Produced in Association with SERIES 24 / Module 02 Solar Thermal Systems


much of the UK’s solar climate, particularly in Scotland and northern England. Commercial and institutional


installations exist but are relatively sparse. Edinburgh Napier University has incorporated solar thermal into the hot water provision for student accommodation. Some NHS facilities have explored solar thermal for pre- heating cold water before it enters calorifier systems, although full integration into hospital-scale domestic hot water systems remains rare. The capacity of new solar thermal


installations globally has declined in recent years. To be clear, the total installed stock continues to grow, but the rate of new deployment is slowing. In 2023, new installations fell to 21 GWth, compared with 22 GWth in 2022, representing a 7% decrease. By contrast, new solar thermal capacity


installed in the UK in 2023 was 66% higher than in 2022 – the largest percentage growth of any country globally, albeit from a comparatively small base. However, new solar thermal installations remain considerably below their historical peak.


Health and safety risks Solar thermal systems are generally safe


in operation, but they introduce specific risks that must be addressed through careful system design, competent installation, and disciplined ongoing maintenance. In the UK context, Legionella risk is


the most significant of these, and it has a direct bearing on the technology’s applicability in certain building types – most notably healthcare facilities. Legionella pneumophila, the bacterium


responsible for Legionnaires’ disease, proliferates most actively in water held at temperatures between approximately 20°C and 45°C. Solar thermal systems, which store significant volumes of heated water, must therefore be designed and operated to avoid prolonged storage or flow within this temperature band. UK regulatory and guidance


frameworks – including the Health and Safety Executive’s Approved Code of Practice L8, the associated Technical Guidance HSG274 Part 2, and the NHS- specific Health Technical Memorandum HTM 04-01 – require that hot water is stored at 60°C or above throughout the storage vessel, that distribution systems deliver water at 50°C or above at all outlets, and that regular thermal pasteurisation cycles are implemented to control bacterial populations. These requirements are technically achievable within a well-designed solar thermal system, but they impose constraints on system configuration, control logic and auxiliary heating arrangements that add complexity and cost. In NHS hospitals and acute healthcare


settings, these constraints are applied with particular stringency under HTM 04-01, which reflects the vulnerability of patient populations to respiratory infections and the high hot water turnover rates characteristic of clinical environments. The unpredictable delivery temperature of solar-only systems – which varies with irradiance, ambient temperature and demand profile – makes it difficult to guarantee continuous compliance with the required storage and delivery temperatures without a reliable and fully integrated auxiliary heating system. For this reason, the compatibility of solar thermal in NHS hospitals is most commonly perceived as being limited to pre-heating applications to raise the temperature of incoming cold water before it enters a conventionally controlled calorifier, rather than solar being relied upon as a primary heat source. Careful hydraulic design would be required to prevent thermosiphon effects from bypassing the calorifier and delivering sub-temperature water to distribution circuits.


Environmental impacts Solar thermal is widely and correctly


characterised as a low-carbon technology, but a balanced and honest environmental assessment requires acknowledging that its lifecycle impacts are not negligible, and that they vary significantly between technology types and manufacturing contexts. Lifecycle carbon analyses consistently


show that the operational carbon intensity of solar thermal heat is very low – typically in the range of 10–25 gCO₂e per kWh for flat-plate collectors, 20–35 gCO2e per kWh for evacuated tube collectors, and 30–60 gCO2e per kWh for CSP systems. These values are substantially lower than those of natural gas boilers (approximately 230–250 gCO2e/kWh), oil boilers (approximately 290 gCO2e/kWh), or even grid electricity in most European markets. The emissions that do arise are concentrated in the manufacturing phase, with energy-intensive processes such as aluminium smelting, copper refining,


and float glass production accounting for the majority of embodied carbon. The manufacture of glycol antifreeze for the primary circuit adds a further contribution, and end-of-life processing – including the recovery and recycling of metals and glass – carries residual impacts that are not always fully credited in lifecycle assessments. Solar thermal systems use


considerably fewer critical and rare-earth minerals than photovoltaic panels or wind turbines, which is a genuine comparative advantage in discussions of material supply chain risk. However, the spectrally selective absorber coatings used in high- performance flat-plate and evacuated tube collectors typically contain titanium, nickel and chromium compounds, the mining and refining of which have environmental and social impacts in producing countries. CSP systems, which require very large quantities of structural steel, reinforced concrete, mirror glass, and nitrate salt for thermal storage, have correspondingly larger material footprints than collector-based systems, though their lifecycle emissions per kWh of heat or electricity delivered remain competitive with other low-carbon technologies. The relatively long operational lifetime


of solar thermal systems – typically 20–30 years for a well-maintained installation – helps to amortise embodied carbon across a large volume of delivered heat, improving the overall lifecycle carbon performance. This makes longevity of installation and quality of maintenance significant factors in environmental as well as economic assessment.


Conclusions Solar thermal is technically viable in


the United Kingdom, and significantly more capable than its modest market share might suggest. Evacuated tube collectors, in particular, perform well under the diffuse irradiance conditions that characterise the UK climate, and the UK’s solar resource – while lower than that of Mediterranean Europe – is sufficient to deliver meaningful contributions to hot water demand throughout the year, with a


Countries with largest solar thermal market growth in 2023


pronounced seasonal peak in spring and summer. The technology faces a complex set


of structural drivers and barriers in the UK context. On the positive side, the persistent elevation of natural gas prices following the 2022 energy crisis has materially improved the economics of solar thermal across all sectors. Off-gas-grid properties in rural and


semi-rural locations, which currently rely on oil or LPG heating, represent a particularly attractive market segment where solar thermal can reduce the volume of delivered fuel required and improve energy security. High-demand applications such as


commercial laundries, food service operations, sports and leisure facilities, and agricultural buildings offer load profiles well matched to solar thermal’s seasonal output characteristics. The growing policy focus on industrial


heat decarbonisation – which accounts for a substantial fraction of UK energy demand – is creating new interest in solar thermal as a source of low-temperature and medium-temperature process heat, particularly in combination with heat pumps in hybrid configurations. On the other side, the barriers to


widespread adoption remain formidable. Heat pumps have absorbed the majority of policy attention and financial support in the built environment sector, and while solar thermal and heat pumps are in principle complementary rather than competing technologies, in practice they compete for the same limited installation slots and consumer budget. Public awareness of solar thermal remains low, particularly among a generation of homeowners who associate solar energy exclusively with photovoltaic panels. The installer base has contracted following the decline in domestic demand, creating skills and capacity constraints that would need to be addressed before any significant market expansion could occur. Looking to 2045–2050, solar thermal is


Image copyright International Energy Agency, Solar Heating & Cooling Programme, Solar Heat World Wide Edition 2024


likely to play a growing role in three areas of the UK energy system. First, industry and commerce will need cost-effective, reliable, scalable low-carbon heat. Solar thermal, combined with thermal storage and hybrid heat pumps, can meet hot water and low-pressure steam demand. Second, expanding district heating networks, supported by the Heat Network Zoning programme, could integrate large solar collector fields with seasonal storage, following Denmark’s successful model and benefiting from economies of scale. Third, as heating, transport and industry electrify, pressure on the grid will increase, boosting demand for technologies that cut peak electricity use. Solar thermal can provide grid-independent heat while reducing electrical demand and improving energy system resilience long term.


Produced in Association with


EIBI | JULY � AUGUST 2026


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