Due to the limitation of solar dryers operating only during sunlight hours, thermal storage emerges as a great solution. It allows the stored heat to be used during the night, ensuring continuous drying and preventing rehydration of the products. During off-sunshine hours, microbial activity may lead to the growth of microorganisms and the extended drying periods can degrade the quality of agricultural products, resulting in poor product quality and spoilage [66,124,125,126,127,128,129,130,131,132].

The integration of a TES unit is needed, and numerous studies have tested it. Some authors divided it into:

• Sensible heat storage (SHS): materials are heated to store excess solar energy, depending on their specific heat capacity, mass and temperature. The best properties of these materials are density, thermal conductivity and stability. For example, materials such as brick, aluminum, gravel, river rocks, concrete, granite and limestone can be used. The rock bed is the most common material for sensible storage used in solar dryer systems [133,134];
• Latent heat storage (LHS): in this kind of material, solar energy is stored during the phase change process. The phase change materials (PCM) can be organic (such as paraffin, like wax n-alkanes and methyl groups) or non-paraffin types (like fatty acids, glycols, alcohols and esters), inorganic (salt hydrates and metallic) or eutectic composition [128,135];
• Thermo-chemical energy storage (TCES): it is based on the principle that all chemical reactions either absorb or release heat. This process stores energy by using high-energy chemical processes. In this case, the heat stored depends on the amount of storage material, the endothermic heat of the reaction and the extent of conversion [136,137,138].

The dryer can operate on solar energy, but for additional heating, auxiliary units are used. The most common are fuelled with fossil fuel or biomass to reach and maintain the required temperature. Despite their effectiveness, their availability is limited, and they are associated with environmental pollution issues. Amer and Gottschalk [139] used electric resistances as auxiliary units in fresh chamomile drying; Matouk et al. [140] used them for onion slices; and Hossain et al. [141] used them for tomato slices [142]. Ferreira et al. [143] applied 20 incandescent lamps, 100 W each, for drying banana slices. Suherman et al. [144] used SUS (stainless steel) plates as heat collectors for solar radiation and an LPG (liquefied petroleum gas) burner in seaweed drying. Many studies have been conducted on this type of dryer in various contexts [145].

Solar dryers with PV assistance are probably the most widely used. Thermal energy can be obtained from solar radiation by using solar collectors and it is converted via PV panels into direct current electricity [146]. These kinds of systems have a huge variety of possible configurations and can range from the simplest forms, such as powering fans to provide air circulation, to making a significant contribution to the decarbonization of electricity production. The PV-ventilated system is very common in greenhouse dryers [147,148,149,150,151,152]. The integrated arrangement for applying thermal energy as well as electrical energy with a PV module is referred to as a hybrid PV/T system [18,73,153,154,155,156,157,158,159,160,161]. The integration of PV panels with solar dryers ensures a continuous and reliable power supply, reducing dependency on the grid and further promoting sustainability in the drying process.

Some authors defend that combining a solar thermal energy source, such as solar thermal collectors with a heat pump dryer, will assist in reducing the operation cost of drying and producing products of high quality [162]. The aim of installing a heat pump is to solve the problem of the intermittent availability of solar radiation. Depending on weather conditions, four working modes can be chosen [163,164,165,166,167,168,169,170,171,172,173,174]:

• Solar energy heating mode, when solar radiation is sufficient during the daytime;
• Heat pump heating mode when solar radiation is unavailable;
• Solar-assisted heat pump heating mode, when solar radiation is insufficient during the daytime;
• Heat pump dehumidification mode when ambient humidity is high.

Beyond all the drying designs associated with heat pump systems, some authors also consider solar systems with chemical heat pumps (CHP) and solar systems with dehumidification systems [13].

The chemical reactions in a CHP system are generally reversible, enabling the alteration of the temperature level of the thermal energy stored by chemical substances [175,176]. These reactions are crucial for absorbing and releasing heat. Typically, the main components include an evacuation system, a storage tank, a chemical heat pump and a drying chamber. CHP can be categorized into solid–gas [177] and liquid–gas.

A solid–gas chemical heat pump unit consists of a reactor or adsorber, an evaporator and a condenser. Liquid–gas systems have at least two reactors: endothermic and exothermic. The high storage capacity, low heat loss and long-term storage of reactants and products are the principal advantages of CHP [178].

Regarding dehumidification systems, in general, fresh products have high moisture contents. Using a desiccant material, such as silica, alumina, pillared clay, or zeolite [179,180,181], may consume low energy and produce dry air to improve drying performance. The pressure difference of generated water vapor, even at low temperatures, can improve driving force that is proportional to the evaporation rate. As a result, energy efficiency can be potentially improved while maintaining product quality [182].

Heat pump dryers come in different types and their performance varies depending on the type. The ability to control the temperature of the drying air and humidity while recovering energy from exhaust is one of the primary advantages of heat pump dryers; however, the environmental impacts is still not well known [183].

This kind of dryer uses solar radiation in combination with a low-potential energy source, such as geothermal or wastewater. The installation allows for the combination of conventional or nonconventional energy sources [184]. According to Ivanova and Andonov [185], it is possible to achieve continuous drying, even during the night, enabled by additional heating of the air during movement in the collector using this source of energy, in a clean and cost-effective mode, as renewable energies are used. The system includes a stainless-steel body, heat exchanger, piping, dehumidifier, blower and trays [186].

Based on the design, construction material used, energy backup systems and auxiliary heating units, several variants of solar dryers for drying foods have been described. These diverse configurations allow for customized solutions to suit specific drying requirements, optimizing energy efficiency and ensuring consistent drying performance across different applications. The integration of low-potential energy sources with solar radiation enhances the versatility and reliability of solar dryers, making them more sustainable and resilient in various operating conditions.

Several studies are being conducted to test different techniques for improving solar dryers, including the use of thermal storage materials, deep bed drying methods, enhanced solar collector designs and energy hybridization. As we can observe, there are a wide variety of solar dehydrators with different shapes and operating modes.