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2008-09-02 14:44:56

CO₂ Heat Pumps for space & water heating

We are building a 12000 square feet commercial building which is going to be a "net-zero” one both for electricity and heating. We will install a PV/Thermal solar system and a geothermal system that will provide all the electrical and thermal energy needed. The system is backed up by zinc/bromine regenerative fuel cell (energy storage). The system is totally CFD free (liquid desiccant dehumidification) and we are looking for a heat pump using a natural refrigerant to back up the solar heating/cooling/geothermal system. We are looking for four 5 ton R744 heat pumps water to water.

Please let me know if you have a reliable source for such a heat pump.

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Dear Andrew,

Reading your request, I thought it might be a good idea to clarify some issues regarding the use of CO₂ heat pumps in hot water and space heating, as I’ve received quite a lot of similar requests:

HEATING SANITARY WATER

Sanitary water needs to be heated up from a low temperature (i.e. +10°C) up to storage temperature (60 to 80°C or more). A conventional heat pump, for example a heat pump using R134a as a refrigerant, releasing heat at constant temperature, requires a high condensing temperature, at the same level, at least, of final temperature of water.

The above is not true in absolute, as it is possible to design, for instance, a system with R134a as a refrigerant where also subcooling and superheating of refrigerant are used for heating water, obtaining a significant improvement in efficiency. Final temperature of water could be in that case slightly higher than condensing temperature. It means however that heat pump should be designed for heating water in one passage (once though) while most of the heat pumps today are not of this type. Heat pumps using CO₂ as a refrigerant are well suited for heating sanitary water in one passage from a relatively low temperature up to high temperature, as an example from +10°C up to 70°C or more, as the efficiency depends significantly from outlet temperature of refrigerant at the end of heat rejection process, in other words it depends strongly on inlet temperature of the water.

In combination with a stratification storage vessels – containing directly the water to be used – a CO₂ water heating heat pump has another significant advantage over state of the art heat pumps, as it is possible to have readily available a lot of water at high and constant temperature, as cold water entering the storage vessel doesn’t mix with warm water reducing its temperature.

Conventional heat pumps with HFC refrigerants, being not “once through” type, cannot be used with a stratification vessel, and they are normally used with a mixing storage vessel. The direct use of stored water has another advantage : for some time there is a huge “power” available. For instance if a relatively small heat pump with 20 kW output “produce” 3500 liters of warm water at 70°C – heated from 20°C - during a nighttime period of 10 hours and this water is used at 45°C in a hotel from 7,00 to 8,30 in the next morning it corresponds in average to a power of 130 kW, sufficient for about 150 showers of almost 50 liters. It is clear that in practice this is the most reasonable way to use a heat pump for heating water. A mixing type vessel would be enormous (at least 4 times as big) and impractical. Moreover the use of an “inertial” vessel would add inefficiency, due to additional temperature differences, and also there wouldn’t be, with an heat exchanger, the flexibility given by direct use of hot water.

A heat pump with 20 kW output as in the above example, operating with a COP of more than 4,0, would have an electrical power input of about 5 kW, so electricity input required is quite low. So definitely for this application, heating water for sanitary use or in general heating water dissipated in a process, it is better to use a C_O2 heat pump.

SPACE HEATING

It must be stressed that many people, listening about the features of this product, are attracted by high temperatures of water (i.e. 80°C) at the end of process and erroneously believe that it is possible to use a CO₂ heat pump to supply hot water to a space heating system operating, as an example, with radiators, with return temperature relatively high, i.e. 55-60°C.

This is not possible, as a CO₂ heat pump is not suited for high temperature space heating system, due to high return temperature required by these solutions. High return temperature means higher temperature of CO₂ at the end of heat rejection process (i.e with return water at 50°C CO₂ temperature at gas cooler out would be approximately 55°C) and consequently very poor efficiency and capacity.

Nor it is possible or convenient, for the same reason, to use a water heating heat pump as a back up system to lift water temperature of a vessel heated by solar panels. When solar radiation is no more sufficient to obtain a given temperature of the water. There are of course solutions for integration of a heat pump for water heating with solar thermal panels, but certainly a “series” connection of solar panels (meaning that “out” of solar panels is “in” of heat pump) with heat pump is definitely not the right solution.

It is obviously possible to produce an efficient heat pump for space heating using CO₂ as a refrigerant – and some companies are working on that – but it will always be designed for a low temperature heating system (i.e. radiant or floor heating), so with a water return temperature not higher than 30-35°C, as with normal heat pumps for space heating. In case, for low energy houses becoming now the standard, there will be in any case a significant advantage compared with HFC heat pumps, considering that energy required for heating of sanitary water will be predominant.

RE: CO₂ Heat Pumps for space & water heating

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Hello Andrew,

Before I add something to Dr.  Sergio's reply would you please let me know if geothermal energy source is within the property or not. 

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Hallo.

I don't know whether the question is directly applicable to the discusion above, but could somebody please give me an answer on the following question. It will be appreciated.

I would just like to know what is the minimum delta Temp between the water and the gas cooler? For instance...if you have a water inlet temperature of 10 degrees, what is the closest the gas cooler outlet Temp could come to this? I assume that the delta Temp would be in the order of 5 degrees, but would just like to confirm this... But this is applicable to the cirve af the gas cooler temperature gradient. What is the pinch point between CO₂ and water temperatures?

Thank you.

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Hi,
he meant a general solution, which could be ground source or air source. the question I think can be considered for both systems
please give your opinion about that
sergio

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hi, Wynand
the mimimum difference can be very low, as low as 1 K, but when you test one unit you immediately see that in general it is better to work with high DT, as to reach low approach you need to raise high pressure. I have written about that in one paper available also on this website.
the question is quite simple: the best approach is not the minimum, it depends on the combination gas cooler/compressor/evaporator/working conditions. so in practice it is difficult to use formulas in literature (they bring to wrong conclusions). we've solved the problem developing one algorithm that finds the best approach.
sergio

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RE: CO₂ Heat Pumps for space & water heating

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Dear All,

I should have added that we have successfully applied a CO₂ transcritical cycle for space cooling with water heating, but as Dr Girotto describes, space heating is far harder to achieve. In this case I feel a sub critical cycle is probably better suited, especially using ammonia at condensing of 70C.

The "beneficial use of waste heat" is a major goal that we must all strive for. 150 years of throwing all the heat away from condensers has to change.

dave

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Dear All,

I am a newbie in this field of r744 and only recently came across with CO₂ HVAC and DHW systems. And I would like to simulate such systems. And I found out my usual software of choice (EnergyPlus) does not simulate r744. I have seen simulation tools for mobile CO₂ systems but none for buildings.

So what free simulation simulation software is able to model CO₂ transcritical (and cascading cycles) for HVAC and DHW  for buildings?

Thank you in advance
Miguel

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Dr. Girotto.

Thank you for the reply.

I have come across the article that you mention and it made this topic clear to me.

Thank you for your contributions.

Wynand

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A very interesting discussion bringing out potential limitations in the application of CO₂ heat pumps to heat water for space heating. I struggled with this when starting to think about  Total Energy Transcrtical CO₂ Cooling and Heating Systems for office buildings, district  heating and cooling systems and the like.

But in large central Cooling and Heating systems in office buildings, hospitals etc. it will not be a problem because high volume airflows are required for cooling. Thus low approaches are possible at the air entry side to a hot water heating coil. Indeed it would be possible to reduce airflow volumes during the heating cycle in those zones requiring heating because a high air temperature rise would be available. This would reduce the fan power consumption, but in heating mode the fan power consumption is usually useful heat.

This was later confirmed when I came across one of Prof. Gustav Lorentzen's paper where he advocated large two stage CO₂ Heat Pumps with interstage desuperheating and CO₂ liquid subcooling. The water temperature rise would be 30 degrees C from 35 deg C to 65 deg C. I now believe a return water temperature of 30 deg C or even a bit lower to be possble in purpose designed large heating systems. A high water temperature rise has the advantage of lower mass flow and thus smaller piping, and pumps and lower surface area heating coils for a given design.

It is interesting to note that Prof Hans Quack and his team at Dresden University came to the same conclusion when developing their free piston CO₂ expander assisting compression. They found that at the transtion from subcritical to transcritical compression  it  is advisable to  have the compression stage driven by the expander chane from parallel compresion to series compression, ie the expander compressor would become a second stage compressor. So I am now looking around for compressors with an allowable suction pressure of 80 bar. Please let me know if you know of any. There are two stage compressors on the market but I am not certain whether the high stage of such compressors can handle a transcrtical CO₂ fluid through its suction valves. The free piston design certainly can without any problems if not running too fast.

In conclusion I am therefore confident that custom designed large CO₂ heating and cooling systems will be very efficient when considering the system's performance over a whole year running. It may not be so easy when retro fitting these system to existing buildings, but technical solutions are available.

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Hi there,
We are in the process of developing a greenfield site for a medium size food processing plant and would like this plant to be "future proof" and environmentally responsible.
The present plant on the old site utilises NH3 as a first refrigerant to cool glycol to -6C as the secondary coolant which is reticulated. Capacity of present plant approx. 500kW. We also have a chilled water system (R22) for process cooling with a capcity of 350kW.
We require comfort heating and cooling of  the process area as well as hot water at 90C for CIP.
Is it economically feasible to use CO₂ for the above duties? At the moment we use natual gas with a hot water generator to create the hot water.
I look forward to your comments.

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Dear Mr Wehman, my apologies for not responding earlier. Everything you describe can be done with direct CO₂. I suggest you run direct CO₂ at an evaporating temp of about the mean glycol tempearture to evaporate directly in evaporators to replace the glycol circuit. You can chill water with the same evaporating temperature, or a higher temperature depending on the chilled water temperature, or indeed as a direct evaporating circuit depending on the duty.
Space cooling with direct expansion CO₂ is again no problem and generating hot water for CIP, factory washing, domestic hot water and space heating is agian no problem, provided high temperature range low flow designs are aimed for. The final hot water tank configuration, if any, depends on production patterns ( one or two shifts) and hot water usage patterns.

You would ideally arrange for parts of  the plant to be able to run in trans - and sub-critical modes simultaneously to be able to achieve a high wieghted annual COP. New Zealand's moderate climate would lend it self very well to the efficient use of CO₂ refrigeration systems.
 
I am currently in the final stages of design of a transcritical CO₂ plant for a small food processing facility doing exactly what you propose to do, which we hope to have running by March next year. We were pleasantly surprised by the relatively minor cost penalty in our case, but that does depend on a number of factors and it may not always be the case. We expect to reduce energy consumption by at least 40% compared to conventional multiple separate systems and global warming emissions by an estimated 50% by getting rid of all HCFC's and HFC's on site (14 small systems including six R134a air to water heat pumps and three reverse cycle office AC units) and by heating water with CO₂ rather than burning gas.

I trust the above is of some help to you.

With best wishes and kind regards

Yours sincerely

Klaas Visser.