Ireland has minimal indigenous fossil energy resources and as an island is heavily dependent on fossil fuel imports. In addition, fossil fuels (oil, natural gas, coal and peat) accounted for 91.4% of the total primary energy requirement for electricity in 2014, and heat generation is dominated by fossil fuels; natural gas, oil, coal and peat [
1]. The combustion of such fossil fuels in energy generation is a major contributor to greenhouse gas emissions (GHG) in Ireland, accounting for 63.3% (37.06 Mt CO
2-eq) of total national greenhouse gas emissions in 2012 [
2]. Ireland is committed, under the European Union (EU) Renewable Energy Directive (RED) 2009/28/EC, to reduce greenhouse gas emissions and to develop alternative energy sources to reduce dependence on finite fossil fuel resources [
3]. Under this Directive, Ireland’s specific mandatory renewable energy target is 16%, driving the need to develop indigenous renewable energy resources. The National Renewable Energy Action Plan (NREAP) set a target of 40% electricity consumption target from renewables by 2020 [
4], with latest data showing a rate of 22.7% (5479 GWh) was achieved in 2014 [
1]. The majority of renewable energy in Ireland is generated by wind, accounting for 71% of the renewable electricity contribution, with hydro energy accounted for 11%, and biomass accounting for 8% in 2014 [
1]. The majority of the electricity from biomass is generated by co-firing with peat, to offset peat combustion. Peat combustion produces large volumes of CO
2 directly but also indirectly reduces the carbon budget of indigenous peatlands [
5]. The uptake of solar power generation in the Irish renewable energy market to date has been low, accounting for less than 0.3% of total electricity generated in 2012 [
6]. Ireland is currently on track to meet the national renewable electricity target of 40%, but is currently behind schedule in meeting the renewable heat target [
7]. This has implications for meeting the binding 16% renewable energy target; the Sustainable Energy Authority of Ireland (SEAI) has estimated that the cost to Ireland may be in the range of €100 million to €150 million for each percentage point Ireland falls short of the overall 16% renewable energy target (Dáil Éireann Debates (Unrevised), Written Answer No. 597 ‘Renewable Energy Generation Targets’ (1 December 2015)).
1.2. Environmental Impacts of Solar PV Systems
A key driver for renewable energy systems implementation is the perceived carbon benefits which can be achieved when compared to conventional fossil energy systems. However, in determining the tangible environmental benefits of these systems, the entire life cycle must be taken into consideration. For example, biomass is often stated to be a carbon neutral fuel, however, research has shown that production of indigenous energy crops and harvesting and processing of wood biomass in Ireland is not carbon neutral when cultivation, harvesting, processing and transportation are taken into consideration [
22,
23].
Photovoltaic energy systems, directly generating electricity from solar energy, are free from fossil energy consumption and GHG emissions during operation. Therefore such systems would seem to be carbon neutral and cause no environmental burdens [
24]. However, in determining the tangible environmental impacts of these systems, the entire life cycle must be taken into consideration, including PV system manufacture, operation, and disposal and/or recycling. When viewing PV systems in such a holistic manner, it is clear that energy production from such systems is directly linked to GHG emissions and thus these systems are not, in practice, emission free technologies [
25].
Two indicators of sustainability, energy payback time (EPBT) and GHG emission rate, are commonly used to evaluate the energy and environmental performance of PV systems [
26,
27,
28]. The energy payback time is defined as the period required for a renewable energy system to generate the same amount of energy (in terms of primary energy equivalent) that was used to produce the system itself [
29].
GHG emissions, along with other environmental impacts, can be evaluated for PV systems using life cycle assessment (LCA), an internationally accepted tool for assessing the environmental sustainability of a system, product or service and provides the most comprehensive method currently available [
30,
31]. LCA is a method which can be used to analyse the environmental impacts of a system over its entire life-cycle, from raw materials acquisition through production, use and operation, and finally, end-of-life treatment, recycling and final disposal [
32]. Environmental impacts are assessed based on a life cycle data inventory of relevant material and energy flows over the life-time of the system. In this sense, LCA is a “cradle-to-grave” approach as it begins with the gathering of raw materials from the earth to create the product and ends at the point when all materials are returned to the earth. In order to ensure harmonisation and standardisation of LCA studies, the methodology is governed by a set of international standards, ISO 14040 and ISO 14044; ISO 14040 describes the principles and framework for LCA, providing a basic explanation of the standard LCA process [
32], ISO 14044 specifies minimum requirements and provides guidelines for an LCA [
33]. Despite the presence of these LCA standards, studies examining similar PV systems have been found to show a large variance in results due to differences in methods and assumptions [
34].
The environmental impacts of PV systems are dependent on the material and energy requirements in production, operation and maintenance, and disposal/recycling. The relation of the environmental impacts to the functional unit of the system, e.g., unit of electricity produced, is dependent on the energy generated by the system. The annual energy output is determined by a number of factors such as type of PV module, manufacture technologies, module conversion efficiency, installation location (roof top, façade or ground mounted) and pattern (integrated or mounted), array support structure, frame or frameless, application type (stand-alone or grid-connected) and performance ratio [
24].
Several comprehensive reviews have been carried out on studies evaluating the sustainability of PV systems [
24,
35,
36,
37]. The type of PV technology utilised (and resource and energy inputs required), along with the installation location have been identified as important factors in the environmental performance of solar PV systems [
25].
There is a wide range of available solar cell technologies, including; crystalline silicon technologies such as mono-crystalline (mono-Si), poly-crystalline (poly-Si), multi-crystalline (multi-Si) and ribbon multi-crystalline (ribbon-Si), and thin-film technologies such as amorphous silicon (a-Si), cadmium telluride (CdTe) and copper-indium-gallium-diselenide (CIGS). Each of these technologies has different material and processing requirements, leading to distinct emission profiles [
25,
38]. The solar conversion efficiencies of the different types of PV modules varies, which directly affects the energy return of the system [
39].
Manufacturing of PV cells and associated components is an energy intensive process which requires direct fossil fuel use, generally for heating processes, and substantial electricity inputs [
25], the use of which is significant over the life cycle [
39]. The carbon intensity of the electric grid supplying the PV production chain has been shown to significantly affect life cycle GHG emissions [
40], with emissions ranging from 1 g CO
2-eq per kWh electricity generated by PV when the manufacturing process is supplied by 100% renewable electricity, to 218 g CO
2-eq per kWh when the manufacturing process is supplied by coal electricity. This range of GHG emissions is still significantly lower than the carbon intensity of the Irish national grid at 457 g CO
2 per kWh in 2014 [
1].
The location of the solar PV installation is important due to the differing levels of solar insolation across the globe, and on a local scale. The amount of electrical output generated by PV systems generally increases with an increase in solar radiation intensity consequently leading to lower E-PBTs [
39]. Alsema and de Wild-Scholten [
41] found that solar electricity generated in Germany produces approximately 70% higher GHG emissions than the same system installed in Spain.