Analysis of technologies and potentials for heat pump-based process heat supply above 150 °C

Abstract The transition of the manufacturing industry towards carbon neutrality requires a reduction of the emissions from combustion for the supply of process heat. Heat pumps are an efficient alternative technology for supplying heat while improving the overall efficiency and shifting to potentially carbon neutral electricity. The state-of-the-art technology is limited to supply temperatures between 100 °C and 150 °C because of lower efficiency and component limitations. This paper has therefore analyzed two promising concepts for higher supply temperatures and found technically and economically feasible solutions for process heat supply of up to 280 °C. These solutions are using large-scale equipment from oil and gas industries for applications in energy-intensive industries. The suggested systems benefitted from the economy of scale and access to low electricity prices. The concepts outperformed a biogas-based solution, and they were competitive with biomass or natural gas systems with respect to economic performance. It was concluded that an electricity-based heat supply is possible for a wide range of industrial applications and accordingly represents an important contribution to fulfilling the objectives of lower climate impact of energy supply in industry.


Introduction
The ambitions to reduce greenhouse gas emissions do inevitably require sustainable alternatives to fossil fuel-based combustions for supply of process heat to industrial processes. Electricity-driven heat pumps imply the general potential to operate emission free and do thereby represent a sustainable long-term solution for emission free process heat supply.
Currently available heat pump technologies are however limited to supply temperatures of 100 °C to 150 °C, while electric boilers and biomass boilers are often mentioned as alternatives in energy transition strategies. The overall feasibility for heat pump systems in such applications is among others limited by technical component constraints as well as limited thermodynamic performances, resulting in limited operating performances.
Zühlsdorf et al. [1] have therefore analyzed the possibilities for heat pump-based process heat supply at large capacities and temperatures above 150 °C. They evaluated the technical and economic feasibility of two heat pump systems for two case studies. The main results from [1] are summarized by this extended abstract. The article focused on large-scale applications and considered components as known from oil-and gas applications, as these are capable of operating in more challenging conditions and enable exceeding the limitations known from available refrigeration equipment [2]. In addition, the focus was on applications, in which the plant owners have access to electricity at low costs or the possibility to invest in own renewable electricity generators, such as wind farms and photovoltaics, as these are ensuring low levelized cost of electricity [3].
1.2. Analysis of technologies and potentials for heat pump-based process heat supply above 150 • C, Benjamin Zühlsdorf, DTI 9th September 2019, Copenhagen, Denmark

Methods
The study considered two different heat pump systems, namely a cascade multi-stage steam compression system and a reversed Brayton cycle. The cascade multi-stage steam compression system is shown in Figure 1 and consists of bottom cycles that are recovering the heat from the heat sources while providing heat to the evaporator of the top cycle, in which the steam from the evaporator is compressed in several stages. The steam is cooled by liquid injection after each compression stage. The system can supply steam at every pressure level to the system, ensuring an optimal integration into the process and thereby maximum performances.  [1] The less complex layout of the reversed Brayton cycle is shown in Figure 2. The cycle consists of three heat exchangers, as well as a turbocompressor and a turboexpander, which are mounted on the same shaft. The cycle uses CO2 as working fluid and operates completely in the gas phase.
The cycles were modelled with energy and mass balances. Design variables, such as pinch points in the heat exchangers or pressure levels were defined or optimized under consideration of common limitations. The investment cost of the equipment was estimated with cost correlations and validated with estimations obtained from manufacturers. 1.2. Analysis of technologies and potentials for heat pump-based process heat supply above 150 • C, Benjamin Zühlsdorf, DTI Both cycles were evaluated for two case studies. The first case study was alumina production in which 50 MW were supplied to heat thermal oil from 140 °C to 280 °C, while heat was recovered between 110 °C and 60 °C. The second case study was a spray dryer for milk powder production in which an air stream was heated up from 64 °C to 210 °C with a capacity of 8.2 MW, while a heat source at 50 °C was recovered.

Subcooler
Both technologies were evaluated in both cases for a set of economic boundary conditions. Three economic scenarios were considered that corresponded to the fuel cost in Norway, Germany and Denmark in 2020 and one scenario was considered corresponding to the acquisition and operation of own renewables.

Results
The heat pump systems were designed and optimized for both case studies. Table 1 shows the COP and the total capital investment TCI for both cases and both technologies. It may be seen that the COP for the cascade system was estimated to be 1.9 in both cases, while it was 1.7 for the reversed Brayton cycle in the alumina production and 1.6 in the spray dryer case. The investment cost were relatively similar for the two technologies, while the economy of scale yielded considerably lower specific investment cost for the alumina production.  Figure 3 shows the levelized cost of heat for both technologies and both case studies for all economic scenarios and compares them to the alternative heat supply technologies. The levelized cost of heat is divided into the contributions accounting for the investment, the fuel cost and an exemplifying CO2 tax of 50 €/ton to indicate the impact of a potential tax. In the case of the alumina production, the levelized cost of heat reaches as low as 31 €/MWh to 33 €/MWh under consideration of own renewable electricity facilities, while it is between 44 €/MWh and 46 €/MWh for the spray dryer case. In the spray dryer case, the heat pump-based solutions are competitive with a biomass boiler and a natural gas boiler under consideration of the assumed CO2 tax. In the alumina production case, the lowest levelized cost of heat are obtained for the heat pump systems.

Conclusions
The study analyzed a reversed Brayton cycle and a cascade multi-stage steam compression for largescale process heat supply at temperatures above 150 °C. It was pointed out that these temperatures might be reached by components from oil-and gas industries and that low electricity prices, as typically accessible for energy intensive industries or obtainable from acquiring and operating own renewable facilities, may improve the economic performance considerably. The levelized cost of heat for the heat pump-based systems were competitive to the biomass boilers and natural gas boilers for the spray dryer case study and outperformed both for the alumina production case study. This study has accordingly demonstrated, that heat pump systems are a viable alternative for process heat supply in industrial processes at temperatures of up to 280 °C.