1 Introduction
Increasing emissions of greenhouse gases (GHG) due to human activities have led to a substantial increment in GHG atmospheric concentrations. In 2019, they were nearly 43.1 billion tCO
2-eq, a 39.4% increase from 1990 (The world count
2021). Global surface average temperature increases by 0.2 °C per decade (European Commission
2021). Global warming has been proven to increase the frequency and severity of fire weather conditions Abatzoglou and Williams
2016. Climate change has increased forest fire risk across Europe. The year 2020 was more than 1.2 °C hotter than the average in the nineteenth century (ECMWF
2020). In 2021, most European countries suffered the worst heatwave in over 30 years (Severe-weather
2021).
By 2030 and compared with 1990 levels, the EU climate and energy framework targets a 40% cut in total GHG emissions, increasing renewable energy dependence by 32% of total EU energy consumption and improving energy efficiency by 32.5% (Council of the European Unieon
2013).
On 15 October 2016, the Montreal Protocol parties reached an agreement in Kigali to phase down hydrofluorocarbons (HFCs) and approved a timeline for their gradual 80–85% reduction by the late 2040s (Clark and Wagner
2016). These chemicals are ozone friendly, but a GHG thousands of times more potent than carbon dioxide (CO
2). In this regard, R134a is one of the most common HFC refrigerants used in air conditioners and refrigeration applications. A definitive transition to working fluids with a GWP below 150 would mitigate the climate impact caused by these widespread systems (
2020).
Solar energy is the cleanest and most abundant renewable energy source, but the temperature level achieved from this technology is not enough to be used directly for heating. However, it can be combined with heat pumps (solar-assisted heat pumps). Higher evaporating temperatures can increase heat pump energy performance, resulting in lower compressor power consumption. Moreover, photovoltaic/thermal (PV/T) coupled with heat pumps would increase PV/T electrical performance due to an operational temperature reduction.
A few studies have evaluated their potential for energy efficiency increase. Ji et al. (Ji et al.
2007) experimental and numerically studied PV/T direct expansion solar assisted heat pump performance. They proved that a higher solar intensity increases heat gain, output electricity, and PV/T efficiency. Wang et al. (Wang et al.
2018) considered a system that operates with water or air for different purposes (building cooling, heating, and hot water). Water increases an average 13.8% generation efficiency.
Other studies showed that combining a heat pump with multiple evaporator configurations increases system performance. Fang et al. (Fang et al.
2010) used air-cooled heat exchangers as heat pump’s indoor and outdoor coils connected to a PV/T evaporator. This solution incremented the cell average efficiency by 23.8% and the heating coefficient of performance (COP) by 3.5%. Xu et al. (Xu et al.
2011) tested a PV/T fixed truncated parabolic concentrator with six flat strips combined with a heat pump. It reached an average COP of 4.8 for heating water from 30 to 70 °C. Dott et al. (Dott et al.
2012) evaluated different strategies for space heating that increased the system efficiency and seasonal performance.
Regarding solar-assisted heat pump configurations, Bellos et al. (Bellos et al.
2016) concluded that for an electricity cost ranging from 0.2 to 0.23 € per kWh, 20 m
2 PV panels coupled with an air source heat pump were the most economically attractive solution. Instead, when coupled with a water source heat pump, they are convenient for higher electricity costs. Tzivanidis et al. (Tzivanidis et al.
2016) assessed three water source heat pumps, a solar source heat pump, and a solar fan coil heating system from energetic and economic perspectives. The solar-assisted heat pump provides the best thermal comfort and is suitable for demanding weather conditions. Also, it results in the highest collector efficiency with the lowest energy consumption. Bellos et al. (Bellos and Tzivanidis
2017) considered a solar-assisted heat pump with the highest system COP in 20 different European cities, with an average of 35% electricity savings than conventional air-source heat pumps.
In addition, the ejector-compressor combination in solar-assisted heat pump configurations has benefits in system performance. Huang et al. (Huang et al.
2011) performed energy and economic analysis for a solar-assisted ejector system with a vacuum-tube collector. This arrangement can save 17 to 27% electricity, and the payback period decreases with a cooling capacity increase. Dang (Dang
2012) simulated a combined solar-driven ejector-vapour compression cycle with R1234ze(E) and R410A in the ejector and vapour compression cycles, respectively. A vacuum tube solar collector was the heat source of the ejector. Increasing solar heat input benefits the system COP, with 50% and 20% energy savings in heating and cooling modes. Xu et al. (Xu et al.
2017) theoretically proposed an R600a compound ejector-compression refrigeration cycle with a 1.3-m
2 flat plate collector as a heat source. The proposed system has a 24% higher COP than a conventional system. Al-Sayyab et al. (Khalid Shaker Al-Sayyab et al.
2021) compared a compound ejector-heat pump system with an R134a conventional heat pump for simultaneous heating and cooling. The study concluded that R450A increases COP by 7% and 75% in cooling in heating modes.
Research about data centre cooling and district heating networks (DHN) is becoming increasingly crucial regarding cooling and heating applications. In this way, mobile internet users are projected to increase from 3.8 billion in 2019 to 5 billion by 2025. Internet of Things (IoT) connections is expected to double from 12 billion to 25 billion. This increases the demand for data and digital services, with global internet traffic expected to double by 2022 to 4.2 trillion gigabytes (Association
2020). It requires new data centres, increasing power demand by 15% and 20% (Avgerinou et al.
2017). One of the most critical data centre challenges is reducing the operating cost (electricity consumption by servers and cooling system) and the associated carbon footprint. In the European Union, information and communication technology (ICT) represents 2.5% of the total electricity consumption (European Commission
2015). The energy consumed by the data centre is converted into excess heat removed by the cooling system, causing an electricity consumption that represents 40% of the total consumption (Payerle et al.
2015).
A few studies highlight the relevance of improving the cooling system’s energy efficiency by employing excess heat (waste heat recovery). Zhang et al. (Zhang et al.
2015) simulated a data centre water-cooled refrigeration system with thermosyphon for waste heat recovery in Beijing. He et al. (He et al.
2018) concluded that heat pipe-heat pump arrangements for supporting DHN in Chinese cities would reduce 10% energy consumption compared to traditional coal boilers. The proposed technology reduces by 33% and 60% the electricity required for building cooling and heating, respectively. Deymi et al. (Deymi-Dashtebayaz and Namanlo
2019) proposed an air-side, water-side, and combined air or water economiser for Iranian data centres that reduced the cooling requirements by 80%, improving the data centre’s power usage effectiveness (PUE) by 11%. Sheme et al. (Sheme et al.
2018) included two renewable energy sources for data centre cooling that provided more significant surplus hours than when considered independently.
There is a growing interest in waste heat recovery for DHN purposes. Using two energy analysis tools, Lund et al. (Lund et al.
2016) proved a significant economic benefit by integrating large heat pumps in a Danish DHN. Oró et al. (Oró et al.
2019) concluded that an air-to-water heat exchanger in a data centre’s indoor air return duct shows promising thermo-economic results in Barcelona (Spain). Wahlroos et al. (Wahlroos et al.
2017) considered both the data centre and DHN operator’s perspectives. A high waste heat share in the Espoo (Finland) DHN saves costs up to 7.3%. Besides, other studies propose reusing the data centre waste heat through different configurations. Bach et al. (Bach et al.
2016) simulated a large-scale heat pump’s performance connected to a DHN in Copenhagen (Denmark) using waste heat from sewage, drinking water, and seawater. The seasonal COP variation has a minor impact on the overall performance. Pieper et al. (Pieper et al.
2019) considered three heat sources: groundwater, seawater, and air (also the combination). The HP capacity should be at least 60% of the maximum hourly peak demand. Oró et al. (Oró et al.
2018) reused data centre waste heat (liquid cooling on-chip servers configuration) for swimming pool heating in Barcelona. This method saves 6% electricity compared to a traditional air-cooling type with a 60% CO
2-eq emission reduction.
The data centre cooling system and the waste heat utilisation technique mentioned in the previous literature can only operate seasonally. Moreover, these works do not consider environmentally friendly refrigerants (associated with a low global warming potential (GWP), electricity consumption, and PUE). This study proposes a novel solar-driven ejector-heat pump combination for simultaneous data centre cooling and DHN. This system is proposed to reduce energy consumption, carbon footprint, and annual operating costs. The proposed system presents the novelty of the ejector’s pump avoiding by combining PV/T waste heat with the evaporative condenser as a complete ejector driving force. The heat pump simultaneously cools a data centre and injects the heat into a DHN, and the PV/T provides the required electricity. Data centre waste heat recovery could significantly reduce energy consumption and carbon footprint.
A single thermodynamic analysis of systems may lead to a high system cost. A preoperative evaluation can be obtained by simultaneously considering also economic parameters. In the literature review, techno-economic analyses of such systems have not been reported. Therefore, this work focuses on operational parameters such as solar intensity, condenser temperatures, and low GWP refrigerants on the overall system’s energy performance and total cost (capital, operational, and maintenance costs). This can give a complete overview of the proposed system potential under different external conditions. A techno-economic evaluation is performed for several low GWP refrigerants and compared with R134a based on the COP, data centre cooling efficiency, and total cost rate minimisation. Shell-and-tube heat exchangers are considered and sized for the evaporator, generator, and condenser.
The main objective of the current work is to propose a techno-economic comparison for a compound ejector-heat pump system with PV/T (photovoltaic thermal) waste heat and the most appropriate low GWP alternative refrigerant to R134a.
The rest of the paper is organised as follows to achieve these objectives. Section 2 presents the system configuration and low GWP refrigerants. Section 3 describes the system modelling, including thermodynamic and economic equations. Section 4 discusses the main results of the study. Finally, Sect. 5 summarises the most relevant conclusions of the work.
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