Thermo dynamics for hot water that runs off atmosphere

[steve k]

The heat is magicked from the compressor, from the heat of compression. Imagine a bicycle pump, why do these get hot in use? kinetic energy to heat enegry.
All the evaporator plate/panal is for is to allow the refrigerant to boil and change state from a liquid to a gas, so that the vapour compression cycle can start again.

The gas does not care weather it is 10 deg C, 50 deg C or -10 deg C as long as it can change state in there.

I will try to simplify Jason's explanation:

When a fridge (which is essentially what these are) is switched off the gas in side them will be in a saturated condition; part liquid, part gas.
When we switch it on here's what happens, the compressor starts and does what it does i.e. compresses the gas in the system, as the gas is compressed it heats up and comes out of the compressor in the hot gas discharge line. It is this hot gas line that wraps around the cylinder full of cold water, as the heat flows from the hot gas to the cold water obviously the gas cools down, and then condenses from a gas into a liquid.
The trouble now is that we have liquid refrigerant that we need to turn back into a gas so that it can be compressed again. Then we can utilize the heat of compression to heat our bloody water up. Are you all with me so far???

To do this we send the liquid refrigerant through an expansion valve that will allow the liquid to expand and lower its pressure (think spraying mister muscle in the kitchen) this happens just before the evaporator (the panel), now that the liquid refrigerant is in small particles, like a mist, it will now boil/evaporate easily back into a gas using any heat around as long as it is above the gases boiling point of -26 deg C.

The process can start again..

It's all clear in my head:sifone:, I hope it will help someone understand it all a bit better.

 

[Gavin A]

ok, the heat is effectively created by the compressor, but the energy isn't.

there is the same energy difference in joules between the incoming and outgoing gasses to the energie unit from the panels as there is between the incoming and outgoing heating fluid to the tank plus part of the electrical input to the compressor.

but that's clear from what I originally wrote anyway, in terms of the compressor concentrating low grade heat energy into higher grade heat energy.

and of course it matters what temperature the incoming gas is at, as this dictates how much energy is available - why else would the efficiency and heat output from heat pumps rise with temperature of the incoming gasses?

the worrying thing to me about this conversation is that both of you install heatpumps, and I don't yet.

 

[Gavin A]

I have installed a few of these systems and these are the facts:

Ambient air temp - 6 deg C
Water entering temp - 8 deg C
200l of water 8 deg to 55 deg = 2 hours
amp draw = 9amps

Now i'm no mathematician, but what does this calculate into?

Steve

200l at a little under 50deg temp rise needs around 11kWh of energy input.

9amps x 230 v = 2kW x 2 hours = 4kWh

COP = 11 / 4 = 2.75

a COP of 2.75 at 6 deg c ambient air temperature is reasonable, but not particularly impressive for a heat pump tbh, though it at least demonstrates that it's considerably better than an immersion heater, and maybe just about on a par with a condensing gas boiler.

So possibly worth it for those on full electric heating.

something has been bugging me about these figures, and I've just worked out what it is.

These are the ratings for the 200l energie unit

electrical input 390 - 550Wpower output 1690 - 2900W
anti legionella heater 1200W

assuming you were using the energie system, it's pretty clear from these figures that you must have had the 1200W immersion switched on when you were heating the system up from cold, as there's no other way you could have got anything like 5.5kW heat output from it, or 2kW electrical input. I'd been thinking the electrical input figures seemed extremely high.

http://www.thermogroupuk.com/thermogroup_pdfs/TDY_ECO_Data_Sheets_v1.0.pdf

 

[steve k]

the worrying thing to me about this conversation is that both of you install heatpumps, and I don't yet.

Why? I am an air conditioning engineer and have been installing "heat pumps" for 25 years>

assuming you were using the energie system

You have assumed incorrectly, perhaps the ones we installed have a larger compressor and a more efficient heat exchanger.

Steve

 

[Gavin A]

Why? I am an air conditioning engineer and have been installing "heat pumps" for 25 years>

have a read through Jason's posts and your recent post then, and the answer should be obvious. You don't seem to be understanding, or at least you're not describing very well where the actual energy input originates in these systems, or allocating any importance to it.

it will now boil/evaporate easily back into a gas using any heat around as long as it is above the gases boiling point of -26 deg C.

yes, this is where the actual additional energy input into the entire system (over the electrical input) comes from, not from the compression / expansion cycle itself. This energy input to the otherwise closed cycle* of the heatpump comes from the panel collecting energy in the form of heat from both the air and the solar radiation, and is directly proportional to the actual heat energy output from the other side of the compressor as is clearly shown in the graph posted.

You have assumed incorrectly, perhaps the ones we installed have a larger compressor and a more efficient heat exchanger.

Steve

fair enough.



*well, not entirely closed as the energy is obviously being extracted at the other side into the hot water tank, which balances out the heat energy input from the panel + the most of electrical energy input.

 

[steve k]

I am lost for words so I shall gracefully bow out of this discussion and look forward to reading it instead. I will also carry on installing these things without worrying too much about the if's, but's and why's of a simple fridge that simply works..

Steve

 

[The Solar King]

Normal Solar Thermal has zero costs for energy production and pretty low running costs - a fluid change every five years and a pump if you are unlucky during it's lifetime.

not entirely true - the electricity to power the pump can easily cost £5-10 a year depending on the set up, efficiency of the pump etc. Unless it's a PV powered pump, or I guess if the customer also has solar PV installed.

The pump on a solar thermal system will not operate before there is sufficient temperature differential to ensure that more energy is being transferred into the hot water cylinder than the pump is consuming giving a permanent net gain. How you account this will depend on the calculation used for the performance of the ST system. It should be showing the net energy provided.

There is a lot of hard work going on with DECC to make normal ST viable under the proposals for the RHI. If this is successful and IF the questions raised about thermo dynamic equipment can be answered by the manufacturers, then at some stage it may be included. However, the complexities currently involved in the RHI consultation over issues such as bi-valency, and issues surrounding what this technology is finally defined as may still exclude it.

snigger... the same numpties who killed the solar thermal industry immediately the tory government got into power by pulling the plug on the previous grant scheme then taking 3 years to fail to implement RHI? That's not my definition of hard work to make ST viable, quite the opposite, but yes maybe they will eventually come up with something that makes it viable again.

​

Anyone who has bothered to actively engage in the consultation process and negotiation surrounding the domestic RHI would find this comment somewhat at odds with reality. Patrick Allcorn of DECC who is lead on the domestic RHI is certainly no numpty, far from it, and has only had responsibility since earlier this year. He is doing everything he can to drive the process to make the RHI a success. This is a hugely complex piece of work covering different technologies. It is confronted by several policy constraints that make this a daunting task. If there is a stumbling block, it will be the Treasury.

He is not the only person working extremely hard. Interested trade bodies have been walking the extra mile in consultations and negotiations to provide evidence based information, and solutions to issues raised. This is especially so for solar thermal for which the impact assessment shows a zero uptake under the initial proposals. I am hopeful of a positive outcome as DECC acknowledge the position outlined in the consultation document is not that desired.

With regard to market size, yes it will be niche and therefore very small. The bottom line is how much it costs to run for the energy produced. We still do not know this. Hot water is the one energy element in properties that cannot be insulated away. When costs are known a fair comparison can be made between technologies. My money is still on traditional ST. There are other technologies around that are developing fast (and cost effectively) such as waste water heat recovery that reduce daily requirements still further. This will also affect equipment choice in the future.

Use in conjunction with other technologies means it would need to to be compatible with a multi-coil thermal store or hot water storage vessel, not just a dedicated tank. If it is not compatible, you are possibly in to a two tank design, with all the space implications that entails.

that entirely misses the advantage these systems have over solar thermal in that they do not need a secondary heat source other than an immersion for the very few days of the year when the system can't operate due to extreme low temperatures. So no they don't need a second coil to be combined with another heat source​

Using an immersion as an auxiliary heat source may not be the most cost effective method of providing a secondary heat source. Until we have validated performance data we do not know how much 'very few days' is. It may be much more than anticipated. The question of how the system is controlled to satisfy part G of the Building Regs also need to be addressed.

One of the mistakes often made when assessing the suitability of a property for for renewables is a failure to take a holistic approach to total energy requirements due to the compartmentalisation of skills and registrations for differing technologies. There is also an inherent assumption that the equipment performs in a cost effective manner which we still do not know. One of the question sets within the RHI consultation is to do with bivalency. There is recognition that in some circumstances two or more technologies working in conjunction with each other are more effective than they are on their own. This could well be the case here. How technologies interact is important to getting the best from each.
​

 

[Gavin A]

I am lost for words so I shall gracefully bow out of this discussion and look forward to reading it instead. I will also carry on installing these things without worrying too much about the if's, but's and why's of a simple fridge that simply works..

Steve

oh don't be like that ffs.

I don't understand why it is even slightly controversial to say that the actual energy input into this system that gives a COP of over 1:1 is from the panel absorbing energy from the air and solar radiation.

yes the compressor then raises the temperature to something more useful, but the actual energy input itself comes from the panel absorbing heat from the air and solar radiation.

This should be a simple statement of fact, not the basis of a 2 page argument at the end of which a time served air conditioning engineer flounces from the thread.

 

[Gavin A]

The pump on a solar thermal system will not operate before there is sufficient temperature differential to ensure that more energy is being transferred into the hot water cylinder than the pump is consuming giving a permanent net gain. How you account this will depend on the calculation used for the performance of the ST system. It should be showing the net energy provided.​

that's not zero cost though is it.

Anyone who has bothered to actively engage in the consultation process and negotiation surrounding the domestic RHI would find this comment somewhat at odds with reality. Patrick Allcorn of DECC who is lead on the domestic RHI is certainly no numpty, far from it, and has only had responsibility since earlier this year. He is doing everything he can to drive the process to make the RHI a success. This is a hugely complex piece of work covering different technologies. It is confronted by several policy constraints that make this a daunting task. If there is a stumbling block, it will be the Treasury.

He is not the only person working extremely hard. Interested trade bodies have been walking the extra mile in consultations and negotiations to provide evidence based information, and solutions to issues raised. This is especially so for solar thermal for which the impact assessment shows a zero uptake under the initial proposals. I am hopeful of a positive outcome as DECC acknowledge the position outlined in the consultation document is not that desired.

I attempted to engage, but got no response at all to my email asking them to clarify exactly how they were planning to calculate the 7 year RHI payments, and asking for a worked example (actually I supplied them with 2 alternative worked examples for the 2 possible ways of interpreting it to make it easy for them), but yes I have completed the 75 consultation questions last night.

And anyone who's read the consultation document that took them 2.5 years to produce could only conclude that it was written by people who don't really know what they're doing, especially as it followed on from 2 years of the industry being told / hinted at that everything was going to have to be metered, then presumably as a result of the trials of the meters they seem to have changed their minds. What did they discover in the trials that wasn't obvious before them to anyone who knows anything about heat metering?

If they aren't clueless then they've spent a good 3 years doing a very good impression of an organisation that is, when they should have been rolling out a workable scheme instead.

I particularly liked the bit in the consultation where they blamed the economic climate for the reduction in domestic renewable heat installations since 2010, nicely ignoring the fact that the recession started in 2008, and the downturn in the renewable heat market only happened 2 years later when DECC pulled the LCBP funding a year early, then spent the next 2 years repeatedly delaying the launch of the RHI scheme, during which time they attempted to partially rectify their previous mistake by launching a temporary grant scheme to replace the one they'd killed 18 months earlier. But of course, it's the recession that was to blame not DECC.

 

[The Solar King]

There were a number of mistakes made in DECC in the past. Yes there are things written that niggle. Those who got right up their own backsides over heat metering have now 'left' DECC. It used to be that you did not need to be the sharpest knife in the drawer to work in DECC. The clever people went to DEFRA when the two departments were set up. Fortunately there have been some changes with the reduction in staff numbers.

The greatest shame is that Patrick Allcorn did not get the lead on this on day one. We then might have had a more workable and attractive non-domestic RHI. Yes, I do rate the guy, but do not envy him his task. If the RHI is not everything everyone wants, it will not be for lack of effort from those involved, but more the intervention of the treasury or political masters.

Sorry you did not get the information you sought. Sometimes it is finding the right person to give the answer. It is a pity you were unable to attend one of the road shows that DECC put on around the country. Patrick Allcorn would have been quite willing to provide the information you were looking for. He has been very accommodating in his approach. He genuinely wants as many as possible to respond to the consultation.

With luck, there may be a degree of separation between the RHI and the Green Deal. I get the feeling parts of DECC view GD as having a few shortcomings.

If it was how the 20 year payment was compressed in to seven, the calculation is based on a net present value discounted cash flow giving a 6 to 8% return. My own trade body worked this through to look at various scenarios and the optimum we can propose for ST based on the constraints facing DECC. (namely the cap imposed by off-shore wind). The original consultants report to DECC for the RHI suggested ST needed a tariff of around 95p/kWh! This is clearly bonkers and included a high level of so called barrier costs that may exist for other technologies but not ST. However 17.3p is way too low. If we could achieve a level equivalent to an up to 4kWp FIT over 7 years, ST will be viable.

There was an interesting meeting on deeming last week where a new appendix to SAP was presented which may be know as GDsap. Like RdSAP, it's a "front end" that goes onto SAP and modifies some of the inputs. In the case of GdSAP it takes real occupancy into account, and therefore should benefit Solar Thermal.

Hot water use is calculated based on what the occupants tell you about their hot water use, or if they can't tell you how many showers a day they have you calculate it based on the actual number of occupants (a bit like the new MCS).

Maybe we will finally get a hot water usage calc everyone agrees on and is used uniformly across technologies. Currently, how much hot water a household uses and at what temperature depends on which technology and MCS document is being used!

If you want a copy, please pm me.

 

[Gavin A]

There were a number of mistakes made in DECC in the past. Yes there are things written that niggle. Those who got right up their own backsides over heat metering have now 'left' DECC. It used to be that you did not need to be the sharpest knife in the drawer to work in DECC. The clever people went to DEFRA when the two departments were set up. Fortunately there have been some changes with the reduction in staff numbers.

I hope they rot on the dole, like those they've forced onto the dole with their ignorant policies and years of procrastination. If the idiots truly have gone then that at least is something to be thankful for I suppose.

If it was how the 20 year payment was compressed in to seven, the calculation is based on a net present value discounted cash flow giving a 6 to 8% return. My own trade body worked this through to look at various scenarios and the optimum we can propose for ST based on the constraints facing DECC. (namely the cap imposed by off-shore wind). The original consultants report to DECC for the RHI suggested ST needed a tariff of around 95p/kWh! This is clearly bonkers and included a high level of so called barrier costs that may exist for other technologies but not ST. However 17.3p is way too low. If we could achieve a level equivalent to an up to 4kWp FIT over 7 years, ST will be viable.

I still have zero idea what this means in practice. Why DECC can't actually give worked examples is beyond me - if you do know for sure what they're actually proposing, I'd appreciate it if you'd tell me which of the following worked examples is correct.

so a standardish 5m2 of solar thermal panels generating 868kWh* a year get's 17.3p x 868kWh = £150 a year for 7 years = £1050

or does it get 20 years worth of heat output at 868kWh a year x 17.3p per kWh = £3003.28 total split over 7 years = £429 per year for 7 years

There was an interesting meeting on deeming last week where a new appendix to SAP was presented which may be know as GDsap. Like RdSAP, it's a "front end" that goes onto SAP and modifies some of the inputs. In the case of GdSAP it takes real occupancy into account, and therefore should benefit Solar Thermal.

Hot water use is calculated based on what the occupants tell you about their hot water use, or if they can't tell you how many showers a day they have you calculate it based on the actual number of occupants (a bit like the new MCS).

Maybe we will finally get a hot water usage calc everyone agrees on and is used uniformly across technologies. Currently, how much hot water a household uses and at what temperature depends on which technology and MCS document is being used!

If you want a copy, please pm me.

This about sums up the sort of idiocy and incompetence I'm referring to. How hard is it really to come up with an agreed common standard for how much hot water a house should be expected to use. This is not rocket science, it's basic stuff that should have been agreed years ago. I do include BRE in my bunch of incompetent numpties assessment btw for coming up with sap assessments based on floor area instead of anything actually related to likely water use such as number of bedrooms, bathrooms, occupancy levels etc. SAP has obviously never been fit for purpose for solar PV or solar water heating purposes (amongst other things), yet instead of starting from scratch and coming up with something sensible all we get are slightly rehashed versions of the same outdated / wrong methodology.

If they are now finally going to sort that out then that at least can be one item I'll remove from my list of complaints, but for every day they dither more good people end up on the dole as a direct cause and effect to their dithering.

ps We've now lost more than half our staff as a direct result of DECC's FIT cuts policies combined with the RHI depts decision to postpone the launch of domestic RHI from this september, which we were expecting to cushion the impact of the August and October FIT cuts when they actually did the FIT consultation. So yes I'm extremely angry at the incompetence / negligence I've witnessed from all sides of DECC over the last 3 years - if I actually went to a meeting with them I doubt I'd be able to restrain myself from kicking the **** out of them tbh, which is partly why I haven't been to any meetings with them. I did go to one meeting with them prior to the first round of FIT cuts, and the arrogant, ignorant posh git from DECC at that nearly made me implode with fury. I'm generally not a violent man, but I'd make an exception for him.

 

[Gavin A]

sorry for the rant, but the impact of their policies is really coming to a head right now, and I suspect that's a fairly industry wide thing.

On a more positive note, from what you're saying it does sound like they actually might have someone in charge now who might be able to get this thing launched, so do you see the timetable within the RHI consultation as actually being something that's likely to happen - ie a spring launch for the legislation and RHI levels, and late summer / autumn launch for the scheme proper?

I'm glad it sounds like there have been some people / organisations working away in the background to try to sort this mess out and get something workable out of it, hopefully that will bear some fruits when the final scheme is announced.

 

[jason121]

oh don't be like that ffs.



yes the compressor then raises the temperature to something more useful, but the actual energy input itself comes from the panel absorbing heat from the air and solar radiation.

QUOTE]

And what happens to the heat at night time when no sun in the sky and its -5 outside ?????

 

[Gavin A]

And what happens to the heat at night time when no sun in the sky and its -5 outside ?????

that'd be it absorbing heat from the air without the solar radiation component, hence the difference in the daytime and night time energy output on the graph I posted.

At night it's just like an ASHP except it relies on natural air circulation over a large surface area instead of large volumes of air being forced by a fan over a smaller surface area.

I was thinking I might have somehow misinterpreted your postings, but I see I haven't.

 

[jason121]

and how does the system manage to keep the temp at 55 at night were is the heat coming from

 

[Gavin A]

and how does the system manage to keep the temp at 55 at night were is the heat coming from

As I said earlier, I think we're talking at cross purposes because of confusion over the difference between heat and temperature. The compressor creates the higher temperatures, but it doesn't create the heat energy itself* (as energy can't be created or destroyed, just changed from one form to another), this heat energy input originates from the panels, or more properly, from the absorption of energy from the air blowing over the panels and when available from radiated solar energy.

Understanding this matters because someone with some time on their hands (and probably a bit more data than I've found so far) could then use this to calculate the actual additional heat that can be expected to be produced over and above the electrical input from such a system given different levels of air temperature, wind speed and sunlight levels. As I keep saying, the heat exiting the compressor is directly proportional to the heat input to the panel, as is clearly shown on the graphs I posted.


*although the compressor does also directly converter electrical energy into heat, which is why I've repeatedly referred to the additional heat energy on top of the electrical input.

 

[jason121]

but were does the 55 degrees come from to heat the water on a cold night

 

[Gavin A]

ok, I'm getting a bit bored of this now. If you're not on a wind up then I suggest you go and learn how the ASHP technology you install works, as this works on the same principle just with the panel replacing the fan unit.

If you're asking how it heats the water to 55 when the air temperature is below its lowest operating temperatures then I'd expect it would use the inbuilt 1.2kW immersion, though I've not installed a unit and am not 100% sure of the control settings etc.

 

[jason121]

thermodynamic panels works on vapour compression cycle to create heat = heap pump.

 

[jason121]

This is why MCS have suspended them and classed them as a heat pump, heat pumps under the RHI must be able to provide space heating and hot water.
They can be used if filled with water/glycol mix and be registered under MCS (no idea how they work like that)

not disputing on the low COP but they are a heat pump just like Ground and air