Why don't air cooled chillers operate as a heat pump? 2

[greenen]

Hi all,

I am a new junior engineer and have a question about air cooled chillers and heat pumps. Basically, why don't manufactures offer air cooled chillers that can heat the building water as well as chill it for air conditioning purposes?

From what I know about heat pumps is that they are simply air conditioners that operate in reverse. In heating mode, they cool the outside air and reject the heat indoors? Why can't a chiller act in the same way? Is it a case that a chiller could operate as a heat pump but some sort of efficiency barrier makes it impractical?

Thank you for your replies.

 

[LittleInch]

Mainly cost and operation.

To get your chiller system to heat water instead of air, you need a different heat exchanger. However unless you have a steady stream of cool water that needs heating, your chiller won't work very well or at all. Generally if its hot weather, the hot water demand on a building is quite low and no where near enough to remove the heat from a building compared to using an air cooled heat exchanger where the air temperature is relatively constant, but where there is an unlimited supply. Work out the heat being removed from your chiller and then work out how much water you would need to take away the same quantity of heat, assuming some sort of start temperature and finish temp and then compare to hot water usage in the building. I think you'll find that there is a distinct discrepancy

To have two heat exchangers - one for heating water and one for heating air just makes it all too expensive and unfortunately CAPEX is usually king over OPEX

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[greenen]

Thanks for the prompt reply!

I think I might have miss written my post. I did not mean that the chiller would heat the hot water for sinks and the kitchen but rather it would heat hot water to run through the air handling units to heat the building. This would turn a two pipe cooling only system into a four pipe heating and cooling system. The chiller would use the same water side heat exchanger to heat and cool the water. It would use a closed loop outdoor coil to reject/absorb the heat from the air. Thus the "air-cooled" part. The category of equipment I was thinking about is the Trane Series R RATC.

One of the reasons I am thinking about this is I live in a climate with ~1000 cooling degree days but ~6000 (temperate) heating degree days. As such it makes sense to have an efficient heating system. I was wondering, for large buildings, is there an energy efficient alternative to heat your building besides a boiler? I guess geothermal heat pumps would fall into that category, but they seem crazy expensive.

Thanks again.

 

[LittleInch]

Reverse cycle systems do exist. I think the thing that doesn't work well is that in reality you can only cool the outside air down to around 5C before if freezes up your HX. If the outside air is say 10C, then you need to shift an awful lot of air through your HX to extract it. That takes a lot of power and a big HX. Hence you can put in lots of energy to extract only a small amount of heat.

Below 10C air temp then you need other forms of heating as well, so the whole thing starts to double up and cost more and more. As said unfortunately a lot of decisions are based on short term capex, running costs and saving energy come a second to cost of building.

Far better spending money on insulation and natural ventilation and heating incoming air with stale but warm air exhausting the building. Companies like this do what I think you're looking at. No I don't work for them, but my son did for a while.

http://www.breathingbuildings.com/home

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[willard3]

Consider that the maximum efficiency of a Carnot cycle (what a heat-pump is) in the low 30% range and a condensing boiler operates at 85-95 % efficiency.

 

[GMcD]

What Willard said, and as well, here in the cold North, the LEED addicts have created the market for a lot of air cooled heat pump chillers (basically just refined heat recovery chillers) with a number of European brands showing up like Aermec, Clima-Cool, etc. The LEED (ASHRAE 90.1) energy modeling rules result in a lot of heat pumps being used as the magic bullets to get those precious LEED Energy Credits, but be careful what you wish for - here in BC the natural gas costs are running just under 3 cents/kWh while even our low cost electrical from BC Hydro is running just over 9 cents/kWh, so that electric heat pump better have some darn fine COP's at low outdoor ambient temps before running a gas boiler (right, he said sarcastically). Locally the School Districts are turning off the electric heat pumps of various flavours (air cooled, water two water geo) to save money because overall, once the school does require heating, it's cheaper to run the condensing gas boilers and get on with life, plus the maintenance costs for the exotic air cooled heat pumps are expensive.

 

[HVACMech]

Caution on natural ventilation. Verify you are maintaining the required IAQ requirements for your AHJ.

 

[Drazen]

a gmcd elaborated, there are business and elaboration costs.

basically you can turn any chiller into heat pump even by simple addition of diverting valves on refrigerant circle, but issue of real cost effectiveness of that depends on almost everything: construction of heat exchanges, feasible temperature regimes.

noone but hvac engineers understand relations between feasible temperature regime one secondary/consumption circles and heat pump efficiency and how to actually relate building demands to effective secondary circles.

but as we are very much pulled out of story by incompetent architects and building legislators, there is more and more of outrageously stupid legislation.

as gmcd mentioned in some projects it is good to install heat pumps to satisfy formal demands, than to install good old boiler afterwards. i know of projects where boilers are added after building permit procedure, like retrofit, just months after building start-up, to ensure moderate fuel and maintenance costs.

 

[willard3]

Consider also that elec cost/million btus (heat pump) is 3-5 times what gas costs /million btus (boiler).

 

[greenen]

Thanks all for the clear answers. If I understand things right, turning a chiller into a heat pump is possible and some people do it, but it will not necessarily save me any money.

 

[LittleInch]

That's it in a nutshell.

The other is below a certain temperature (air or soil), it becomes not possible as otherwise you freeze the air or the ground.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ReverseCycle]

It's a worthwhile question, especially if you are in a location with no reticulated supply of natural gas, LPG for boilers.

Further to that, you might want to consider heat recovery type chillers rather than standard heat pumps. There is more than likely a situation where you will require simultaneous heating and cooling.

On smaller scale systems, you are seeing the VRF/VRV players enter the market. For hotels where refrigerant gas volumes and leakage is an issue, the VRF players have a HX branch box to convert to chilled/heating hot water where required.