Evaluation of Integrated Concepts with CO2 for Heating

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energies Article Evaluation of Integrated Concepts with CO2 for Heating, Cooling and Hot Water Production Silje Smitt 1,*, Ángel Pardiñas 2 and Armin Hafner 1 􏰰􏰱􏰲􏰰􏰳 􏰵􏰶􏰷 􏰸􏰹􏰺􏰻􏰼􏰲􏰽 Citation: Smitt,S.;Pardiñas,Á.; Hafner, A. Evaluation of Integrated Concepts with CO2 for Heating, Cooling and Hot Water Production. Energies2021,14,4103. https:// doi.org/10.3390/en14144103 Academic Editor: Dimitris Katsaprakakis Received: 11 June 2021 Accepted: 30 June 2021 Published: 7 July 2021 Abstract: The hotel sector is characterized by high thermal demands and a large carbon footprint, which greatly contributes to the global warming effect. Consequently, there is a need to investigate solutions that can reduce energy usage within this sector by means of environmentally friendly and sustainable technologies. Integrated CO2 heat pump systems for heating, cooling, and hot water production in hotels have demonstrated promising results. This paper theoretically compares the energy consumption, environmental impact, and cost of three different design concepts for integrated CO2 units equipped with thermal storage. The main characteristics of the evaluated designs are single- stage compression, parallel compression, and ejector-supported parallel compression. Furthermore, two separate hot water charging strategies were implemented and investigated over a large span of ambient temperatures and loads. The evaluations were carried out by considering eight different European locations, ranging from Scandinavia to the Mediterranean. The results revealed that the ejector-supported parallel compression design was superior in terms of annual COP, which was found to be in the range of 4.27 to 5.01 for the Scandinavian locations and 5.03 to 5.71 for the other European locations. When accounting for investment cost and electricity prices, the payback period at the Scandinavian locations was 6.3 to 7.7 years. Payback periods of 3 and 4.5 to 7.5 were obtained for hotels located in the temperate and Mediterranean climates, respectively. The investigation also revealed that the hot water charging strategy, rather than the specific CO2 heat pump design, is the least expensive measure to enhance performance. Keywords: heat pump; system design; heating and cooling; hotels; CO2; thermal storage; numerical modeling; concept evaluation 1. Introduction Different actors are involved in a global and intersectoral effort to achieve the 2- degree goal of the Paris Agreement by limiting CO2 emissions through efficiency and reduction of energy demands [1]. Energy use in buildings involves approximately 18% of greenhouse gas emissions globally. A staggering one-third of these emissions are linked to commercial buildings, such as hotels [2]. Similar numbers are given for Europe, with the commercial sector being responsible for one-third of the total energy consumption and related emissions in buildings [3]. Thus, measures to increase efficiency by improved technology, management, and integration of demands in the hotel sector will significantly contribute towards realizing the goals of the Paris Agreement. Estimations indicate a potential in energy saving within the commercial sector of approximately 30% [4,5]. This is particularly important as the tourism sector is estimated to increase by 3.8% annually until 2030 [6]. The dominant thermal demands in hotels include domestic hot water (DHW) produc- tion, space heating (SH), and cooling, with the share between them depending on hotel location or quality of construction (level of insulation), among other factors. For SH and 1 Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjørn Hejes vei 1D, 7491 Trondheim, Norway; armin.hafner@ntnu.no SINTEF Energy Research, Kolbjørn Hejes vei 1D, 7465 Trondheim, Norway; angel.a.pardinas@sintef.no 2 * Correspondence: silje.smitt@ntnu.no Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Energies 2021, 14, 4103. https://doi.org/10.3390/en14144103 https://www.mdpi.com/journal/energies

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