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Heat Pumps: Decarbonizing the Process Industries

| By Jörg Freckmann, MAN Energy Solutions

As the chemical process industries push to decarbonize their businesses, heat pumps are emerging as a go-to alternative to fossil fuels. Efficient and cost-effective, heat pumps are a rapidly expanding solution for industrial heat demand

In the drive to meet global sustainability targets, there remains a major challenge in decarbonizing many process industries, which are responsible for around a fifth of all greenhouse gas emissions. Electrification, while possible, can present substantial efficiency challenges, which leads to the broad belief that many industrial sectors will be overly difficult to decarbonize.

In considering an alternative to electrification, the latest generation of high-temperature heat pumps offers an attractive approach. Large-scale heat pumps present an efficient mechanism to convert so-called “waste” heat or even ambient heat sources, such as the atmosphere or bodies of water, into valuable high-quality process heat.

Many facilities in the chemical process industries (CPI) rely on steam, the production of which requires a great deal of heat. Overall, steam production represents about two thirds of energy demand for the process industry sector. However, modern heat pumps are fully capable of delivering steam at temperatures up to 300ºC and at industrial volumes and pressures, even when using low-temperature heat sources. This article introduces industrial heat pump use and discusses the decarbonization potential for users of heat pumps.

 

The heart of the heat pump

Compression. At the heart of the latest generation of large-scale heat pumps is an efficient, robust and reliable compressor. Various kinds of compressors may be deployed for industrial heat pump applications, including scroll, reciprocating, screw and both inline and radially geared centrifugal types. In large industrial applications exceeding 10 MWth, centrifugal compressors are invariably used. As an example, integrally geared compressors can be deployed for process steam applications (Figure 1). Some types of these centrifugal compressors feature a bull gear able to drive pinion shafts and attached impellers for efficient compression. Designed to meet stringent industrial standards, radially geared centrifugal compressors have a strong track record and are commonly used elsewhere in the chemical and petrochemical industries.

FIGURE 1. Effective compression is the cornerstone of large-scale heat pump installations. Integrally-geared compressors are often deployed in such applications

Refrigerants.Various refrigerants can be employed for heat pump cycles, and the thermodynamic properties of the working fluid can significantly influence the heat pump process. Making the correct choice of refrigerant is a key consideration in optimization of the thermodynamic process. Qualities such as critical temperature and pressure and evaporation/condensation enthalpy are indeed critical. But other characteristics, such as compatibility with component materials like metals or seals, and chemical stability, as well as safety and environmental factors are also important, as are costs. Commonly used refrigerants fall into three broad groups — natural, hydrocarbons and synthetic.

The performance of a heat pump with a particular refrigerant is measured as the coefficient of performance (COP), which indicates that refrigerants like ammonia, butane and propane are among the most efficient naturally occurring refrigerants. COP is calculated as the ratio between the rejected heat and compressor work. Ammonia is generally considered to have the best COP for steam production, with the highest power density. Future research focusing on heat pump refrigerants, such as azeotropic mixtures, is needed to explore potential advances aimed at further optimization of process steam generation.

 

Meeting industrial demands

In comparison with district heating, processes found in the CPI typically require higher temperatures and pressures. Typically, these processes are met by conventional electrical or fossil-fuel-powered steam boilers. However, the flexibility afforded by heat pumps makes them attractive to production processes in those kinds of industries. As such, conventional boilers can be replaced by industrial heat pumps, especially as one of their main advantages is the ability to use waste heat and ambient energy to deliver more heat output than the electrical energy consumed.

During a conventional heat pump cycle used for industrial process-steam production, an evaporator is located at a heat source, which is followed by a compressor. The working fluid, once heated and compressed, is passed to a condenser located at the heat sink where the feedwater is evaporated into steam. Finally, a throttle valve or expander completes the closed-loop heat pump cycle. Depending on the process steam conditions — and the refrigerant and specific heat pump configuration used — a simple heat pump cycle can be installed where required steam conditions are below 2 bars, given that simple high-temperature cycles are limited to around 150°C.

FIGURE 2. Steam can be produced using a heat pump setup that may include additional steam-compression capacity

In cases where the process demands higher steam pressures, a secondary compressor stage can be added, either integrated into the heat pump compressor or installed as a separate motor-driven unit (Figure 2). Many industrial heat applications require multi-stage compression, which can be achieved by integrally geared compressors. As with many aspects of heat pump technology, integrally geared compressors are a proven technology with millions of hours of service across a substantial spread of industrial applications. Steam compressors deployed in conjunction with a high-temperature heat pump can see steam conditions reach temperatures of up to 300°C and pressures of 60 bars. A wide range of process steam requirements can thus be generated using a steam-production heat pump (Figure 3).

FIGURE 3. This figure shows the typical arrangement of a heat pump with an additional steam compressor, which can allow the system to achieve higher steam temperatures and pressures

Steam pressure between the heat pump and the steam compressor can be varied to achieve the best overall efficiency. Lower interim pressures are generally associated with better efficiency, but do require a larger steam-compression component. Actively cooling the steam during the compression phase by injecting water may also be used to boost overall efficiency and also allows a smaller heat pump to be employed in producing process steam. In any event, the combination of heat pump and steam compressor typically yields a COP ranging from just under 2 to over 3, depending on the refrigerant and the cycle configuration.

The inherent flexibility of the heat pump system gives it ample scope to support the cost-effective decarbonization of industrial sectors that are typically considered as particularly challenging targets. Indeed, power-to-heat technologies are the most efficient approach to achieving decarbonization goals, specifically through the integration of industrial high-temperature heat pumps. In particular, the compressor heat pump can be powered directly using renewable energy, making any heating or cooling capacity essentially carbon-free. Many industrial sites have the scope for onsite renewables or can sign contracts for the supply of low-carbon renewable electricity. Furthermore, the heat pump approach allows the effective use of both waste heat and ambient sources of energy. Heat pumps can thus deliver high-pressure steam by recovering process energy from cooling water or other process streams. Many industrial sites are also located adjacent to large bodies of water, making that element of heat pump deployment also feasible.

 

Heat pumps in play

Heat pumps are not a new concept for the CPI and are already commonly used in some processes. For example, during propylene production, heat pumps have been used for many years in the recovery and purification phase of the propane dehydrogenation (PDH) process.

High-temperature heat pumps clearly offer a proven way to convert renewable electricity and low-quality heat sources into useful high-value process heat and steam for industrial applications. This approach delivers multiple benefits — in particular a route to both decarbonizing process heat and electrifying heat production. These qualities displace fossil fuels, in turn reducing primary energy costs, as well as improving the security of energy supply and avoiding extreme price volatility. These are factors that have characterized the energy market over recent years. Improved energy efficiency in industrial processes not only leads to substantial primary energy savings and subsequent reduction of CO2 emissions, though. Much renewable generation capacity is variable, given its derivation from wind and solar energy. During periods of surplus renewable energy, heat pumps can be adjusted to draw additional power and effectively soak up this excess. This improves the stability of the grid and helps operators maintain voltage and frequency control. These faculties potentially represent another revenue stream for heat pump operators. Similarly, heat pump operations may be reduced during periods when renewable-energy output is low in comparison with demand. In each case, the heat storage capacity of the system must be considered, but these novel operational modes are certainly feasible.

 

A sound business decision

Medium- and larger-sized plants offer a significant potential for large high-temperature heat pump deployments, and are good candidates for heat pump use because of the large decarbonization potential available in the CPI. However, heat pumps inevitably face competition from the well-established fossil-fuel heating systems currently used in the CPI. Despite their environmental advantages and proven capabilities, heat pumps understandably face some skepticism regarding their financial viability. A cost comparison between heat pumps and conventional boilers effectively dispels any concerns, given the cost of fossil fuels and their associated carbon emissions certificates, which are major cost considerations.

For example, a heat pump system using ammonia as a working fluid could generate 85 ton/h of superheated steam at 3.5 bars and 200°C using a cooling water circuit in a petrochemical production process as a heat source with a temperature of around 60°C. This configuration can achieve a COP of 2.8. Under these conditions, the heat pump would run for 8,500 h/yr for its 30-yr estimated lifespan. Although the capital expenditure (CAPEX) for a heat pump system is higher, by a factor of about three, the operational expenditure (OPEX) is far lower than for a gas-fired boiler. In this scenario, the return on investment for the heat pump is around 5.5 years, while a levelized cost of electricity (LCOE) comparison makes the heat pump solution a solid business decision in comparison with a fossil-fueled alternative (Figure 4). Heat pumps are certainly highly efficient — for the supply of process-steam, they can achieve a COP of up to 3. For reference, an electric boiler would typically have a COP of less than 1.

FIGURE 4. The LCOH (steam production) is compared between a heat pump and gas-fired boilers. The basis for this comparison assumes an estimated electricity price of $80/MWh and a gas price of $35.31/MWh

Given the ability of heat pumps to offer heat (as hot water or steam) or cooling for multiple processes, it is an approach well suited to numerous industrial sectors. This is especially the case as many industries are making substantial efforts to reduce their direct and indirect greenhouse gas emissions. The route to a net zero industry will rely on improving efficiency and limiting the need for fossil-derived energy wherever possible. Operating boilers with low-carbon fuels like biogas or green hydrogen is one approach, but new process technologies that enable renewable-driven power-to-heat capabilities offers far more scope. ■

Edited by Mary Page Bailey

All images provided by MAN Energy Solutions

Author

Jörg Freckmann is a senior manager in strategic business development for heat pumps, energy storage and petrochemical applications at MAN Energy Solutions ([email protected]). Prior to his current role, he spent several years working as sales manager for gas and steam turbines for power generation applications across many industry sectors, including pulp and paper, waste-to-energy, biomass, concentrated solar power plants and mechanical drive applications in the chemical and petrochemical industries. He studied mechanical engineering at the Ruhr University Bochum, Germany, specializing in power generation, turbomachinery and gas and steam turbines.