Thermo dynamics for hot water that runs off atmosphere

[steve k]

I have a refrigeration question as well, how does the solar panel heat pump prevent the superheat figures of the refrigeration system going off the chart on a hot day?

The gas isn't in the evaporator long enough for it to be affected by the outside temperature.
When I commissioned the system I first posted about the systems had a saturated suction temperature of -12 deg C and a pipe temp at compressor inlet of 4 deg C, giving a compressor superheat of 16k. I didn't measure the evaporater superheat as the pipe run was so short.
I would not expect this to change whatever the weather / ambient conditions.

**Edit**

Colder outside temps could cause more problems for the refrigeration system than the hot. This is due the possibility of un-evaporated liquid refrigerant finding it's way back to the compressor.
To get around this possible problem, the systems I have installed have the suction line coiled around the compressor to boil off any remaining liquid refrigerant.

Steve

Steve

 

[steve k]

a pic to illustrate

Steve

 

[steve k]

how is this equipment going to avoid icing and what defrost cycle is available?

It can't....if you were to incorporate a hot gas defrost system into this, as in a commercial freezer, the refrigerant would shoot through the evaporator at about 70 deg C, the same as the hot surface temperature discussed in another post at a pressure of appx 300psi.

I have pressure tested the evaporators at 250psi with no ill effects.

Steve

 

[The Solar King]

So running at sub zero ambient temperatures, the panel will ice up?

 

[steve k]

60-70 deg on the evaporator is very very high compared to standard refrigeration systems. The negative effect this will have is on the compressor, not the refrigerant itself. I would have grave concerns about the the long term reliability of this system.

The compressor regularly deals with temperatures like these, it's what they do. Compress low (temp)pressure gas into high (temp)pressure gas.
It is this hot gas that heats up the water.

Steve

 

[steve k]

So running at sub zero ambient temperatures, the panel will ice up?

I have seen them icing up at 6 deg C ambient. Remember the gas in the evaporator is at -12 deg C.

 

[Gavin A]

60-70 deg on the evaporator is very very high compared to standard refrigeration systems. The negative effect this will have is on the compressor, not the refrigerant itself. I would have grave concerns about the the long term reliability of this system.

that would be stagnation temperatures though, ie when the compressor wasn't running, and that would be extreme top end. tbh I've never known PV panels to get anything like that hot in the UK, usually more like 50-55 deg top end due to the air cooling which this will also have. Thinking about it some more, I'd expect these panels would stagnate at a lower temperature than that due to it's greater surface area (fins) and conductivity to the rear of the panel for greater cooling of the rear (back of a PV panel can be around 35deg while the front is at 50 due to the poor conductivity, I'd expect this to be far more even in an aluminium panel, so the heat loss to air on the back would be that much greater).

In operation I'd be surprised if it ever went much above 25-30 deg flow temperature into the compressor even in full sun, though this depends on the flow rate and return temperature.

 

[The Solar King]

Steve
Thanks for your knowledge and honesty in these replies. This is really refreshing in trying to get to the bottom of the efficacy of this equipment. From your reply, does in your opinion this mean there is a minimum ambient temperature at which this equipment should operate? Is there a point at which there is no energy gain for the energy input to run the system? I had been told the COP effectively fell to one at around this temperature, which would mean an auxiliary back up source of water heating would then be more effective if in terms of energy cost (eg gas) if it was cheaper than electricity.

 

[steve k]

In operation I'd be surprised if it ever went much above 25-30 deg flow temperature into the compressor even in full sun, though this depends on the flow rate and return temperature.

If you ever saw this kind of temp flowing into the compressor, you need to call a fridgeman. Your expansion valve is knackered.....

 

[The Solar King]

ST panels get way hotter than this during stagnation, hence the avoidance of soldered joints. Temperatures can exceed 180degC.

 

[steve k]

From your reply, does in your opinion this mean there is a minimum ambient temperature at which this equipment should operate?

R134a boils at -26 deg C, given that you need appx 10k superheat to get a refrigeration effect, this would suggest that the systems will function at -15 degC ambient

 

[Gavin A]

Its is good to see some more technical discussion of this technology, even without firm performance data. At the other end of the scale, how is this equipment going to avoid icing and what defrost cycle is available?

Actually I reckon that where used for water heating only at least it doesn't need a defrost cycle, as it's probably simpler and more cost effective to just switch to immersion if required once the COP becomes lower than 1:1, then rely on the sun to defrost the panel - this is one advantage these systems have over ASHP where the sun will never defrost the ASHP while outdoor air temps are low, whereas I'd expect the sun would be able to defrost these panels fairly quickly even in -10 ambient air temperatures, which I've certainly seen happening with PV panels.

The running costs element is critical to its viability. 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.

There is a lot of hard work going on with DECC to make normal ST viable under the proposals for the RHI.

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.

I think the one thing the opinions expressed in this thread shows is the true market for appropriate use of this equipment IF it's performance can be validated is very very small.

I agree that it should be a niche product,but I'm not sure I'd agree on the size of the potential niche being 'very small'.

all new houses by 2016 ought to have the sort of low energy requirements that would make this sort of technology potentially viable for both heating and hot water purposes as long as the government doesn't water down it's plans any further (ok, maybe only the smaller houses / flats, with ASHP being more suited for larger houses). That's a 100,000 houses a year potential market for both water and space heating, or for water heating alone.

There's also around 500,000 homes that are all electric heated, where this would almost certainly be a cost effective alternative for the water heating side of things at least (if installed at a reasonable price), and surprisingly, that number has been risking over recent years due mainly to the number of flats / 'studio apartments' being built with such low heat demands that they're opting for all electric heating to save the expense of a wet gas heating system. I'd think that many of these stuido apartments could well be candidates for themodynamic panels to be viable cost effective alternatives for both water and space heating, where there's only 1-2 rads and low heat requirements due to only 1-2 external surfaces with reasonably high insulation values.

I'd also think this could be a particularly viable option for water heating only for those with PV installed where the running costs could be close to zero if the water is set to heat only during the day.

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.

 

[Gavin A]

ST panels get way hotter than this during stagnation, hence the avoidance of soldered joints. Temperatures can exceed 180degC.

yes, but they are insulated from the air, this panel is specifically not insulated at all, quite the opposite, it's highly conductive to air as it's entirely made of aluminium.

 

[Gavin A]

If you ever saw this kind of temp flowing into the compressor, you need to call a fridgeman. Your expansion valve is knackered.....

oh erm, right, sorry I've not hugely looked into the exact functioning of the heat pump side of things, tbh I don't think I've seen a proper diagram of how they're laid out, and I've not actually worked with heatpumps (at least not the internal side of them).

I'll bow to your knowledge on that side of things as you're actually installing the things from the looks of it.

Do you have any data on what the operating temperatures of flow and return from the panel actually are in operation in full sunlight? Knowing this would help greatly to understand the likely efficiency of the panel in terms of absorbing heat from sunlight vs losing a proportion of that heat to the atmosphere if the panel's operating hotter than the surrounding air. If the panel is operating at lower temperatures than the surrounding air even in full sunlight, then the panel should be in the region of 90-95% efficient at the conversion of radiant solar energy into heat depending on it's absorption factor / efficiency.

I'd generally been assuming they'd lose some of this heat to the air, but looking at your figures earlier, it looks fairly obvious that they must actually still be absorbing heat from the outside air as well, which means they must be operating below ambient air temperatures, and therefore not be losing solar energy to the air at all.

In which case my assumptions behind my guess of maximum operating temperature are obviously way off as this would be below the maximum ambient air temperature not above it.

 

[jason121]

Your missing the point of heat pump operation they dont use the sun the defrost, they use hot gas to defrost and the buffer tank will aid the heating system while they defrost.