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Comment PDF Heat Transfer

Improve Energy Efficiency Using Heat Pumps

By Sebastiano Giardinella |

Heat pumps are an efficient way to provide space and process heating. Information on their types, and how to assess their potential benefits is provided here

Approximately 40% of the energy that is used in buildings in the U.S. goes toward keeping indoor spaces comfortable. Heat pumps provide a proven, efficient way to provide this service.

Whereas a heater can only convert less than 100% of the energy put into the device into useful heat, a heat pump can provide many multiples of the device energy input by combining the heat rejected from said device with heat available from a typically free source. This translates into a much lower energy cost over the heat-pump lifecycle, which pays for its typically higher initial cost when compared with a heater.

Heat pump technology applications have been growing, from typical refrigeration, space heating/cooling, to industrial or hybrid uses in the chemical process industries (CPI).

This article discusses heat pumps, their types, working principle and components, and provides an overview of the process for defining, specifying and evaluating a heat pump project.

 

Heat pump basics

Heat pumps are devices or systems that extract heat from at least one source and transfer it to at least one sink at a higher temperature. A basic heat pump is based on the refrigeration cycle. Figure 1 illustrates the basic heat-pump steps over a generic pressure-enthalpy diagram:

Compression (1–2). Vapor is compressed to a higher pressure, Pcond, where condensation occurs.

Condensation (2–3). The vapor at a pressure of Pcond is condensed at the corresponding saturation temperature, Tcond, by transferring its heat to the sink, at a temperature of Th.

Expansion (3–4). The liquid is expanded in a flow restrictor from the condenser pressure, Pcond, to the evaporator pressure, Pevap.

Evaporation (4–1). The liquid, at a pressure of Pevap, is evaporated at the corresponding saturation temperature, Tevap, by receiving heat from the source, at a temperature of Tc.

FIGURE 1. A pressure-enthalpy diagram for a basic refrigeration cycle is shown here

Both in the evaporator and in the condenser, there is a temperature difference with the surroundings to allow heat transfer to or from the working fluid. Hence, the evaporator works at a lower temperature than the source, and the condenser works at a higher temperature than the sink.

To compress the gas, additional energy needs to be supplied to the heat pump in the form of either power (typically electrical) or heat, depending on the type of heat pump as described below.

The total heat that is delivered to the high temperature sink is the sum of the heat being transferred from the low temperature source plus the work used in compression. The measure of the system performance depends on whether the heat pump is used for heating or cooling, as described below.

 

Heat pump performance

For heating service, the useful thermal energy that is delivered to the higher temperature sink is the total energy supplied to the heat pump resulting from the combination of heat taken from the lower-temperature source plus the energy used to drive the heat pump, minus the energy losses in the system, described by Equation (1):

Eu = Es + Ed – L (1)

 

Where:

Eu = Total useful thermal energy

Es = Heat taken from low-temperature source

Ed = Energy used to drive the heat pump

L = energy losses in the system

(all values in consistent units)

The efficiency of the heat pump is measured by the ratio of useful thermal energy delivered, to the driving energy supplied to it. This ratio is known as the coefficient of performance (COP):

 

COP = Eu / Ed (2)

 

Equation (2) can be expressed in different forms depending on the type and configuration of the heat pump, as described further below.

Assuming a typical vapor compression heat-pump system, such as that shown on Figure 1, where heat, Qc, is transferred from a source at a lower temperature, Tc, to a sink at a higher temperature, Th, electrical power W is used to drive a compressor, and the net useful heat delivered to the sink is Qh, the COP can be expressed as:

COP = Qh /W (3)

 

Where:

Qh = net heat delivered to high temperature sink

W = power supplied to the heat pump

 

The theoretical maximum coefficient of performance that can be achieved by the basic vapor compression cycle in Figure 1 is given by Equation (4):

 

COP < Th /( ThTc) (4)

 

Where:

Tc = Source temperature (absolute units)

Th = Sink temperature (absolute units)

 

It is common for heat pumps to operate with COPs well below the theoretical maximum. There is continuous effort by heat pump manufacturers to design systems that are capable of reaching closer to the theoretical maximum COP.

For cooling service, the useful energy in Equation (2), Eu, can be expressed as the heat extracted from the low-temperature source. Assuming the same vapor compression cycle on Figure 1, the COP for cooling then becomes:

 

COPcooling = Qc /W (5)

 

The theoretical maximum cooling COP that can be achieved by the basic vapor compression cycle in Figure 1 is given by:

 

COP cooling < Tc /( ThTc) (6)

 

Heat-pump classification

Figure 2 presents a general classification chart for heat pump systems, segregated into system, technology, model and driving energy.

FIGURE 2. This diagram shows the classification of heat pumps (adapted from Ref. 2)

Based on system. Heat pumps can be classified into closed and open systems.

Closed systems have the working fluid confined within the boundaries of the system, with no material entering or exiting it.

Open systems, on the other hand, involve material exchange with the exterior. The working fluid in these types of systems is used to transfer the heat, and a portion of it is taken from, and delivered to, the surroundings.

Based on technology. Closed systems are classified into compression or sorption-based systems, whereas open systems are of either thermal or mechanical vapor-recompression type. Figures 3, 4 and 5 show different basic configurations for each technology type.

Vapor-compression heat pumps, shown in Figure 3 (in this case, depicting an electrically driven compressor) use work to raise the pressure of the gas.

FIGURE 3. This diagram shows a mechanical (vapor-compression) heat pump

Sorption heat pumps, such as the absorption system shown on Figure 4, utilize a solvent with an absorption and desorption (generator) step. Depending on the sequence of the flow, the system may be classified as either an absorption heat pump or a heat transformer.

FIGURE 4. An adsorption heat pump is shown here

In an absorption heat pump, the solvent absorbs the gas from the evaporator, allowing the fluid to be pumped to elevate its pressure. The rich solvent is then subject to heat at the generator, which desorbs the gas at a higher pressure before the condenser. The lean solvent is returned to the absorber through a valve that reduces its pressure, and it is cooled to the initial absorption temperature.

Heat transformers operate on the same principle as absorption heat pumps, but in reverse: the condenser and generator operate at a low pressure, whereas the absorber and evaporator operate at a high pressure.

In comparison to vapor-compression heat pumps, sorption types utilize heat as the main driving energy, with a typically low consumption of electricity for pumping. Figure 4 shows combustion (for example, from natural gas) as a heat input to the system, but other suitable heat streams, such as waste heat, can be utilized. Also, depending on the configuration and integration with the surroundings, heat from the pump can be utilized for different temperature services: this includes heat from the evaporator, heat removed from the absorber, and heat losses (for example as fluegas) from the generator.

Figure 5 shows a thermal vapor-recompression (steam-jet) heat pump. This technology also uses a combination of thermal energy (in the boiler) and work (at the pump). In this case, the system is open as some of the steam is vented and make-up is added to the evaporator.

FIGURE 5. This heat pump features an ejector (steam-jet)

Mechanical vapor-recompression heat pumps differ from thermal vapor-recompression heat pumps in that they use a mechanical driver to move the vapor to a higher pressure.

Based on driving energy. The driving energy refers to the energy that generates the motion of the working fluid from the low to the high-pressure side of the cycle. This energy can be sourced from process heat, steam, combustion, engines, or motors, depending on the technology and model.

 

Heat sources

Typical heat pump sources include air, ground (geothermal), water, solar-assisted, waste heat or a hybrid.

Air-source heat pumps (ASHPs) are subject to seasonal variation of ambient temperature. Their advantages are relatively low costs and ease of installation.

Ground (geothermal) source heat pumps (GSHPs) are subject to lower seasonal temperature variations, but at a usually higher initial cost than air source heat pumps due to the excavation required to reach the desired depths and install the piping needed to exchange heat with the ground.

Water-source heat pumps (WSHPs) allow for a higher temperature difference between the source and the sink, provided a significant water reservoir is nearby. Permitting requirements, freezing during winter, and limited geographical access to large volumes of water limit the use of these types of sources. However, their configuration can use hot water from other sources, such as process water, as described below.

Solar-assisted heat pumps combine the heat from another source with that from solar thermal collectors for applications such as water heating that require moderately higher temperatures.

Waste-heat heat pumps may use a variety of different heat sources, such as those available on industrial facilities. The temperature, quantity, and availability of different sources of waste heat will dictate the configuration, type of evaporator (for example, gas or liquid hot fluid), and working fluid to be used in the system.

Hybrid-source heat pumps combine different sources for either single or multiple applications.

 

Other configurations

Figures 3, 4 and 5 show single-stage configurations with one evaporator and one condenser. However, heat pumps can be configured in many different manners to improve performance, capture heat from different sources, deliver heat to different sinks, or deliver heat to higher-temperature differentials. Figure 6 shows some examples of different configurations of vapor-compression heat pumps:

  • A multiple-stage heat pump where intermediate flash tanks are placed after each expansion stage to separate vapor going to compression from liquid going to the evaporator (Figure 6a)

FIGURE 6a. An air-to-air heat pump with two compression stages is shown here

  • A cascade arrangement, where a low-temperature heat pump using one working fluid delivers heat to a high-temperature heat pump using another working fluid, which delivers heat to the sink. In this arrangement, the first loop’s condenser acts as the second loop’s evaporator (Figure 6b)

FIGURE 6b. Shown here is an air-to-air heat pump with two effects in series. A heat exchanger acts as a condenser for the low-temperature cycle and as an evaporator for the high-temperature cycle

  • Systems involving different evaporators to capture heat from different sources, such as geothermal, solar collectors and waste heat (Figure 6c)

FIGURE 6c. This ground-to-air heat pump has solar and process heat boosts

  • Systems involving different condensers to deliver heat to different sinks (Figure 6d)

FIGURE 6d. In this heat pump, heat is transfered to a process stream and air in parallel

  • A reversible heat pump used for space heating during winter and cooling during summer, where the direction of the flow is reversed so that the evaporator and condenser switch roles depending on the ambient temperature (Figure 6e)

FIGURE 6e. Here, a reversible heat pump is shown that transfers heat from a ground source to air during winter and vise versa during summer

 

Operating ranges & refrigerants

Figure 7 shows typical operating ranges for commercially available vapor compression heat pumps utilizing different compressor types and working fluids. These ranges are under continuous improvement, given research efforts into increasing the efficiency of the components, utilizing new solvents, or enhancing the system configuration. As an example, efforts are underway to increase temperatures, in order to enable heat pumps to reach industrial heating (for example, low-pressure steam) ranges.

FIGURE 7. This chart shows the typical operating ranges for heat pumps (adapted from Ref. 2)

When selecting refrigerants, aside from the pressure, temperature and efficiency considerations inherent to the system, other criteria to consider include toxicity, flammability, environmental considerations and cost.

The International Institute of Refrigeration estimates that R-22 and R-410A are currently “the main refrigerants used for ASHPs” [3].

R-22, along with other Hydrochlorofluorocarbon (HCFC) refrigerants, are being phased out due to ozone depleting potential. For instance, in the U.S., the production and import of R-22 and R-142b has been banned for new units since 2010, and for servicing of existing units since 2020 [4].

Hydrofluorocarbon (HFC) refrigerants are replacing HCFCs, but some of the options available also possess environmental concerns due to their Global Warming Potential (GWP). For instance, R-410A has a GWP of 1,924 [5].

Refrigerants such as ammonia or CO2 are mostly used in industrial settings due to safety and pressure concerns. Propane is utilized in some refrigeration applications, especially in industrial settings where electrical installations comply with electrical area classification requirements.

There is wide research into developing non-toxic, low-GWP refrigerants, but many of these are flammable and thus subject to safety considerations. Different tables are available to compare their performance and characteristics, subdividing them into low, medium or high pressure, or into low, medium and high GWP.

Some of these newer refrigerants are marketed as direct replacements to their phased-out counterparts, which is especially important to maintain usage of equipment designed for the latter. When replacing the working fluid, it is always recommended to consult with the heat-pump supplier or service technician.

 

Heat-pump process design

Heat-pump design typically involves the following steps: 1) Defining the service heat load and temperature; 2) Assessing the sources for the heat pump; 3) Defining the design capacity of the heat pump; 4) Specifying and selecting the heat pump.

Step 1. The first step in designing a heat-pump system consists of identifying the required service (heating or cooling), heat load and temperature.

The temperature is defined in accordance with the required application. For instance, for space heating, the temperature is typically selected around the comfort temperature. Water heating or industrial applications typically require higher temperatures.

Step 2. The required heat pump load is calculated to maintain a net zero energy balance around the volume of the space or system receiving the service:

 

0 = Qheat pump + Qheat sources + Qtransfers to/from environment + Qprocess (7)

 

Heat sources may include heat rejection from equipment, machinery or appliances, or heat emanated from occupants.

Heat transfers to and from the environment may include heat radiated from the environment (for example, solar radiation), heat losses to the environment (when the system is at a higher temperature than the environment), or heat gains from the environment (when the system is at a lower temperature than the environment).

Process heat encompasses other services used for process streams (for example, water heating or cooling, process cooling and so on).

These heat-pump requirements typically involve heating, ventilation and air conditioning (HVAC) calculations that consider space distribution, types of equipment or appliances, insulation, and other contributors to the heat-balance equation, as well as seasonal variations in heat loads and environmental conditions. Commercial or open-source tools are available to assess heating or cooling requirements.

In Equation (7), especially in space heating or cooling service, passive design features directly affect the heat requirements of the heat pump. Measures such as solar passive design and increased insulation reduce the heat pump requirement and are typically evaluated in an economic optimization study that seeks to reduce the overall cost of the service throughout the project life horizon.

The second step is to assess the available source(s) for the heat pump and determine its temperature and operating conditions. As discussed previously, heat sources may come from air, ground, water, waste heat or combinations. The evaluation of the heat source includes temperature profile (daily and seasonal), hot fluid phase (liquid, gas) and available heat (especially when considering waste heat, water, or ground sources, which may have flowrate or regeneration constraints).

Step 3. The next step is to define the capacity of the heat pump. The capacity is typically defined as a margin above the calculated heat load considering seasonal variations and is set at the worst condition for the expected service (for example, for an ASHP used for space heating, the heat pump capacity is defined by the minimum expected ambient air temperature and the maximum desired room temperature, and vice versa for space cooling).

Step 4. The heat-pump specification then comprises the required capacity, heat load evaluation, source type and temperatures. The specification may also include the heat-pump type, a desired minimum coefficient of performance, some technical, service and guarantee considerations, and a preference for working fluid (for example, a non-toxic, non-flammable, low-GWP refrigerant). There may be some tradeoff between the desired specifications (for instance, a low-GWP refrigerant may lead to a lower coefficient of performance or a higher cost than a higher GWP alternative), so care must be taken to discuss with potential suppliers about the availability of suitable options.

Selection of the final device or system depends on adherence to the specifications by the potential suppliers, along with a tradeoff between capital and operating costs, and other technical aspects. Often, each supplier will quote the standard system within its catalog that will cover the service requirements in the specifications.

It is a good practice to evaluate different types of heat pumps (for example, comparing an air-source heat pump against a ground-source heat pump), given that higher initial capital costs may be compensated by reduced operations costs due to higher coefficients of performance. A lifecycle cost analysis (see Ref. 6) can be used to compare different alternatives to determine the minimum total cost over the system lifespan.

 

An example calculation

An industrial process requires 3,000 kW to raise the temperature of a stream to 45°C. Two alternatives are under evaluation: 1) use a natural-gas-fired heater with 80% efficiency; or 2) use a heat pump to transfer heat from a source at 24°C. The proposed heat pump is a closed one-stage vapor-compression cycle with an evaporator operating at 18°C and a condenser operating at 50°C. When reading the refrigerant properties at a pressure-enthalpy diagram similar to Figure 1, the following specific enthalpies are determined:

h1 (evaporator outlet): 1,461 kJ/kg

h2 (compressor discharge): 1,630 kJ/kg

h3 (condenser outlet): 421 kJ/kg

h4 (expansion valve outlet):421 kJ/kg

The plant manager wishes to compare the annual energy expense of both options, assuming that the plant is required to operate 8,640 hours per year, and that the energy rates are $4 per million Btu ($0.014/kWh) of natural gas, and $0.04 per kilowatt-hour of electricity.

Option 1: Natural-gas-fired heater. For option 1, the fuel heat consumption is the heat load divided by the heater efficiency:

 

Fuel heat consumption, option 1 = 3,000 kW/0.80 = 3,750 kW

 

The total annual energy consumption is:

3,750 kW × 8,640 h/yr = 32,400,000 kWh/yr

 

The total annual energy cost is:

 

32,400,000 kWh/yr × $0.014/kWh = $453,600/yr

 

Option 2: Heat pump. For option 2, the electrical power consumption is the heat load divided by the coefficient of performance. To obtain the coefficient of performance, Equation (3) is rewritten to express the condenser heat rate and compressor power as the working fluid flowrate multiplied by the enthalpy difference in each equipment, assuming no losses in piping:

 

COP = Qh / W = [m× (h2h3)]/[m( h2h1)]

 

Where m is the mass flowrate of the working fluid.

Since the flowrate through the compressor is the same as the flowrate through the condenser, then for this configuration:

 

COP = (h2h3)]/(h2h1) = (1,630 kJ/kg – 421 kJ/kg)/(1,630 kJ/kg – 1,461 kJ/kg)

 

COP = 1,209 kJ/kg/169 kJ/kg = 7.154

 

Then, the electrical power consumption for option 2 is:

 

W = Qh/COP = 3,000 kW/7.154 = 419 kW

 

The total annual energy consumption is:

 

419 kW x 8,640 h/yr = 3,620,160 kWh/yr

 

The total annual energy cost is:

 

3,620,160 kWh/yr × $0.04 kWh = $144,806/yr

 

This is significantly lower than the annual cost for option 1.

 

Concluding remarks

Heat-pump technology is mature, reliable and safe. It is an efficient means to provide heat to a growing number of uses at increasing temperatures. Savings in heating expenses in the long run often outweigh the initial capital investment of a heat pump over heaters. New working fluids are continuously being developed, first to replace ozone-depleting refrigerants, and currently to provide non-toxic, low-GWP, and ideally non-flammable alternatives to available refrigerants.

 

Acknowledgements

The author would like to thank Mr. Philippe Nellissen for sharing the figures of his authorship in this article, and Dr. John R. Abelson for his feedback to improve the article.

 

References

1. Secretary of Energy Granholm, Accelerating Cold Climate Heat Pump Technology and Adoption in the Midwest, January 26, 2022, www.youtube.com/watch?v=z5oeTxVpb2A

2. Nellissen, Philippe “Energy Efficiency in industrial processes – the role of heat pumps” European Heat Pump Association, February 6, 2015, Retrieved from: www.ehpa.org/about/news/article/energy-efficiency-in-industrial-processes-the-role-of-heat-pumps/

3. International Institute of Refrigeration (2021, January) “Air Source Heat Pumps for Space Heating and Cooling”

4. US EPA “Phasing out HCFC Refrigerants to Protect the Ozone Layer”.

5. Fifth Assessment Report (AR5) of the IPCC (Intergovernmental Panel on Climate Change).

6. Giardinella, Sebastiano; Baumeister, Zabdyk and Baumeister, Alberto, Using Lifecycle Cost Analysis for Best Project Value, Chem. Eng., December 2020, pp. 32–39.

 

Author

Sebastiano Giardinella is a visiting assistant research scientist and project engineer at the Illinois Sustainable Technology Center, Prairie Research Institute, University of Illinois at Urbana-Champaign (1 Hazelwood Dr., Champaign, IL 61820; Phone: +1-217-953-1424; Email: sg00@illinois.edu; URL: www.istc.illinois.edu) where he is involved in energy-storage and carbon-capture projects. He previously co-founded the Ecotek group of companies (www.ecotekgrp.com/en), where he has performed feasibility studies, corporate management, project management and process engineering consulting in projects for the chemical and energy industry. He is a project management professional (PMP), has a M.Sc. in renewable energy development from Heriot-Watt University, a Master’s degree in project management from Universidad Latina de Panamá, and a degree in chemical engineering from Universidad Simón Bolívar. He has written technical publications for Chemical Engineering magazine and other international associations, and holds one patent for an energy storage system and method.

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