# Feasibility Analysis of Industrial High-Temperature Heat Pumps in Greece
## 1 Introduction  
High-temperature heat pumps (HTHP) are promising solutions for producing process heat at temperatures over 100 \({}^{\circ}\)C and they can be exploited in the industrial sector. The existing available systems can operate up to 150 \({}^{\circ}\)C [1, 2], while achieving 200 \({}^{\circ}\)C is expected to be feasible in the near future [3, 4, 5]. HTHPs can exploit low-grade waste heat streams (WHS) from various industrial processes [6], or they can be driven by ambient heat sources. These systems can substitute conventional furnaces and boilers, leading to a significant reduction in CO2 emissions and also offering reduced operational costs due to their high efficiency [6]. Therefore, HTHPs constitute a sustainable solution for decarbonizing industrial processes and transitioning towards a cleaner energy future. Furthermore, they are also suitable for district heating applications [7, 8]. Numerous industries can exploit HTHPs, includingthe petrochemical and general chemical industries, paper and pulp industry, plastics and rubber industries and textile industry. However, HTHPs face some challenges which affect their further adoption. Firstly, there is a need to develop commercial and efficient systems capable of operating at higher temperatures up to 200 \({}^{\circ}\)C or more [9]. In this direction, the selection of appropriate working fluids that are both environmentally friendly and present high performance is required [6]. Moreover, advanced materials and cycle configurations have to be examined for technological objects to overcome current limitations [10]. Lastly, the economic feasibility of HTHPs depends on regulatory frameworks, electricity/gas prices and various other general parameters that vary among different countries [11]. Aiming to develop HTHPs for industrial decarbonization through electrification, significant research has been undertaken in the last few years. In the case of ultra-low waste heat streams, the use of cascade HTHPs has been suggested as a suitable solution, according to [PERSON] and [PERSON] [12]. Specifically, they studied various working fluids, and they identified R1234 ze, R1224 yd(Z), R1233 zd(E) and R1336 mzz(Z) as the most suitable options. Also, they stated that the cascade configuration is preferable when there is a temperature lift over 70 K. The use of transcritical HTHPs is a promising choice for heat production at high temperatures. [PERSON] et al. [13] studied a transcritical HTHP with CO\({}_{2}\) as the working fluid and they achieved a coefficient of performance (COP) of 2.22 at 200 \({}^{\circ}\)C hot air production. The incorporation of eject devices in transcritical HTHPs has been studied by [PERSON] et al. [14]. In addition to the eject devices, they also used two evaporators and this configuration was concluded to outperform the simpler cycles. Another idea is a hybrid system that combines mechanical compression and an absorption heat pump to achieve high temperatures. [PERSON] et al. [15] studied this concept and they concluded that this unit can process heat production up to 200 \({}^{\circ}\)C, with compression rates playing a critical role in the unit's performance. The application of zeotropic mixtures as working fluids in HTHPs is another very promising solution. This idea aims to exploit the glide of the mixtures during the phase change, aiming to enhance the heat transfer conditions in the heat exchangers and to minimize the exergy destruction. In this direction, [PERSON] et al. [5] found that the application of cyclopentane/R1336 mzz(Z) mixture is a suitable choice for HTHPs that produce process heat up to 200 \({}^{\circ}\)C. In another study, [PERSON] et al. [1] suggested the mixture HFC32/HFO1234 yf for HTHPs that produce heat up to 150 \({}^{\circ}\)C. The HTHP using this mixture presented a 14% higher performance compared to that operating with pure working fluids.