of value here.

It strikes me that the size pf the ice bank may be the issue here because the little I know about heat pumps is that the closer the input and out put temperatures are the greater the COP.

What if the OP was to have a larger heat store of low grade heat as opposed to his 600 litre tank?

For his underfloor heating say the optimum temperature is 35 degrees C and the COP of his heat pump is 4, what is the ideal temp of his heat store?

Condenser

Carlow52 wrote: »

Condenser,
In fairness to Moan 69's ideas I think it behooves you to explain a bit further why the system wont work.


I don't doubt for a moment that you 'know your stuff' here but you need to consider the audience.

Given that the solar heat from a revenue cost perspective [as opposed to a Capital cost] is essentially free heat, and collecting it from the panels seems more efficient than letting the sun heat the soil in a ground source system, the presence of 'free' energy as defined must be of value here.

It strikes me that the size pf the ice bank may be the issue here because the little I know about heat pumps is that the closer the input and out put temperatures are the greater the COP.

What if the OP was to have a larger heat store of low grade heat as opposed to his 600 litre tank?

For his underfloor heating say the optimum temperature is 35 degrees C and the COP of his heat pump is 4, what is the ideal temp of his heat store?

Sorry Carlow, I was keeping it as basic as I could and felt I explained the reasons why.

The size of the collector (ice bank) isn't the issue. Its the rate its replenished at. Even if it was 10 times the size it would need a method to replenish the energy at a rate not far behind that which you can extract it.

You can also not raise the temp of this tank too high as you will create problems too. The rate of energy extraction from the tank would need to be fairly stable, so having the tank at 20C at one stage and 10C at another will not work as this will affect the refrigerant cycle adversely and move outside the optimum conditions for the evaporator. The only way to overcome this would be to have a variable speed pump that would control the flow through the evaporator to match various temps, so as to consistently supply the evaporator with the amount of kw's it needs. (The evaporator is the cooling side and heat extraction area of the refrigeration circuit)

On top of that the temp of the ice bank would not be able to exceed 20C as this can lead to further problems such as over superheating the refrigerant being drawn back into the compressor, this will remove the cooling effect it has on the compressor and lead to burnout. And while your point about the evaporating and condensing pressures being closer and achieving a higher COP are true to a point, there is a point where pressures become too close, affect compressor performance and can also lead to failure.

All in all the cost of digging an area and laying some pipework is far out weighed by the cost of solar on the scale required to match the heat extraction requirements and thats before you go into problems relating to long winter nights, snow, below freezing temps etc.

I know that may be technical but there really is no way of making it less technical than that.

moan 77

Hi condenser, firstly are you involved in the sale or installation of heat pump of any kind.
I agree with some of your points, but to say 600 liters of water can be brought down in temp in a few minutes, I would question where you've based this on. I do agree the tank temp should not be excessive but I worked on milk tanks and beer chillers with high ambient starting points +30 and they've taking longer than a few minutes to come down in temp.

As for ground collectors when you start extracting heat from the ground it starts to freeze and if it's freezing above, below and to the sides, I wonder if the ground will be able to replace the heat load required, as for air sourced heat pump units once temp drops below 0c they will struggle.
Can I ask you what is the kind of temperatures 2 x 3 sq of evcauted tube solar panels would heat water too in the spring then in summer, autum and finally winter.

Condenser

I am, but first and foremost I'm a refrigeration engineer and any views I put forward on this board are from a purely application and efficiency point of view. If you doubt the impartiality of my views I have no problem stepping out.

On your points. Lets stick with the 12kw example. To have sufficient energy coming to the evaporator you'll need about 2,500 litres per hour of water at 10C just to remain constantly at 9kw. This water will return to the tank at about 7C. The rate of extraction and the and the return water temp will ensure the tank drops quite rapidly. It only has to drop 5C to drasticly change the set up the expansion valve etc requires.

Beer coolers and Milk tanks are another matter. Firstly, their primary purpose is to reduce a product to a certain temp and maintain within a 2/3C range. No new substantial energy is added to the process at any point besides the intial period and in the case of milk it has 3hrs to reduce this in temp. The condensing (heat rejection side of the system) is not hindered in any way. They can dump heat to air as required and control condensing temp by cycling on and off fan motors.

As for ground source units, the ground does not begin to freeze unless the collector is undersized. If you extract too much energy per linear meter or per sqm the ground will freeze but thats down to poor design. Air source heatpumps drop in energy output by 3% for every degree dropped. Some are worse than others though as the outdoor units are undersized, with fins that are too close together and therefore frost over much more quickly needing more defrosting.

I'm not going to pretend to be an expert on solar because I'm not and there are others far more qualified here to answer that than I. I will make the points though that a. The temp they can attain doesn't matter, its the energy output they can achieve and b. the output when theres snow or at night will be next to nothing and there is no alternative place to turn for energy in the system you propose.

moan 77

Hi Condenser,

I don't doubt you or your information for one minute, I'm glad that they are like minded people on boards, that is why I'm here. I will ask the question on temp ranges in the different seasons and the kw the panels can supply in the same seasons, I understand what your saying, but I've seen some horror story's with ground and air sourced heat pumps, my heat load would be in the region of 6kw for the house but I'm waiting for my heating expert to come back to me on this. The house is 250 sq meters with 2 x adults and three teenagers, Underfloor on both floors with each room on it's own stat, the house is quite airtight and very well insulated. I plan to use a scroll made in N. Ireland but its has ecomiser fitted like an inverter. I'll wait till I have all the required info before going ahead but as it's quite at the moment I might use it as a project for my apprentices.

Thanks again for you input and if you have any more to add or clarify just post away.

Condenser

If i was you I'd play around in mini scale but no more than that, if nothing more than a education for your apprentices. I don't know which field you're in but they'll gain something out of watching how the system reacts anyway.
One other point, in your drawing you have the 300L buffer as the store for both DHW and ufh water. That is very poor practice and will automatically drop the COP of the HP by 40% and cut the lifetime of the system by at least half. You should always seperate the DHW and UFH water in to two seperate tanks if using a buffer.

moan 77

Hi Condenser, you were saying the following in your last post to me, as I don't know what you mean can you explain how I should have it done. I will have a 4 kw multi fuel stove, 6 sq meters of vacuum tubes and maybe a heat pump or a gas condensing boiler.

One other point, in your drawing you have the 300L buffer as the store for both DHW and ufh water. That is very poor practice and will automatically drop the COP of the HP by 40% and cut the lifetime of the system by at least half. You should always seperate the DHW and UFH water in to two seperate tanks if using a buffer.

Thank you for all your help to date.

Condenser

moan 77 wrote: »

Hi Condenser, you were saying the following in your last post to me, as I don't know what you mean can you explain how I should have it done. I will have a 4 kw multi fuel stove, 6 sq meters of vacuum tubes and maybe a heat pump or a gas condensing boiler.

One other point, in your drawing you have the 300L buffer as the store for both DHW and ufh water. That is very poor practice and will automatically drop the COP of the HP by 40% and cut the lifetime of the system by at least half. You should always seperate the DHW and UFH water in to two seperate tanks if using a buffer.

Thank you for all your help to date.

If you combine your buffer tank and dhw tank in one, the heatpump will need to produce water at 50 degrees plus consistently for that tank in order have sufficient hot water on hand at any stage it is required.
Your ufh should be operating between 30-35C. With a HP for every degree you increase the output temp the COP will drop 2%. That means that because you need to produce 50C constantly instead of the 30C you should need, that the COP will drop by about 40%. Which means poor efficiency and higher electric bills. It'll also mean your compressor will have about half the life it would otherwise have had.
To do it right you need to have the DHW in one tank and the water for the UFH in the other. That way you maximise your COP on the one thing you will be running 90% of the time to serve.

beyondpassive

If using a heat pump, you could consider recovering some of the large amount of solar heat that is dumped in summer by large low demand housing. You need about 14sq.m of solar thermal in one particular house that comes to mind. That would generate about 6000kWh more heat in the 3 summer months than you need for the house which might only have an annual space heat demand of 2000kWh for space heating and 3000kWh for water use for the entire year. By incorporating interseasonal stores such as stone, water or even phase change material (PCM) such as parafin or calcium chloride hydrates, you can store up to 50% of that heat under a highly insulated raft.

Of course as usual it is important to concentrate on reducing the heat demand first before bolting on expensive 'eco-bling' and the risk of using grid based electricty which is not robust enough to withstand the coming upheavals. The ideal system is one where the house can still be comfortable year round, even if the gas or electricity grid is down.