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Heat pumps

Topic last reviewed: 10 April 2013

Sectors: Downstream, Midstream, Upstream

A heat pump is a device that can be used to boost the temperature of low grade heat (6–70°C) to higher levels (normally 20–100°C), using a relatively small supply of high quality drive energy (electricity, fuel or waste heat). Heat pumps can also be used for cooling, in which case the heat is transferred from the application being cooled to surroundings at a higher temperature. Almost all heat pumps are either based on a mechanical vapour compression system (see Figure 1 (a)) or on a physical, chemical absorption cycle (see Figure 1 (b)); however, in some cases, chemical adsorption is also used. More recently, heat pumps using alternative thermodynamic cycles and processes, such as Stirling and Vuilleumier cycles, single-phase cycles (e.g. with air or CO2), hybrid systems (combining the vapour compression and absorption cycles), and electromagnetic and acoustic processes are entering the market or have reached technical maturity (Reference 3). Vapour compression heat pumps can be driven by electricity or by an internal combustion engine, or may use process fluid itself as the working fluid in an open cycle (e.g. mechanical vapour re-compressors). Traditionally, the most common working fluids have been chlorofluorocarbons (CFCs), but given their high ozone-depleting potential and other adverse environmental impacts, these are now being replaced by more environmental friendly ‘natural working fluids’ such as ammonia, propane, water, carbon dioxide and air. Absorption heat pumps are thermally driven, which means that heat, rather than mechanical energy, is supplied to drive the cycle.

Heat pumps have several useful applications in the industry. For example, ground-source heat pumps use the ground—which maintains a relatively constant temperature of 6–20°C at 3 m or more below the surface—either as a heat source or as a sink, depending on the season. Another application for industry is upgrading low-grade heat to a higher, more useful temperature.

Electrically-driven heat pumps used for heating buildings typically supply 100 kWh of heat with just 20–40 kWh of electricity. Many industrial heat pumps can achieve even higher performance, and supply the same amount of heat with only 3–10 kWh of electricity (Reference 3). If the heat pumps use waste or renewable heat, they will consume less primary energy than conventional heating systems. Therefore, heat pumps are an important technology for reducing energy use and emissions. The section on ‘Key metrics’ (below) provides more information on the coefficient of performance (COP) for heat pumps.

While heat pumps have been commercialized for use in residential and commercial building applications, their use is much less common in industrial installations. However, as environmental regulations become stricter, industrial heat pumps may become an important technology to save fuel, reduce emissions and improve efficiency.

For a more detailed description of this technology in typical applications, please refer to:

Technology maturity

Commercially available?: Yes 
Offshore viability: No 
Brownfield retrofit?: Yes 
Years experience in the industry: 11-20 

Key metrics

Range of application:
Capacity of up to a few MWth (0.5 to 4 MW) per heat pump (Reference 12, Table 6)
Guideline capital costs: Greenfield: data not available for oil and gas industry installations. Typical cost range of closed-cycle mechanical heat pump systems range from US$50,000/MMBtu to US$200,000/MMBtu (2009, Ref. 7)
Guideline operational costs: Less fuel and energy used than traditional heating and cooling systems, saving operational costs
Typical scope of work description: For any heat pump application, a feasibility study and detailed engineering design should be performed. Use of heat-integration methods, such as Pinch Analysis, provide the analytical tools to help identify and evaluate heat pumping opportunities. Heat pumps are more likely to be economically viable for use in regions with extreme temperature differentials between process and ambient temperatures, as well as in regions where both heating and cooling are required depending on the season (such that the heat pump can replace both boiler and A/C).  

Decision drivers

Technical: Heat (or heat sink) resource availability and temperature
Heat pump temperature limit
Footprint: plot area required for geothermal heat pump system
Lack of high output temperature applications in industrial processes
Uncertainty about long-term reliability
Operational: Heat fluctuation (if using waste heat)
Water availability (if using open loop ground heat)
Commercial: Economic rule of thumb: see the literature (Leonardo Energy 2007, Reference 9) for analysis examples.

Additional comments

Efficiency: The performance of a heat pump at a given set of temperature conditions is referred to as the 'coefficient of performance’ (COP) for an electrically driven heat pump, or as the 'primary energy ratio' (PER) for an engine or thermally-driven heat pump. COP is defined as the ratio of heat delivered by the heat pump to the energy supplied to operate the compressor. PER is defined as the ratio of heat delivered by the heat pump to the supplied energy, which is the higher heating value (HHV) of the fuel supplied. Typical COP and PER ranges are provided in Table 1.

 

Table 1: Typical COP/PER ranges for heat pumps with different drive energies (HPC, Reference 3)

Heat pump type   Buildings   Industry  
 COP PER  COP PER
Electric (compression)  2.5 – 5.0    3.0 – 8.0  
Engine (compression)    1.0 – 2.0    1.0 – 2.0
Thermal (absorption)   1.0 – 1.8     1.1 – 1.8

Heat pump efficiencies decrease as the desired temperature lift increases (Reference 12).

Alternative technologies

The following items are technologies that provide similar benefits (high efficiency power generation) and may be considered as alternatives to heat pumps:

  • Gas (or biogas)-fired burners/steam generators/boilers
  • Solar-powered absorption chillers or heating systems
  • Combined heat and power (CHP)
  • Heat exchangers (between high enthalpy fluids needing cooling and streams needing heating)

Operational issues/risks

  • Heat fluctuation (if using waste heat)
  • Integration with the existing industrial systems
  • Water (if using open loop ground heat)
  • System complexity, potential maintenance issues
  • In the case of ground source heat pumps, the drilling and completion costs could be a high expense

Opportunities/business case

Opportunities:

  • Heat pump with higher output temperature (e.g. 150°C or above) for industrial uses (while maintaining reasonable efficiency)
  • Ground source heat pump for cooling

Industry case studies

Distillation column heat pump application

Reference: Leonardo Energy, Application Note: Industrial Heat Pumps (see Reference 9).

A case study was prepared to address the high energy costs associated with operating a steam-heated reboiler on a distillation column through the use of heat pump technology. The simplest alternative to conventional column design is to replace the steam heating in the reboiler with a condensing refrigerant at a relatively high pressure, and replace the condenser coolant with an evaporating refrigerant at low pressure. Thus, the reboiler becomes the condenser and the column condenser becomes the evaporator in a heat pump system. The column design is unchanged, but the heat exchanger design is different from a conventional distillation column operational standpoint. Figure 2 illustrates the heat pump concept for the distillation column application.

Figure 2: Distillation unit with closed loop heat pumping cycle

 

Baseline scenario: Base design for efficiency comparison is a conventional distillation column with steam-heated reboiler, using natural gas to generate steam.

Energy efficiency project activity: Use a condensing refrigerant in the reboiler to replace steam heat, and the evaporating refrigerant in the column condenser to replace the cooling medium. The refrigerant cycle then effectively acts as a heat pump to provide both heat and cooling capacity.

Performance specifications:

  • Column top pressure / temperature: 175 mbar / 124°C
  • Column bottom pressure / temperature: 199 mbar / 135.5°C
  • Boil-up rate: 36,000 kg/hr
  • Heat of vaporization (top): 74.5 kcal/kg
  • Energy required for boil-up: 3,120 kW

Table 2: Energy consumption comparison

Energy demands Conventional column Column with heat pump
Energy required for boil-up 3.120 kW -
Turbo-blower duty - 310 kW
Reboiler duty - 23 kW
Total energy amount 3,120 kW 333 kW
% Energy reduction   89%

Table 3: Estimated costs comparison:

Energy costs Conventional column Column with heat pump
Steam cost, €/hr (basis: €25/T; year 2011 costs) 140 1
Electricity cost (basis: € 0.06/T; year 2011 costs), €/hr   18.6
Total cost, € / hr (year 2011 costs) 140 19.6
Total cost, € / yr (year 2011 costs) 1,125,000 157,500
% Cost savings   86%

(end Reference 9)


References:

  1. Thermea Energiesysteme (2008). ‘High-Temperature Large Scale Heat Pumps for Industrial Use’.
  2. U.S. Department of Energy (website). ‘Heat Pump Systems’.
  3. IEA Heat Pump Centre (website). ‘About Heat Pumps’.
  4. -REMOVED-
  5. U.S. Department of Energy (2009). ‘Ground-Source Heat Pumps: Overview of Market Status, Barriers to Adoption, and Options for Overcoming Barriers’. Final Report, 3 February 2009.
  6. Articlesbase (website). ‘Ground Source Heat Pumps and Brownfield Reclamation’.
  7. U.S. Department of Energy (2003). ‘Industrial Heat Pumps for Steam and Fuel Savings’. A Best Practices Steam Technical Brief.
  8. CEA Technologies, Inc. (2007). ‘The Heat Pump Reference Guide’. Final Guide, 27 March 2007.
  9. Leonardo Energy (2011). ‘Application Note: Industrial Heat Pumps’. European Copper Institute, Publication No Cu0118, September 2011.
  10. Mehnert, E. (2004). ‘The Environmental Effects of Ground-Source Heat Pumps – A Preliminary Overview’. Illinois State Geological Survey, Open-File Series Report 2004-2.
  11. Jakobs, R. M. (2010). ‘Status and Outlook: Industrial Heat Pumps’. Information Centre on Heat Pumps and Refrigeration, Germany; presentation given at the 3rd European Heat Pump Forum, Brussels, 20 May 2010.
  12. U.S. Department of Energy (2008). ‘Waste Heat Recovery: Technology and Opportunities in U.S. Industry’.