Industry consumes about 32–35% of global energy and is responsible for 37% of the world’s greenhouse gas (GHG) emissions [1]. Approximately two-thirds of the GHG emissions in the industries in the European Union (EU) are created as a result of the use of fossil fuels for heat and electricity generation. Those industries include chemical, petrochemicals, iron, steel, cement, paper, pulp, minerals and metal [2]. Replacing fossil fuel energy with only 1% renewable energy sources could potentially reduce GHG emissions by 0.82% [3]. The European Commission (EC) has set up a target to reduce GHG emissions, compared to 1990 levels, by 30% until 2030 and 100% until 2050 by increasing the use of renewable energy sources by 32% and improving energy efficiency by 32.5% [4].

To promote the sustainable development goals of the EC, in 2019, industries in the EU launched a public-private partnership association named Sustainable Process Industry through Resource and Energy Efficiency (SPIRE). It represents 20% of the total EU manufacturing sector involving a great number of stakeholders and industries like cement, ceramics, chemicals, minerals and ores, non-ferrous metals, steeland water. The main aim of SPIRE is to improve resource efficiency and sustainability in industrial processes, regions and cities [5]. Their vision complies with the EU climate change plan to reduce fossil energy intensity by 30%, non-renewables primary raw material intensity by 20%, and GHG emissions by 40% and 100% by 2030 and 2050, respectively ^[4][6][7][4,6,7]. To help governments to implement a circular economy across Europe, SPIRE invests in the use of solar energy as one of the most promising renewable energy resources due to its economic viability, unlimited supply and environmental advantages [4].

Solar heating for industrial processes (SHIP) systems are alternative methods for replacing fossil-fuel based heating and cooling that have proven to be cost-effective and environmentally friendly. They represent 60% of the overall EU industrial energy demand and 26% of EU total energy consumption; and have been used in different industries for water heating, steam generation, dehydration, refrigeration and air conditioning ^{[8][9][10][11]}[8,9,10,11]. Different types of collectors like flat plate, evacuated tube, parabolic trough and linear Fresnel have been used in SHIP systems to provide the heat supply to designated processes and maintain the desired temperatures ^[1][12][1,12]. The restraints of SHIP systems include inconsistency in solar irradiance and a temperature limit of a maximum 150 °C, so further technical improvements on these technologies like developing new solar Fresnel collectors, parabolic trough collectors, and thermal storage systems are essential to meet the higher temperatures and cooling demands in industrial processes ^[1][12][13][1,12,13]. For instance, the new solar linear Fresnel collectors (LFC) with a single tracking system were originally designed by Industrial Solar GmbH [14] to generate a temperature of up to 400 °C for process heating, cooling and polygonation.

The SPIRE association aims to integrate, demonstrate and validate systems and technologies that are capable of achieving key resources and energy efficiency across all SPIRE sectors. They include non-metallic minerals (cement and ceramics), chemicals, basic metals (non-ferrous metals, and iron and steel) and water industries [5]. About 111 SHIP systems are documented in the EU, of which 10 are integrated into SPIRE industries and the rest into non-SPIRE industries. The use of SHIP systems in various SPIRE industries and processes is discussed. It reviews their location, type of collectors used and operating temperature, and identifies their limitations.

The non-metallic industry includes manufacturing of cement, ceramics, concrete, glass and stone products ^[15][21] but only cement and ceramics industries are members of the SPIRE association and discussed below. Only one SHIP system in the non-metallic industry was found and it is located in Austria. It is assembled in the concrete industry and uses flat plate collectors to supply hot water at temperatures of 25–45 °C for drying and heating purposes [12].

The use of solar thermal energy in the cement industry has been extensively discussed ^{[16][17][18][19]}[22,23,24,25]. A number of different solar components have been used in Portland cement production like a Solar limestone calciner ^[16][22] or solar furnace to concentrate the solar irradiance of raw materials ^[17][23]. Tregambi et al. ^[16][22] mixed solar limestone calciner with commercial clay to produce Portland cement samples at 1500 °C over 15 min. The new samples were compared to the reference sample, which was a mix of fresh lime and commercial clay. It was found that there was no significant difference in terms of quality between the two samples; however, the sample containing the solar calciner clearly demonstrated significant economic and environmental advantages. Another study used a solar furnace to concentrate solar irradiance on raw materials for the production of Portland cement clinker ^[17][23]. The authors used two solar cycles for producing grey and white clinkers at different sequent dwell times of 5 min with temperature ranges of 900–950 °C and 1250–1300 °C and 15 min with a temperature range of 1500–1550 °C. The subsequent results were compared to the reference clinker that was used to produce cement with consistency that complies with the EN 197-1 standards. It was found that the grey clinker showed similar properties to the reference sample, which was not the case with the white clinker due to its low absorbance of solar energy. Moumin et al. ^[18][24] used a solar calciner in a cement plant and considered the arrangement of the mirrors in the heliostat field of the system, which could be potentially used in the cement industry in Spain. The energy balance of the solar calciner based on different demands and GHG emissions scenarios by 2050 was analysed. Considering the irradiance, reactor efficiency and solar multiple, the GHG emissions potentially could be reduced by 14–17% and cost 74-118 EUR/t. It was envisaged that the replacement of the conventional fossil fuel calciners with solar calciners will result in a reduction of GHG emissions by 2–7%. Another study reported that the calcination process consumes 80% of the energy in the cement industry and is responsible for 7% of global GHG emissions ^[19][25]. The authors assessed the environmental sustainability of solar calcination for cement production using life cycle assessment to compare three solar technologies for calcination: (i) a full solar thermal system; (ii) hybrid system in which the solar thermal provides 14% of the thermal energy; and (iii) hybrid system in which the solar thermal provides 33% of the thermal energy, with the conventional fossil fuel-based calciner. It was found that the full solar system was the best alternative to replace fossil fuel-based systems, demonstrating a reduction of 48% in climate change, 75% in fossil depletion, 92% in photochemical ozone formation and 79% in terrestrial ecotoxicity. Based on irradiance in different parts of the world, it was reported that the solar thermal system could be integrated up to 36% in the cement industries reducing the climate change impact by 15–40%. However, the drawbacks include more land occupation, higher human toxicity-cancer (102%) and depletion of metals and minerals (6%) due to the involvement of construction processes in the manufacturing of the solar systems.

There is limited information available about the use of the SHIP systems in the ceramic industry. Plaza et al. ^[20][26] reported the use of a 60 kWh solar furnace in low and high temperature processes for raw material drying (150 °C), single firing (1100–1200 °C), double firing (1000–1100 °C) and triple firing (700–1000 °C). It was reported that the solar furnace could be successfully used for raw material drying, and double and triple firing while further investigations are required for the single-firing processes.

Ten SHIP systems are integrated into the chemical industry worldwide, of which two are located in EU countries like Austria and Germany. The only information about the use of solar collectors was found in a SHIP system in Austria that uses a flat plate collector to supply water with a temperature of 60 °C, which is used for cleaning [12].

Haagen et al. ^[21][27] and Frein et al. ^[22][28] used solar energy to meet the high energy demand in pharmaceutical processes in Jordan. Haagen et al. ^[21][27] used a linear solar Fresnel system for direct steam generation that could possibly replace the existing diesel steam generator and be installed on the rooftops. The results showed that the proposed system significantly replaced the existing diesel generator, which resulted in a reduction in the diesel use by 30,000 L annually. Frein et al. ^[22][28] evaluated the performance of the same solar generator during summer and winter times by developing a numerical model to quantify the mass and energy deviations. A number of parameters like pressure distribution, pressure drop and liquid level variation were monitored. The results showed that the system operated effectively except at start-up and shutdown when losses appeared, which led to changing of the control system. The authors found that infrequent cleaning of the system can deviate its performance by 40%. Gaballah et al. ^[23][29] used SHIP systems in the petrochemical industry in China. The solar heating techniques with evacuated tube collectors were used to preheat animal waste for biogas production using two similar bio-digesters: (i) heated by the greenhouse integrated to a solar water heating system with a capillary heat exchanger and (ii) heated by the greenhouse. The results showed that the average slurry temperature for the two systems was 9.5 and 4.9 °C above the ambient temperature while the mean specific biogas production was 247 and 181 L/kg, respectively. It was concluded that solar energy is efficient to achieve the required temperature for the production of biogas for most of the year; however, some of the disadvantages included economic unviability. Gunjo et al. ^[24][30] developed a bent-tube flat plate collector to grow thermophilic bacteria in the anaerobic digestion process for producing biogas. The developed collector achieved a temperature of 61 °C and thermal efficiency of 71%, which was significantly higher compared with a conventional flat plate collector. The other parameters like insolation, ambient temperature and flow rate additionally increased the thermal efficiency of the collector. Biabani et al. ^[25][31] developed a solar-powered microreactor to achieve a temperature of 58–60 °C for the production of biodiesel from waste cooking oil. The overall process was more economically effective, reducing the cost by 90%.

The basic metal industry includes the non-ferrous metals, iron and steel sectors ^[15][21]. A total of seven SHIP systems are used in the industry worldwide, of which 6 are based in the EU countries like Germany and Austria. Four of them use evacuated tube collectors to supply water at 30–90 °C for galvanic bath in steel production, one uses a flat plate system for preheating at 80 °C and drying at 50 °C in a galvanising process and one a flat plate collector for a cooling process in a grinding company [12].

Solar energy in the non–ferrous metal industry has been widely used and its technical, economic and environmental performances evaluated in several studies. The use of solar heating in the electrowinning process for producing copper was assessed by Jannesari and Babaei ^[26][32]. They analysed evacuated tube collectors to obtain higher technical and economic performances and found that for the area of the solar system of 5000–6000 m², approximately 150–250 m³ of the storage tank is required to supply 40–78% of the heat demand. This type of system could reduce GHG emissions by 970 tonnes annually with a payback period of 6–10 years. Another study reviewed applications of concentrated solar energy (CSE) using a solar furnace and Fresnel lens for modifying the surface of metallic materials that use temperatures of 1200–1750 °C ^[27][33]. The use of CSE increased process efficiency while maintaining a good quality of the selected material surface. Quiñones et al. ^[28][34] investigated the economic characteristics of solar thermal energy collectors in the Chilean mining industry, including (i) flat plate, (ii) evacuated tube and (iii) parabolic trough and found the flat plate demonstrated the most economical solution, particularly for the processes with temperatures of 80–90 °C.

Solar energy in the iron and steel industry has been widely used for heating processes. A solar furnace and Fresnel collectors were used for densification of high-speed steel sintering at 1150 °C over 90 and 30 min and found to be more efficient than the conventional tubular furnace with optimum densification at 1290 °C over 10 h ^[29][35]. The advantage of using a solar furnace was also confirmed in heat treatment of EN 1.4136 stainless steel to improve its mechanical properties ^[30][36]. Steel alloyed with nickel (lot A) and copper (lot B) was subjected to the heat of a solar furnace for 20 and 33 min to reach an authentication temperature of 1050 °C. The results showed that the periodic maintenance of austenitisation was 7 min for nickel and 21 min for copper. Heat treatment using a solar furnace increased the hardness by 55% while decreasing the average wear rates. The concentrated solar Fresnel system for heat treatment of steel alloys, X210Cr12 and HS6-5-2-5 with different thicknesses of 15 mm and 7 mm, respectively, showed a reduction in heating time of 13–16 min for X210Cr12 and 17–19 min for HS6-5-2-5 steel ^[31][37]. The influence of the sintering cycle, atmosphere and content of the alloy on the produced steel was studied in the sinter-hardening of a chromium alloyed steel system with applied CSE ^[32][38]. It was found that CSE achieved complete densification of the chromium alloyed steel using lower temperatures and less time compared with the conventional techniques.

Only one SHIP system was recorded worldwide and it is located in Spain [12]. It uses a flat plate collector to supply hot water for distillation with a temperature range of 20–90 °C. Ferry et al. ^[33][39] used extreme compound parabolic concentrators (CPC) in a wastewater treatment plant to power an evaporator at 130 and 150 °C. The CPC provided an evaporation rate of 10 gallons per hour and reduced the wastewater by 80% and GHG emissions by 0.09 tCO₂ per m² compared with a gas fired boiler.

SHIP systems have been integrated into various SPIRE industries with a maximum capacity of 34% and GHG emissions reduction of 15–40% ^[19][25]. Collectors used in these systems include solar furnace and Fresnel lens with operating temperatures of 700–1500 °C. Those collectors were used for material treatment at a high operating temperature, high cost and high space consumption ^[20][26]. The other collectors in those systems like flat plate, evacuated tube, linear Fresnel and parabolic trough were used to mostly supply heat at the lower temperature of up to 150 °C, which is mainly used for processes like drying, water and steam heating, and cleaning ^[12][21][33][12,27,39].

Table 1 summarizes existing SHIP systems in the EU’s SPIRE industries [12]. As shown, a very limited number of flat plate collectors were integrated at a temperature of up to 90 °C in higher irradiance countries like Austria, Germany and Spain.