The use of solar photovoltaics increased in the last decade; however, solar panel efficiency is typically below 20% (Govindasamy and Kumar, 2023; Alharbi and Kais, 2015). The efficiency is further reduced by inverter losses, high temperatures, and an annual degradation of roughly 0.7% due to external factors such as heat, Ultraviolet (UV) radiation, wind, snow, hail, and humidity (Zdyb and Gulkowski, 2020; Kim et al., 2021; da Fonseca et al., 2020). Excess electricity from solar photovoltaic systems can be transmitted to the grid. Alternatively, some solar photovoltaic systems include batteries to store excess energy, but are expensive and may have an environmental impact (Koko, 2022; Abdoos et al., 2025). Inverters typically last 15 years, and solar panels last 20–30 years, although severe weather can shorten these lifespans to 10–12 years (Korpale et al., 2016; Muteri et al., 2020; Rossi et al., 2020; Zhang et al., 2018).

Electric motors are among the primary energy consumers, and are responsible for over 70% of industrial and 53% of global electricity consumption, especially for water pumping in agriculture and urban systems (Stoffel, 2015; de Souza et al., 2024). Water pumps alone account for about 22% of motor energy use (de Souza et al., 2021). Solar pumps, which use renewable energy, represent a feasible alternative toward greener systems (Al-Omari et al., 2025), especially for off-grid systems and applications on rural and agricultural sites (Saha, 2025). However, conventional photovoltaic (PV) water pumps suffer from low overall efficiency: the solar-to-water conversion efficiency rarely exceeds 10%, with panel efficiency below 20% and pump efficiency between 35% and 70% (Korpale et al., 2016; de Souza et al., 2021; Libra et al., 2023; Silvestri et al., 2022; Mutizhongo et al., 2021). Therefore, despite their potential, PV-based solar pumps are constrained by mechanical and electrical losses, underscoring the need for more efficient designs (Pirouz et al., 2025). These constraints have motivated research into solar-thermal alternatives that can capture and utilize solar energy more directly and efficiently.

Solar-thermal systems capture more solar radiation and are highly effective for thermal applications. High-temperature systems that use focused sunlight achieve 60%–70% efficiency (Zhang et al., 2022), while solar water heaters can reach nearly 80% (Herez et al., 2023). The effectiveness of parabolic dishes is among the highest, ranging from 63% to 83%, and those with cavity receivers can reach 90% (Karimi et al., 2018). These devices can also generate power from 20% to 30% of solar energy when combined with Stirling engines (Mahmood and Al-Salih, 2018; Maurya et al., 2022; Rosillo-Calle et al., 2015). The efficiency of commercial solar dish Stirling systems (SDSS) has been found to range from 18% to 32% (Zayed et al., 2021; Malik et al., 2022). The systems can achieve focus-point temperatures above 300 °C, depending on the design, such as the mirror size or the spiral coil (Hafez et al., 2016). Table 1. Summarizes the main characteristics and efficiencies of solar pumping systems under various climatic conditions, reporting pump efficiencies ranging from 34% to 89%, and total system efficiency less than 10% and about 2.3%–8.5%

Despite the extensive literature studies on solar-powered water pumping systems, most studies focus on photovoltaic-driven systems coupled with electric motors, in which cumulative electrical, mechanical, and power-conditioning losses limit overall system performance. While solar-thermal systems have demonstrated high thermal efficiencies and the capability to achieve elevated temperatures, their application to water pumping has largely remained limited to indirect electricity generation via a Stirling engine or motor-assisted solutions. Investigations of motor-free pumping concepts that directly convert concentrated solar thermal energy into hydraulic work remain rare, and studies combining field measurements, laboratory-scale feasibility testing, and long-term numerical simulations for such systems are particularly limited.

Addressing this clear research gap, the present study proposes a novel solar-thermal pump that employs a vapor-pressure piston activated by concentrated solar radiation. It evaluates the technical feasibility of the concept through a combination of field measurements, laboratory-scale experiments, and numerical simulations. The study pursues four objectives: (i) to investigate achievable temperatures of a solar parabolic concentrator mirror, and to characterize the thermal potential of it for generating operational pressures, (ii) to verify piston-driven suction and discharge using a laboratory semi-prototype, (iii) to model long-term thermal performance using TRNSYS simulations across seasonal conditions, (iv) to assess the system’s feasibility for off-grid irrigation applications during peak water demand period under Mediterranean climatic conditions.

The novel proposed solar pump system is designed to extract water from various sources, such as wells, rivers, lakes, and distribution networks, and deliver it to elevated storage tanks or end-use points via piston-driven pressure, without the use of electric motors (Figures 1 a,b). The proposed motor-free thermal pumping system directly utilizes a solar parabolic dish as the primary energy source. This high thermal efficiency enables direct conversion of solar radiation into mechanical work via vapor-pressure generation, eliminating electromechanical conversion losses. However, while solar-thermal collectors exhibit high optical–thermal absorption efficiencies (80%–90%), the overall solar-to-hydraulic efficiency of the proposed system will depend on thermo-mechanical conversion effectiveness.

The system operates in two thermodynamic cycles. During the daytime heating cycle, concentrated solar radiation heats the thermal fluid on the heat exchanger located beneath the piston. The heat exchanger increases water temperature under the piston, and generates vapor pressure, which pushes the piston upward and discharges water to the delivery point (Figure 1c). During the cooling cycle, which occurs at night or under low solar input, vapor condensation leads to a pressure drop beneath the piston, generating suction that refills the tank from the water source (Figure 1d). This adjustable cycle enables the system to utilize both thermal gain and thermal loss, thereby maximizing overall energy use while pumping without external mechanical actuation. The suction head depends on the water vapor pressure at the location. In contrast, the pumping head depends on the heat exchanger temperature beneath the piston, which is directly related to the solar concentrator’s operating temperature. These mechanisms define the feasibility of the proposed motor-free solar-thermal pumping system.

System elements: (1) Solar concentrator; (2) Receiver at focal point; (3) Main Tank; (4) check valves; (5) Small DC pump for circulate of thermal oil to be heated by solar concentrator; (6) Solar PV panel for DC pump; (7) Pipe to circulate thermal oils from tanker to solar dish; (8) heat exchanger beneath the piston; (9) Water vapor; (10) Piston in which move upward for pumping water and downward to suck water and refill the tank; (11) Water that will be pumped; (12) Pipe toward water source such as well, river, urban network; (13) Output pipeline.

To evaluate the thermal performance of the solar parabolic concentrator under real operating conditions and to support the validation of the numerical simulations in TRNSYS, a field measurement was conducted on a parabolic mirror with a reflective area of 11.4 m² installed in Rende (southern Italy), as shown in Figure 2. This allowed determination of achievable focal-point temperatures under Mediterranean climatic conditions, enabling assessment of whether the required thermal input for vapor-pressure piston movement could be achieved. Temperature measurements were taken at the focal point of the concentrator using an infrared thermal camera and conducted at different times of day during clear-sky conditions. In addition to providing experimental evidence of achievable temperature levels, the field measurements were used as a reference for assessing the reliability of the long-term TRNSYS simulations.

In this section, the experimental evaluation of the proposed solar pump system was conducted using a developed semi-prototype for controlled laboratory testing (Figure 3). In the laboratory condition, as direct water vapor generation from a solar parabolic concentrator is not feasible indoors, a high-pressure diaphragm pump was used instead, as presented in part 1 of Figure 4, ensuring consistency between laboratory conditions and the field operation. This approach reproduces the vapor-pressure conditions expected during solar operation, enabling systematic analysis of piston displacement, flow direction, cycle behavior, and potential electric generation, in addition to water pumping.

It is emphasized that the diaphragm pump does not replicate phase change, condensation, or real thermal losses. Its purpose was limited to verifying mechanical piston operation under controlled positive and negative pressure conditions.

Long-term behavior of the solar system can be assessed using TRNSYS simulations. Sara Soltan et al. modeled a solar desalination system using TRNSYS and demonstrated that collector area, water temperature, and flow rate significantly influence overall efficiency (Sultan et al., 2016). Yettou et al. simulated two solar tracking systems in Ghardaïa, Algeria, using TRNSYS, and found a 67% efficiency for a 4.22 m² parabolic trough collector, with two-axis tracking achieving up to 25.4 kWh/day in spring (Yettou et al., 2012). Mohsin J. et al. compared Evacuated Tube and Parabolic Trough Collectors in India using TRNSYS, verifying the efficacy of both systems for solar heating and cooling applications (Mohsin and Jani, 2020). In another study, Juarez-Trujillo et al. used thermal oil, solar collectors, and a heat-pipe condenser to optimize a solar water distillation system utilizing TRNSYS. They demonstrated how altering the thermal tank volume, flow rates, and number of collectors significantly enhanced system efficiency (Juarez-Trujilloa et al., 2013). Y. Krishna et al. modeled a solar water heating system with a parabolic trough collector in India, and a PCM (Phase Change Material) storage tank filled with lauric acid, achieving an average annual collector efficiency of 26%, and demonstrating effective hot water distribution even at night and early hours in the morning (Krishna et al., 2021). José A. León et al. designed a TRNSYS model for a solar system supporting biodiesel production, in which nine parabolic trough collectors (118.8 m²) provided 91% of the required heat, saving 70 MWh and reducing CO₂ emissions by 27.3 tons annually (León et al., 2016). Therefore, to investigate the annual and seasonal performance under Mediterranean climatic conditions, and specifically system performance during agricultural periods and peak irrigation demand, a long-term simulation has been conducted through TRNSYS. Simulation details in TRNSYS are presented in Table 2, and the model architecture is shown in Figure 4.

It should be emphasized that the TRNSYS model developed in this study is limited to thermal analysis of the solar concentrator and piston tank. The simulation evaluates long-term temperature evolution and seasonal operability under Mediterranean climatic conditions; however, it does not include dynamic thermo-hydraulic coupling between vapor generation, piston displacement, and resulting flow rate.

A fully coupled thermo-hydraulic model would require experimentally derived piston motion equations, vapor condensation rates, internal pressure evolution, hydraulic resistance coefficients, and valve flow characteristics. Since these parameters depend on full-scale prototype measurements, they were beyond the scope of the present feasibility study. Therefore, the TRNSYS simulation should be interpreted as an assessment of thermal availability rather than direct hydraulic performance prediction.

To investigate achievable temperatures of the selected solar parabolic concentrator and to characterize its thermal potential for generating operational pressures for vapor-driven pumping, field measurements were conducted on different days and at various hours. Figure 5 exhibits the temperature variations of the receiver on 1st November 2024 at different hours of the day.

The results showed a rapid increase in focal-point temperature during the morning hours, reaching approximately 487 °C at 9:00, 501 °C at 10:00, and 578 °C at 11:00. The maximum temperature of about 605 °C was recorded at noon, followed by a gradual decrease in the afternoon, with temperatures of approximately 597 °C at 13:00, 479 °C at 14:00, 403 °C at 15:00, and 355 °C at 16:00.

These measurements demonstrate that, even in winter, the parabolic concentrator can consistently achieve temperatures well above those required to generate significant water vapor pressure beneath the piston in the novel vapor-driven pumping system. Water vapor pressure increases sharply with temperature, while temperatures in the range of 120 °C–140 °C correspond to theoretical pumping heads of approximately 20–37 m of water (199–362 kPa). Therefore, even after accounting for thermal losses, heat transfer inefficiencies, and hydraulic head losses in the piping system, the observed temperature levels confirm the suitability of concentrated solar thermal energy for driving the proposed vapor-pressure piston mechanism, thereby supporting the thermal feasibility of the proposed solar-thermal pumping concept.

In this part, to verify the feasibility and mechanism of piston-driven suction and discharge, a laboratory-scale semi-prototype of the proposed system was developed and tested under controlled conditions (Figure 6). Since direct vapor generation from a solar concentrator is not feasible in an indoor laboratory environment, a high-pressure diaphragm pump has been used to generate controlled positive and negative pressure cycles acting on the piston. Maintaining a consistent flow direction is essential for continuous hydraulic operation and for potential electricity generation, regardless of the piston movement direction. Therefore, as shown in the laboratory setup, several check valves (one-way valves) have been used to regulate flow direction during piston motion. This configuration guarantees the direction of water flow through the piston cycle (charge and discharge). Moreover, it ensures that the flow direction through the micro-hydro turbine remains unchanged during both upward and downward piston movements.

Figure 6 illustrates the two operating cycles of the piston. During the first cycle (Figures 6a,b), positive pressure drives the piston upward (which, in a full-scale and outdoor environment, can be achieved by water vapor generated via a solar concentrator), resulting in water discharge toward the outlet. During the second cycle (Figures 6c,d), negative pressure generated beneath the piston creates suction (which, in a full-scale and outdoor environment, can be achieved by cooling the piston tank), refilling the system from the water source. The corresponding thermal images confirm the distinct pressure-driven flow paths and validate the effectiveness of the valve arrangement in controlling fluid motion.

The results demonstrate that the proposed design configuration can reliably achieve alternating suction and discharge cycles while preserving a unidirectional hydraulic flow. This experimental verification confirms the fundamental operability of the vapor-pressure-driven piston concept. It supports its feasibility as a motor-free pumping mechanism when coupled with solar-thermal pressure generation.

To evaluate the long-term thermal performance, feasibility, and applications of the proposed solar-thermal system under realistic operating conditions during the peak water-demand period, annual and seasonal simulations were conducted for the case study location in Rende (southern Italy) using TRNSYS. The following results describe thermal behavior beneath the piston and do not directly represent instantaneous flow rate or hydraulic discharge, which will require future coupled thermo-mechanical modeling and prototype validation. Figure 7 presents the simulated hourly temperature profile beneath the piston over one full year, with an annual average piston tank temperature of approximately 347.6 °C. This value is well above the temperature threshold required to generate sufficient vapor pressure for piston-driven pumping and system charging–discharging dynamics, confirming the long-term thermal adequacy of the system.

Figure 8 displays the average hourly temperature profile beneath the piston for four seasons, demonstrating frequent oscillations of the piston tank temperature between 150 °C and 650 °C, with seasonal averages of 356.5 °C and 308.6 °C, respectively. In summer, due to increased solar irradiation and elevated ambient temperatures, although temperature fluctuations remain significant, the minimum operating temperatures increase, resulting in a higher seasonal average temperature of about 393.7 °C. Conversely, winter conditions lead to lower average temperatures, approximately 291.4 °C, caused by reduced solar irradiation and colder external conditions. Despite these variations, simulated winter temperatures remain sufficient to generate vapor pressure, indicating the year-round feasibility of cyclic piston operation.

For irrigation purposes, the feasibility of the novel pump system needs to be evaluated during the peak water demand period, which in southern Italy typically extends from May to August. Figure 9 illustrates the piston tank temperature profile during the irrigation demand period, showing that the system maintains a stable average temperature of approximately 390.8 °C, with sustained high-temperature availability throughout the critical irrigation months. The lower annual average reflects winter operation, whereas the higher average observed during the irrigation period corresponds to increased solar availability between May and August.

Overall, the long-term, seasonal, and irrigation-period simulation results confirm that the proposed system maintains thermal availability and performance under Mediterranean climatic conditions and supports the technical feasibility of the proposed motor-free solar-thermal pumping concept for off-grid agricultural water supply applications.

Based on saturated steam tables, water vapor at 120 °C–140 °C corresponds to pressures of approximately 199–362 kPa, equivalent to theoretical pumping heads of 20–37 m of water column. These values indicate that, from a thermodynamic perspective, the proposed system can deliver irrigation-relevant hydraulic heads. However, precise flow rate and volumetric discharge will be quantified in the future full-scale testing.

While the present study focuses on thermal and mechanical feasibility, full hydraulic performance characterization requires coupled thermo-mechanical modeling and experimental validation of piston displacement, stroke frequency, volumetric chamber capacity, and hydraulic resistance within the suction and discharge lines. In principle, the instantaneous volumetric flow rate (Q) of the system can be expressed as Equation 1: Q = V s × f (1) where V_s represents piston stroke volume, and f denotes cycle frequency, both of which depend on vapor generation rate, condensation dynamics, and thermal inertia.

The overall solar-to-hydraulic efficiency (η_s-h) can be defined as Equation 2: η s − h = ρ   g   H   Q   /   A   G (2) where ρ is water density, g is gravitational acceleration, H is the pumping head, A is the collector aperture area, and G is the incident solar irradiance.

Determination of these parameters requires full-scale prototype testing under real solar-driven vapor conditions. Therefore, the present work establishes the thermal and mechanical boundary conditions necessary for such hydraulic characterization, while comprehensive efficiency quantification remains the objective of future experimental investigation.

Future research will focus on the development and testing of a full-scale prototype under real solar operating conditions, enabling coupled thermo-mechanical and hydraulic performance validation. In particular, the overall solar-to-hydraulic efficiency of the proposed system will be computed through direct measurement of pressure evolution, piston displacement, and volumetric discharge.

Potential limitations such as thermal inertia, condensation time delay, cyclic operation intermittency, and material fatigue under repeated thermal stress will be experimentally investigated to evaluate long-term reliability.

Although the present study concentrates on technical feasibility, economic competitiveness relative to conventional photovoltaic pumping systems remains essential for practical implementation. Since the system is currently at a conceptual and semi-prototype stage, reliable cost estimation is premature. Future work will therefore include a comprehensive techno-economic assessment that incorporates capital expenditure (CAPEX), operational expenditure (OPEX), maintenance requirements, system lifetime, and comparisons with PV-based pumping systems under equivalent hydraulic-demand scenarios.

In addition, an integrated control strategy based on real-time temperature and pressure sensing will be developed to optimize cyclic operation under variable solar conditions.