Produced in Association with SERIES 24 / Module 02 Solar Thermal Systems Solar thermal technology By Jamie Goth, Goth Energy Management Limited


long wave thermal radiation, thereby reducing re-radiation losses. A single layer of low-iron toughened glass covers the absorber assembly, creating a greenhouse effect that traps heat within the collector cavity. Insulation on the rear and sides of the unit reduces conductive losses. When sunlight heats the absorber,


energy is conducted into the fluid circulating through the bonded pipework. A pump moves this heated fluid to an insulated storage cylinder, where a heat exchanger – typically a coiled immersion element – transfers energy into the potable hot water supply. Flat-plate collectors are robust, relatively inexpensive to manufacture, straightforward to install and well suited to climates where ambient temperatures are moderate and solar irradiance is reasonably direct. Flat-plate collectors dominate the


panels convert sunlight directly into electricity through the photoelectric effect, solar thermal systems harness the sun’s energy in the form of heat, often achieving significantly higher conversion efficiencies for thermal end-uses. This heat can serve domestic hot water systems, space heating circuits, industrial process demands, and even electricity generation when combined with high- temperature concentrating systems. Decarbonising heat represents one


S


of the most structurally difficult aspects of the energy transition in temperate climates, and solar thermal offers a mature, proven and technically robust pathway that is becoming increasingly worthy of renewed and serious attention. Solar thermal technologies range from


simple unglazed collectors used to extend swimming pool seasons, through flat- plate and evacuated tube collectors for domestic and commercial applications, to large-scale concentrating solar power installations capable of providing high temperature heat to industrial processes and renewable electricity generation. Globally, the solar thermal installation market is dominated by small scale systems (over 99% of cumulative systems and over 60% of new installations each year). However, large-scale systems (such as district heating networks in Denmark and China) account for 70-80% of cumulative installed capacity. Each category of solar thermal


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technology operates on the same fundamental principle – capturing solar radiation and converting it into usable heat – but the engineering approaches, performance characteristics and


olar thermal technology occupies a distinctive place in the landscape of renewable energy systems. While photovoltaic


economic contexts differ substantially between them. Understanding these differences is essential for engineers and energy professionals evaluating solar thermal as part of a broader decarbonisation strategy, whether at building, campus or district scale. This article explores solar thermal


technologies, examining how each major technology type works; how the global and European markets have developed; and how solar thermal performs within the specific context of the United Kingdom.


How it works At the heart of every solar thermal system


is the principle of solar absorption. When sunlight strikes a surface, a proportion of its incident energy is absorbed and converted into heat. Solar thermal collectors are engineered to maximise this absorption across the solar spectrum while simultaneously minimising heat loss to the surrounding environment. The absorbed heat is transferred into a working fluid. This is typically water, or a water-glycol antifreeze mixture, where climatic conditions present a risk of freezing. The working fluid is circulated by a pump to a storage vessel or heat exchanger, where the heat is either stored or used directly.


Flat-plate collectors Flat-plate collectors are the oldest,


lowest cost and most widely recognised form of solar thermal technology in commercial use. They consist of a dark absorber plate – usually copper or aluminium – bonded to a network of fluid- carrying pipes. The absorber surface is coated with a spectrally selective material that maximises absorption of incoming shortwave solar radiation while suppressing the emission of


residential markets of southern Europe, North Africa, and large parts of Asia. They are less prominent in the small scale, domestic systems in northern Europe and other regions where the heat loss that occurs in winter conditions requires design features better addressed by alternative solar thermal technologies, such as evacuated tube collectors. However, by contrast, Denmark’s large- scale solar thermal district heating networks mainly feature flat-plate collectors due to their lower costs and excellent compatibility with the flow temperatures of the network.


Evacuated tube collectors Evacuated tube collectors represent


a more sophisticated approach to solar thermal collection, and they are particularly well suited to colder and cloudier climates such as that of the northern states of the USA, EU and the UK.Each collector comprises a series of parallel glass tubes, of the order of 2m in length, each containing an inner absorber plate and a sealed heat pipe. The space between the inner and outer glass walls is evacuated to a high vacuum, cutting down convective and conductive heat loss. This vacuum insulation allows evacuated tube collectors to operate efficiently even when ambient temperatures are well below freezing and when solar irradiance is predominantly diffuse rather than direct. Inside each tube, the heat pipe contains


a small quantity of volatile working fluid with phase change temperatures compatible with ambient conditions – ie evaporating at relatively low temperatures when the absorber is heated by sunlight. The vapour rises to the top of the heat pipe, where it enters a condenser in a manifold header running across the top of the collector array. In this manifold, the vapour condenses and releases its latent


Produced in Association with


EIBI | JULY � AUGUST 2026


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