Thermo dynamics for hot water that runs off atmosphere

[Gavin A]

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

they create heat do they?

best write a rebuttal paper to Einstein et al then as he and the whole of physics since have obviously got things wrong.

but yes they work on the vapour compression cycle and as such are heatpumps (and AFAIK nobody on this thread has disputed that), and as shown on this diagram, they absorb heat from the atmosphere in the liquid>vapour phase, which is then raised to a more useful temperature via the compressor.

 

[Gavin A]

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.

that though is a problem with RHI / MCS classifications, not the technology itself.

 

[jason121]

you have just repeated what Steve K said a while back.

 

[jason121]

it can draw heat both from the air and from solar radiation falling on the panel, an ASHP can only draw heat from the air,


You words not mine
This is partly true but again heat is from compressor not from panel like ST

 

[Gavin A]

it can draw heat both from the air and from solar radiation falling on the panel, an ASHP can only draw heat from the air,


You words not mine
This is partly true but again heat is from compressor not from panel like ST

which bit of that statement is false?

where does the heat come from intially? where does the heat come from to evaporate the liquid back to a gas again? It comes from the absorption of heat from the air and sun within the panel.

this is like pulling teeth.

 

[jason121]

yes but only requires a few degrees to boil the gas thats why thay can work at night and dont need a sunny day to reach 55

 

[Gavin A]

yes but only requires a few degrees to boil the gas thats why thay can work at night and dont need a sunny day to reach 55

so the statement is true then?

in which case why have you just spent several hours arguing against it?

Also, degrees are units of temperature not energy, and the boiling of that gas itself will absorb significant quantities of energy in the form of the latent heat of evaporation, which is then released on the condenser on the form of latent heat of condensation, so a few degrees of temperature change around the boiling point of the gas will require the absorption of much greater quantities of heat energy than warming the gas by a few more degrees.

To reiterate it, all the additional energy that comes out of the warm side of the heat pump (on top of the electrical energy) has first been absorbed from the air and sunlight (if available) in the panel, as shown in this diagram.

The compressor takes affect between points 1 and 2, raising the pressure and therefore the temperature of the system, but having no impact on the overall heat energy / entropy of the system. All the increase in heat energy / entropy within the system occurs between points 4>5>1, which in the energie system is in the panel.

eta - Either I or Wiki has got a bit mixed up about whether it's entropy or enthalpy along the bottom of this diagram.

This is also why this graph is as it is - it clearly shows the direct relationship between air temperature and the heat energy output from the other side of the heat pump. And in relation to your query, at -5 at night it will take roughly twice as long to raise the temperature of a full tank of water to 55deg as it will at 35deg outside air temperature in sunlight because the energy output from the system 3.5kW instead of 7.2kW.

There is something odd going on with the day time levels on that graph, as there is potentially 6.4kW solar radiation input to the 4 panels on that system at 1000W/m2, but yet the increased heat output vs no sun ranges from around 2kW at low ambient air temps to 1kW at high ambient air temperatures, so there's obviously something else going on within that system that limits the transfer of solar energy through the system as this is that is the opposite to what would be expected with a normal solar water heating panel. This was the sort of thing I was hoping I might have spent the last few hours discussing rather than the basics of how a heat pump works.

 

[Vegelen]

Energy transfer normally occurs from a hot source (B) to a colder source (A).

A heat pump does the opposite, transfering energy from a cold source (A) to a hotter source (B).

This can only happen once work is imparted on the system to help move energy from A to B.

If no work is done on the system, energy wouldn't flow from A to B, but from B to A until the temperatures between the 2 systems equalised.

A cold refrigerant liquid with a temperature substantially lower than the ambient air temperature outside the panel, enters the solar thermodynamic panel.

This cold refrigerant liquid extracts energy from the warmer ambient air surrounding the panel, causing it to change state.

This gas then enters a compressor (which does work on it) pressurising it, increasing its temperature.

The hot, pressurised gas then passes though a coil in a water filled tank.

The water inside the tank is colder than the gas in the coil, thus energy flows from the hot pressurised gas to the cold water.

Over time this incremental energy transfer from hot to cold, raises the tanks water's temperature.

This pressurised gas gets passed through an expansion valve, which reduces its temperature and pressure further, reverting the refrigerant back to its liquid state.

This liquid then gets passed into the solar thermodynamic panel and the cycle recommences.

An energy transfer is taking place from a cold source (ambient air) to a hotter source (water in the tank) by virtue of work being done on the system.

If the ambient air temperature drops to -5C, energy will still be transfered from the ambient air to the refrigerant liquid (at a far slower rate) because the ambient air is at a higher temperature than the refrigerant liquid it surrounds (-15C).

We think of air at -5C as having "little or no energy" but it has lots of energy relative to absolute zero.

Jason121 uses the word "heat", when he should be using the words energy / energy transfer.

 

[Marvo]

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.

The compressor cares what temperature the returning gas is, all compressors are designed to work within certain superheat ranges. Most commercial compressors rely on there being a small amount of saturated vapour returning back on the suction line for sub-cooling. If a solar panel in direct sun is being used as an evaporator how is it possible to control the system superheat and ensure the lifespan of the compressor?

 

[Gavin A]

There is something odd going on with the day time levels on that graph, as there is potentially 6.4kW solar radiation input to the 4 panels on that system at 1000W/m2, but yet the increased heat output vs no sun ranges from around 2kW at low ambient air temps to 1kW at high ambient air temperatures, so there's obviously something else going on within that system that limits the transfer of solar energy through the system as this is that is the opposite to what would be expected with a normal solar water heating panel.

I think I've now answered my own question.



I'll try to explain...

A is the maximum energy increase from air at -5 deg, assuming all liquid is vapourised. Note, no increase in temperature or pressure of the vapour
B is a rough estimate of the maximum energy increase from and air temperature of 30, and solar input. Note significant increase in both temperature and pressure.
C is the amount of additional pressure required from the compressor to raise the vapour to 55deg from point A
D is the amount of additional pressure required from the compressor to raise the vapour to 55deg from point B
E is the end of the condenser and point where the high pressure fluid passes through the expansion valve

The line at the top between D>C is effectively the additional heat energy output from the heat pump resulting from the additional energy input from the higher air temps and solar.

Now compare that diagram with this one

The top diagram explains (to me at least) why the additional solar input can only result in a relatively minor increase in actual energy output vs the available solar energy to the panels, and why the additional input from the solar decreases as the input from the air increases.

What it also shows though is that there is a significant additional benefit from the solar input though in terms of doing the majority of the work of the compressor by actively pressurising the vapour and raising it's temperature before it enters the comnpressor, resulting in the compressor needing much less energy input.

This explains the high top end COP figure claims for the Energie systems theoretically, which matches with their energy performance figures

Absorbed Power 0,9 - 1,8 kW ; Thermal Power 3,6 - 7,3 kW

The low end absorbed power figure of 0.9kW will be at the highest energy output figure of 7.3kW because much of the compressors work is being done by the additional energy input in the form of solar energy heating the vapour in a closed system and pressurising it.

7.3 / 0.9 = a max COP of 8.1, which actually does look plausible from the diagram above.



* I fully acknowledge that the diagram is very rough and ready, and I don't know what the exact figures are for the working pressures, what temperature the solar input will raise the panel to etc but I hope it works as an illustration of the principle on which these panels operate.

 

[Gavin A]

Actually thinking about it, this is probably a more accurate diagram, as the pressure would be pretty much equal across the low pressure side of the system so in full sunlight and high ambient air temps, rather than the pressure dropping all the way down then being raised up by the extra energy input it would simply not fall as far as it wouldn't need to fall that far in order to extract the heat from the air and solar input. It will effectively sit at a dynamic equilibrium point instead - point F on this diagram.



The energie system's main benefits over an ASHP then is that in full sunlight the system will be able to reach a dynamic equilibrium temperature that is almost at the ambient outside air temperature. This means the operating pressure on the low pressure side of the system is significantly raised, and the energy input required in the compressor can be as much as halved.

On top of this the actual energy output from the system can be increase by up to 12% at the top end in summer, and around 35% in the depths of winter.

These benefits would obviously be diminished for space heating purposes if they're largely not being used during the day, but they should be very real benefits for water heating systems as long as they're set to largely heat the water during daytime hours when they'll benefit the most from the solar input.

 

[Gavin A]

Those diagrams are a bit complex and took me a while to sus out, but for those not wanting to spend that time working it out, essentially the theoretical data seems to match well with the performance data supplied by Energie, so I see no reason to doubt the energie data I've given isn't accurate.

One note of caution though would be that the output relative to the air temperature alone will be far more dependent upon wind speeds than for standard fan driven ASHPs, as if the air isn't moving rapidly across the panel then it will become cooled by the panel, so the air surrounding the panel will be at a lower temperature than the actual ambient air temperature.

This is going to be one of the factors that makes it impossible for energie units to be tested under standard ASHP test procedures, as they need to be tested at different different combinations of air temps, wind speeds and solar input levels. This ought to be possible to do, but it could well be that the standard ASHP test labs aren't kitted out to do it - they'd probably need to use solar PV test labs to carry out the tests as they do have to simulate all those factors when testing solar PV panels under STC.

 

[The Solar King]

Gavin
re RHI.
The calculation of 17.3p/kWh over 7 years is equal to I think 8.3p over 20 years. The 8.3p relates to the subsidy level paid for offshore wind over 20 years which is the most expensive subsidy in ROCS. This gets referred to as the 'value for money' cap.

Clearly 17.3p times whatever figure of output you choose (I know my 4.4sqM system actually produces 1200kWh/a) does not provide a realistic incentive. Even if you factor in the savings relating to the efficiency of the back-up fuel used, (as the new MCS ST standard will), you still only get to a notional output with a gas boiler of 1600kWh/a.

One of the areas examined has been the additional energy savings generated by changing the hot water cylinder. Moving to a part L compliant cylinder adds a large additional saving due to the reduction in standing loss which is currently not accounted for when calculating the total benefits of ST. A tank scrappage scheme giving an upfront grant of £600.00 in addition to to 17.3p would give the equivalent of around 27p. This would have the benefit of an upfront payment in addition to the 17.3p. This is among our proposals.

The other side of this is deeming. How much energy does a system produce? It is OK having a decent tariff, but useless if it is deemed at too low a level. The methodology of MCS 3001 draft may form a basis coupled to an agreed daily hot water requirement, possibly GDSap.

With regard to not losing ones rag with DECC, after the FITs debacle this time last year, as a trade body, the STA decided to actively enter a constructive dialogue with them. Through our then chair, we had been at the forefront of challenging the decisions taken over FITs in the courts. However continuing a combative approach was not going to gain anything. It also meant DECC had not listened to sound advice in the past which could have avoided the worst of what happened over FITs. It is just as well we did. FITs was set to be capped on the same 'value for money' basis as already mentioned. Due to the tireless efforts of Ray Noble who I think had a camp bed in DECC, we ended up with the scheme now in place which is as good as could be hoped for. There have been extensive ongoing discussions over large scale PV. I am not close to this, but would surmise from recent press releases the lack of announcement on ROCs and those schemes at a lesser level is political and due to intense lobbying by the enemies of renewables. The subsidy level required for large scale PV is less than 2 ROCS and cheaper than onshore wind. The cost effectiveness of large scale PV, and associated developments starts to undermine the case and subsidies (sorry tax incentives) for unconventional gas.

What this dialogue does mean is there is a route through our expert working groups to engage on all policy and technical fronts with DECC, Gemserve, BRE, Government ministers, Politicians and whoever else we need to address. Everyone has the same frustrations, and many share your views on DECC's past performance. There is also now a much better understanding of the constraints under which DECC operates which allows work within the framework of the possible.

That we have this dialogue has been hugely helpful with regard to the domestic RHI. We are after all the experts in the field (as are those in trade bodies representing other technologies).

The expertise so readily shown in this forum by several contributors would be more than welcome within the STA and could then feed directly in to the work on both policy and technical standards.

If anyone wants to know more, please PM me or talk to our CEO Paul Barwell. Contact details are on the STA website.

 

[Gavin A]

I was a member of BPVA. I quit following their failure to notify their membership about the G83/2 changes or do anything to assist me in raising this point with Ofgem despite me asking them to do so a month before the deadline. In the end, I and others from on this and other forums formed virtually all, if not all of the objections to the policy relating to close geographic area defintion and the requirements for stage 2 applications (no STA involvement either as far as I could see), and only as a result of our intervention we've actually got that policy changed to something that is eminently more workable.

SO I'm not against productive involvement in the decision making process, and if STA are open to actually taking advice from and taking up initiatives proposed by their membership, and are actively engaging in the decision making processes, then I would consider joining, as long as I don't get sat next to that stuck up ----- from DECC at a meeting.

 

[The Solar King]

Gavin

You don't even have to sit next to me!!