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Why aren't there more heat pumps in process plants?

  1. Feb 8, 2015 #1

    rollingstein

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    In process plants many heating applications will use steam at approx 120 C whereas the standard cooling utility is cooling water at approx. 30 C.

    In the dozens of plants I've seen I cannot recall having seen even one Heat Pump. Why is this so? Is the capex too high? Or are compressors too inefficient?

    An ideal heat pump raising heat from say 60 C to 120 C ought to have a COP of 6.5. Isn't that a lot of saving on steam & cooling tower costs both? What gives? Is coal / gas so much cheaper than electricity needed for the compressors?
     
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  3. Feb 8, 2015 #2

    Bystander

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    Or, are heat exchangers too expensive to insert two heat exchangers between heat pump and process streams where a single heat exchanger between two process streams is currently doing a less efficient job?
     
  4. Feb 8, 2015 #3

    rollingstein

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    Most often its not a single HEX even now. There's two. One having steam vs process fluid to heat. And another having CW & process fluid to cool.

    The situations where you transfer heat directly from one process fluid to another are perfectly fine. Those are not what I'm targeting. Those don't add directly to boiler load nor cooling tower load.
     
  5. Feb 8, 2015 #4

    russ_watters

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    You are overestimating the COP. For HVAC applications, with much lower DTs, they don't get much above 4.5. And per unit of energy, electricity costs a lot more than natural gas.
     
  6. Feb 8, 2015 #5

    rollingstein

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    I only meant it as the ideal i.e. Carnot efficiency. Actual will be lower I agree.

    A rough estimate. Say 1 kg steam gives you 2260 kJ/kg heat. Assume a COP of 4.5. The energy needed to drive the compressor is 0.13 kWhr. At industrial power of say $ 0.07 per unit that would be 0.009 $/kg-steam-equivalent. i.e. A little less than a cent.

    Does anyone know what the industrial costing of low pressure steam is? It might indeed be cheaper than a cent I suspect.
     
    Last edited: Feb 8, 2015
  7. Feb 9, 2015 #6
    This may be of interest to you:
    Industrial Heat Pumps for Steam and Fuel Savings
    http://www.google.ca/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&uact=8&ved=0CDkQFjAA&url=http://www1.eere.energy.gov/industry/bestpractices/pdfs/heatpump.pdf&ei=VzbZVPnWM8L2UIrhgIAI&usg=AFQjCNGV1W3JrVGuWbbZwbbW2A_4yDActg&sig2=EG_OoqOzk80GouBqfRwRYg&bvm=bv.85464276,d.d24


    Typically it is either:
    The temperature rise needed is too high, resulting in excessive costs and minimal value
    The volume of available fluid from which to recover heat is too low

    Principally, the payback period is too long. If the energy costs don't pay for the purchase, installation, and maintenance of the pump within a range of 1-5 years, a plant generally won't look at it. Most of the time it's about balancing available capital and projects with long payback periods will generally be passed up, even if they technically create more efficient process.
     
  8. Feb 9, 2015 #7

    russ_watters

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    About a cent and a half. But again, with that large of a delta-T, you won't even get anywhere close to 4.5 COP. Also, 7 cents a kWh is a little on the low side.
     
  9. Feb 9, 2015 #8

    rollingstein

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    I used EIA aggregate data for electric prices in USA in Nov. 2014. Industrial average is given at 6.67 cents per kWhr. For some states like Washington, Nevada, Iowa or Utah industrial consumers are paying less than 5 cents per kWhr. Is there a hidden factor I'm missing?

    http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a
     
  10. Feb 9, 2015 #9

    rollingstein

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    For a 60C-120C heat-pump I get Carnot Efficiency as 6.55. At 70% actual effeciency that's mean an effective COP of 4.5.

    How bad is the actual difference from Carnot? Any estimates?

    If not 4.5 what's a more realistic COP to use?
     
  11. Feb 9, 2015 #10

    rollingstein

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    Thanks for the link! The first point sure makes sense.

    But not so sure about the second point: If you look at the cooling water needs of most chemical plants there's a huge amount of heat being extracted out ,say, in the 50 C to 110 C range. Lots of reactions etc. are run in that range & many being exothermic need cooling. Often products need cooling before they can be loaded into tank cars etc.

    So I'm not sure why low volumes are a problem. Perhaps I misunderstand your explanation.

    Agree that payback periods are crucial. Is there a good way to estimate the cost of (say) a kWhr of heat pump capacity. Anecdotally do you have any rough estimates?
     
  12. Feb 9, 2015 #11

    russ_watters

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    The wiki on heat pumps has several tables. They don't go as high a temperature as you are talking (because nobody makes them), but you can extrapolate from the difference between real and carnot efficiency for lower temperatures.

    One thing that's also important here is you didn't say what the source of the heat would be. Ambient air? Groundwater? For example, a residential air source heat pump with a temperature difference between source and sink of 30 C has about a 4.5 COP. Given that you haven't said what the source is and the number you used was 60C, I'm not sure you are understanding the COP properly. The low temp is the temp of the heat source, which must be some sort of ambient heat, either from air or groundwater usually, but it also could be waste heat (room exhaust, at 20C). So the source is more likely to be 5C than 60C. That would give a Carnot COP of 3.4.
     
    Last edited: Feb 9, 2015
  13. Feb 9, 2015 #12

    rollingstein

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    Oh sorry, I should have clarified. No neither ambient nor groundwater but a "richer" source in the form of hot process streams.

    In most process plants there are streams that need cooling that are fairly hot (say 90 C) & are currently dumping all their heat to cooling towers.

    Ergo a ready heat source at T much higher than ambient or groundwater is available. Besides heat transfer coefficients will be high relative to GW or air.
     
  14. Feb 9, 2015 #13

    russ_watters

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    What kind of "process plants" are you referring to?

    If they have waste streams that are that hot, there may be other ways of recovering the heat that are cheaper without the refrigerant circuit; using a glycol/water solution or even a direct heat exchanger. It would depend on the particulars of the processes.
     
  15. Feb 10, 2015 #14

    rollingstein

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    The one's I had in mind were all pharma intermediates, fine chemicals, specialties etc. e.g. producing resins, detergents, or intermediates like pentaerythritol, BHT, BHA styrene oxide, etc. i.e. Much smaller than a refinery but not tiny. Reactions & distillation being the two very common unit operations.

    Which other ways do you have in mind? If I have a large liquid stream at say 90 C that needs cooling to 60 C but OTOH I've no process needing heat at such low temperatures how can you use a direct HEX? Currently dumping the low quality heat to a cooling tower is the only viable option. And that's what most sites seem to do.
     
  16. Feb 10, 2015 #15

    rollingstein

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    Nope. I meant pumping heat up from a hot fluid at around 90 C that needs to be cooled down to a fluid at say 100 C that needs to be heated up. Hence the 60-to-120 duty accounting for deltaT across heat exchangers etc.
     
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