**1. Introduction**

Energy is one of the sectors that pollute and harm the environment the most [1]. A key challenge is to address global environmental problems by supporting energy and environmental conditions in parallel. For sustainable development, it is important to change practices and technologies. Total energy use and efficiency are significant motivating factors for assessing the environmental effect of energy use on the environment. Therefore, it is essential to follow the principles of sustainable development strictly [2]. Aiming at a cleaner and better future, the negative impacts of energy use on the environment can be minimized by implementing the usage of renewable energy sources or by adopting environmentally friendly technologies.

Electricity and heat generation contribute to almost half of the global annual CO2 emissions [2]. One of the major contributors to climate change is emissions of CO2 from the energy sector, which were a major topic for discussions at the 21st Conference of Parties (COP21) in Paris from 30 November to 11 December 2015. As an outcome of this conference, initiatives have been taken to address these issues of annual emissions so that global temperature rise will be below 2 ◦C, and preferably below 1.5 ◦C. This target can be achieved by substituting fossil fuels, specifically coal, oil, and natural gas, with renewable energy sources [3].

Heat pumps offer an energy-efficient solution to heating and air conditioning as they can use renewable electricity and low value heat that can often be taken freely from surroundings. Since heat pumps rely on transmission of heat rather than generation of heat, they do so at one-quarter of the operational cost of conventional heating or cooling technologies, depending on the efficiency of the heat pump, which is often given as the coefficient of performance (COP).

With the electricity markets introducing more and cheaper electricity from renewable sources, heat pumps are gaining market share, replacing traditional fuel-based heating. This is currently happening in the industry as well, with higher temperatures—e.g., above 120 ◦C—being a challenge. This is where high temperature heat pumps [4] become of interest. The Stirling-cycle based heat pump has already been shown to be efficient at high temperatures and high temperature lifts (see, for instance, the previous work by the authors [5]).

The manufacturing phase of a heat pump is a key contribution phase for determining the environmental impacts arising throughout a product life cycle [6,7]. However, the importance of the manufacturing phase in a life cycle assessment (LCA) is dependent on the heat pump, its capacity, main components, and efficiency [8]. The environmental impacts of the operational phase are sometimes less than the impacts caused by the production and assembling phase. For cases such as these, an LCA study is greatly suggested to recognize and quantify the environmental impact hotspots along the complete life cycle of process units or products.

According to Linke et al. [9,10], to make improvements in manufacturing processes and to attain environmental benefits, companies should add Environmental Impact Assessment (EIA) to their manufacturing phase of products. EIA is a necessary step for the planning of any technical structure to gain clear insight into the likely environmental impact of the structure. EIA techniques are designed to minimize or avoid the adverse effect of development, a process, or a product on the environment.

A comprehensive study was conducted by Stamford et al. [11] to investigate the life cycle environmental and economic sustainability of Stirling engine micro-CHP (combined heat and power) systems and compare it with conventional energy provision from natural gas boiler and grid electricity. Another study addressed the environmental impacts of domestic Stirling engine micro-CHP integrated with solar photovoltaics and battery storage. They concluded that relative environmental impacts can be reduced by 35–100% by replacing grid electricity and a gas boiler by such integrated system [12]. Other relevant work includes two studies that estimated the CO2 reduction achievable by Stirling engine and internal combustion engine-based CHP systems, but they did not follow a life cycle approach [13,14].

In this paper, the environmental footprint of an industrial size Stirling cycle-based heat pump is compared to that of natural gas or oil-fired boilers. Environmental footprint as the name suggests is defined as environmental impacts associated with any entity, process, or product. It considers the resources a person/product/process utilizes and the resulting emissions to land, air, and water. The study was made using the SimaPro software [15] for LCA. The construction phase, 1, 8, or 15 years of use, and the decommissioning and recycling are considered.

LCA is increasingly becoming standard procedure environmental footprint analysis and comparison of processes or products. One novelty of this paper is to apply it to an energy technology that has very recently found application at an industrial scale. Sufficient real-time data have recently been produced to make this study possible. While hardly any work on LCA applied to Stirling cycle-based energy technology has been reported in the open literature, this paper addresses a reversed Stirling cycle-based heat pump.

#### **2. Materials and Methods**

The description of the system and a description of the LCA-methodology used in this work are given in the two next sections. The goal and scope, as well as a description of the system boundaries, are given in the third and fourth sections.

## *2.1. System Description*

The heat pump studied for this study is a double-acting alpha configuration Stirling cycle heat pump. A Stirling engine is driven at different temperatures by periodic compression and expansion of a working fluid (for this study, helium gas) such that the net transmission of heat energy results in mechanical work. When operated as a heat pump, the process runs in reverse: work (electricity) is used to yield high-temperature heat from low-temperature heat.

The heat pump is comprised of main components such as the heater, regenerator, cooler, and compression and expansion cylinders arranged in a Franchot configuration. The internal heat exchangers include heating, cooling, and regenerating sections in the same unit. The heat exchanger is constituted of stainless-steel tubes while the regenerator is made of a metallic mesh of stainless steel. The heat pump in this study as shown in Figure 1 is used to recover heat and use this heat along with the electrical input (250 kW) to generate steam at 10 bar at an output of 500 kW [16].

**Figure 1.** The HighLift HTHP from Olvondo Technology installed in the heat pump room at pharmaceutical company AstraZeneca, Gothenburg, Sweden. The nominal heat output from the heat pump is between 450 and 500 kW.
