Enginerd
27-05-2007, 05:51 AM
Hello All,
I want to design an extremely efficient geothermal heat pump system for my residence. I think I have quite a bit worked out, but I still have a few questions. Any input is welcome---including criticism.
First, I'm not a refrigeration engineer, tech, etc. I design electronic stuff. As I researched high efficiency options for cooling/heating my home I became more fascinated with the refrigeration cycle. I also came to the conclusion that packaged water-source heat pumps are designed for broad operating conditions limiting their efficiency for a given application.
I'll keep my day job, but this stuff is cool! Just when I think I have my arms around it, a new layer is revealed prompting more research. I've been on this task for about a month and this forum has been a gold mine---not to mention that the posters seem to be genuinely nice and knowledgeable people.
This project is not about saving money on the installation. It is about learning something new while trying to meet the challenge of lofty design criteria.
Climate
I live in central Texas where we have a design wet bulb of 79F. We have 3175 cooling degree-days and 1586 heating degree-days. For this reason I am mostly concerned with optimizing the system for best cooling efficiency.
Design Parameters
Water-to-water heat pump
K5 scroll compressor using R410A
78F entering water-source temp at 3.5GPM/ton
45F evaporating temp
Low as possible condensing temp
Short refrigerant circuit-- the compressor, condenser and evaporator are packaged close to each other.
No reversing valve in the refrigerant circuit. The evaporator and condenser heat exchangers never swap functions. The "reversing" occurs via valves in the water lines.Maximize Cooling COP
When I first looked into maximizing COP I thought the secret was subcooling---lots of it. Then I learned that the p-h chart unveils the real culprit, compression ratio. So, to lower the compression ratio for a given evaporator temperature the condensing temperature must be lowered. Having said that, I know that some minimum amount of subcooling is required to insure 100% liquid to the TXV---more on that later.
As I understand it, to get the lowest possible condensing temperature I need to maximize the condensing surface area of the heat exchanger. In a typical condensing coil the top portion of the coil de-superheats the gas, the bottom area of the coil provides some amount of liquid subcooling, and condensing occurs in what's left in the middle.
Unless I'm already off base this is where my question comes in.
What is the best water-source heat exchanger arrangement for minimum condensing temperature?
1) One gigantic heat exchanger (HE) to insure plenty of condensing area?
2) Two properly sized HEs: one for de-superheating and condensing with a separate heat exchanger for subcooling/receiving to prevent flooding of the condensing HE?
3) Three HEs: one each for de-superheating, condensing and subcooling?
4) De-superheating via liquid refrigerant injection into the hot gas using arrangements 1 or 2 above.
Subcooling
For the sake of discussion, let's assume that I am able to get the approach temperature of the water source and condensing temperatures near zero. I would have no means to further subcool the liquid refrigerant via the water source, which would be bad for TXV performance.
Enter alternate subcooling techniques...
1) Use a liquid refrigerant pump (increased pressure at constant temp = subcooling)
2) Use a small heat exchanger between the output of the evaporator and the liquid line to the TXV (decreased temperature at constant pressure = subcooling)
Liquid Refrigerant Pump
This appears to be the easy way to do it. I am guaranteed enough subcooling at the expense of pumping energy. Another potential benefit is the ability to divert a fraction of this pump's output into the hot gas before it enters the condensing heat exchanger thereby quickly removing superheat and allowing more of the heat exchanger's surface to be used for condensing.
Suction Return Heat Exchanger
This is where I start to loose it (if I haven't already). On the surface it seems free since (I think) any enthalpy removed from the liquid line is transferred to the suction line. If this is true then all I have to worry about is maintaining enough pressure differential across the TXV? If the differential is marginal I could use an EEV in place of the TXV.
If I used this technique wouldn't it be best to place the TXV bulb on the suction line on the output side of the heat exchanger rather than the output of the evaporator? Assuming the net enthalpy gain/loss is zero, then maintaining ~5F superheat at the output of the subcooling heat exchanger means that more of the evaporator is wet --right? If so, doesn't this increase the capacity of the evaporator if not the system efficiency?
I have more questions about other areas of possible improvement, but I'll make those separate posts.
Thank you for your comments!
Ray
I want to design an extremely efficient geothermal heat pump system for my residence. I think I have quite a bit worked out, but I still have a few questions. Any input is welcome---including criticism.
First, I'm not a refrigeration engineer, tech, etc. I design electronic stuff. As I researched high efficiency options for cooling/heating my home I became more fascinated with the refrigeration cycle. I also came to the conclusion that packaged water-source heat pumps are designed for broad operating conditions limiting their efficiency for a given application.
I'll keep my day job, but this stuff is cool! Just when I think I have my arms around it, a new layer is revealed prompting more research. I've been on this task for about a month and this forum has been a gold mine---not to mention that the posters seem to be genuinely nice and knowledgeable people.
This project is not about saving money on the installation. It is about learning something new while trying to meet the challenge of lofty design criteria.
Climate
I live in central Texas where we have a design wet bulb of 79F. We have 3175 cooling degree-days and 1586 heating degree-days. For this reason I am mostly concerned with optimizing the system for best cooling efficiency.
Design Parameters
Water-to-water heat pump
K5 scroll compressor using R410A
78F entering water-source temp at 3.5GPM/ton
45F evaporating temp
Low as possible condensing temp
Short refrigerant circuit-- the compressor, condenser and evaporator are packaged close to each other.
No reversing valve in the refrigerant circuit. The evaporator and condenser heat exchangers never swap functions. The "reversing" occurs via valves in the water lines.Maximize Cooling COP
When I first looked into maximizing COP I thought the secret was subcooling---lots of it. Then I learned that the p-h chart unveils the real culprit, compression ratio. So, to lower the compression ratio for a given evaporator temperature the condensing temperature must be lowered. Having said that, I know that some minimum amount of subcooling is required to insure 100% liquid to the TXV---more on that later.
As I understand it, to get the lowest possible condensing temperature I need to maximize the condensing surface area of the heat exchanger. In a typical condensing coil the top portion of the coil de-superheats the gas, the bottom area of the coil provides some amount of liquid subcooling, and condensing occurs in what's left in the middle.
Unless I'm already off base this is where my question comes in.
What is the best water-source heat exchanger arrangement for minimum condensing temperature?
1) One gigantic heat exchanger (HE) to insure plenty of condensing area?
2) Two properly sized HEs: one for de-superheating and condensing with a separate heat exchanger for subcooling/receiving to prevent flooding of the condensing HE?
3) Three HEs: one each for de-superheating, condensing and subcooling?
4) De-superheating via liquid refrigerant injection into the hot gas using arrangements 1 or 2 above.
Subcooling
For the sake of discussion, let's assume that I am able to get the approach temperature of the water source and condensing temperatures near zero. I would have no means to further subcool the liquid refrigerant via the water source, which would be bad for TXV performance.
Enter alternate subcooling techniques...
1) Use a liquid refrigerant pump (increased pressure at constant temp = subcooling)
2) Use a small heat exchanger between the output of the evaporator and the liquid line to the TXV (decreased temperature at constant pressure = subcooling)
Liquid Refrigerant Pump
This appears to be the easy way to do it. I am guaranteed enough subcooling at the expense of pumping energy. Another potential benefit is the ability to divert a fraction of this pump's output into the hot gas before it enters the condensing heat exchanger thereby quickly removing superheat and allowing more of the heat exchanger's surface to be used for condensing.
Suction Return Heat Exchanger
This is where I start to loose it (if I haven't already). On the surface it seems free since (I think) any enthalpy removed from the liquid line is transferred to the suction line. If this is true then all I have to worry about is maintaining enough pressure differential across the TXV? If the differential is marginal I could use an EEV in place of the TXV.
If I used this technique wouldn't it be best to place the TXV bulb on the suction line on the output side of the heat exchanger rather than the output of the evaporator? Assuming the net enthalpy gain/loss is zero, then maintaining ~5F superheat at the output of the subcooling heat exchanger means that more of the evaporator is wet --right? If so, doesn't this increase the capacity of the evaporator if not the system efficiency?
I have more questions about other areas of possible improvement, but I'll make those separate posts.
Thank you for your comments!
Ray