Conventional Flat Plate Solar Collector Research Articles

Energy and exergy analysis of conventional and modified flat-plate solar collector with dual spiral flow tubes: A comparative study

ABSTRACT The most fundamental collectors for water heating are flat plate solar collectors (FPSCs). They are inexpensive, simple to build, and need little maintenance. Conventional FPSCs have relatively low efficiency. More study is needed to increase the efficiency of FPSC by using novel designs. To enhance thermal performance, various strategies have been implemented through design modifications of flow tubes. In this study, dual spiral-shaped flow tubes were constructed instead of the usual FPSCs number of riser tubes and headers and tested with three different mass flow rates (20.9, 29.9, and 42.1 l/h). The optimum instantaneous efficiency achieved is 70.8% at a flow rate of 42.1 l/h, representing a 13.7% improvement in efficiency compared to the conventional collector and lowering the mass flow rate results in an enhanced exergy efficiency of 3.51% at 20.9 l/h. The modified collector achieves its highest instantaneous efficiency when the outlet water temperature is measured at 53.4°C. The modified collector boosts heat transfers by enlarging the water’s surface contact area and prolonging its flow time, resulting in greater heat absorption and significantly improved efficiency. When all other parameters are kept the same as in a conventional design, highly encouraging results in a modified collector have been recorded.

Investigation of a dual-spiral flat plate solar water heater under natural convection: an experimental study

In residential and industrial areas, a flat plate solar collector (FPSC) is an important technology that allows for direct solar energy utilisation for heating water. Conventional FPSCs have relatively low efficiency. More study is needed to increase the efficiency of FPSC by using novel designs. Modifications to the design of solar collectors are always a viable option for improving thermal efficiency significantly. In this study, dual spiral-shaped flow tubes were constructed instead of the usual flat plate solar collector's number of riser tubes and headers and tested with three different mass flow rates (0.0058, 0.0083, and 0.0117 kg/s). The optimum instantaneous efficiency achieved is 70.8% at a flow rate of 0.0117 kg/s, representing a 13.7% improvement in efficiency compared to the conventional collector. The modified collector achieves its highest instantaneous efficiency when the outlet water temperature is measured at 53.4°C. The modified collector improves heat transfer by increasing surface contact area of water and increasing the flow residence time. This results in a higher absorption of heat by the water, leading to a significant enhancement in the efficiency. When all other parameters are kept the same as in a conventional design, highly encouraging results in modified solar collector have been recorded.

Thermal performance of solar flat plate collector using energy storage phase change materials

AbstractThe present study has been carried out to improve the overall efficiency of a conventional flat plate solar collector (FPSC) using two different heat storage phase change materials (PCMs). Two grades of paraffin wax—Paraffin‐P116 (PCM‐1) and Paraffin‐5838 (PCM‐2) as PCM are selected for the analysis based on their high heat fusion rate, low thermal conductivity, and suitable phase transition temperature. The present code has been validated with experimental results and it showed a maximum error of 6.43% in predicting the water outlet temperature. Absorber plate temperature, useful heat transfer rate, water outlet temperature and efficiencies are estimated to improve the performance of FPSC with and without PCM for various flow rates (15 lph and 30 lph) with two different weights of PCM (9 kg and 14 kg) for three different locations in India, [Chennai (13.0827° N, 80.2707° E), Bengaluru (12.9716° N, 77.5946° E), and Delhi (28.7041° N, 77.1025° E)]. The maximum increment in the efficiency of FPSC with the use of PCM‐1 is 19.59% and with the use of PCM‐2 is 16.53%. Using 14 kg of PCM‐1 with the conventional FPSC at water inlet flow rate of 15 lph increases the overall thermal efficiency up to 19.59% compared with conventional FPSC. The maximum increase in the percentage of efficiency with PCM‐1 from conventional FPSC for different cases of 15 lph‐09 kg is 16.34%, 15 lph‐14 kg is 19.59%, 30 lph‐09 kg is 14.57% and 30 lph‐14 kg is 18.45%. From the present study, it can be concluded that the overall efficiency of FPSC is increased with the use of 14 kg of PCM‐1 with conventional FPSC at the inlet water flow rate of 15 lph.

Enhancement and experimental study on thermal behaviour of heat pipe based solar absorber by using CuO nanofluid

Technological growth in thermal science found that the awareness of solar thermal energy improved widely in various applications and spotted issues on conventional flat plate solar collectors operating with water fluid: lower thermal efficiency, limited thermal performance during low sunlight, and unavoidable heat loss for extended plate surface. This research attempts to enhance the thermal performance of solar collectors modified with heat pipe solar absorber (HPSA) evaluated by 0.010, 0.015, and 0.02 volume fractions of CuO nanofluid at 18 Lpm. The effect of CuO on varied flow rate on temperature gain, heat transfer coefficient, and thermal efficiency of HPSA is experimentally studied, and its findings are compared with water fluid. The HPSA operates with 0.015 volume CuO nanofluid with a higher rate of flow, proving better thermal performance and offering a maximum temperature gain of 68?C with a better heat transfer coefficient of 81.5W/m2K results enhanced thermal efficiency of 85.2%, which are higher than the water fluid operated HPSA system. An optimum operating parameter of HPSA is suggested for heat exchanger applications.

SunSync Innovation: Empowering Traditional Solar Flat Plate Collectors with Autonomous Sun-Tracking for Tea Leaf Drying

Aim: This research focused on post-harvest technology for preserving tea leaves. The research introduces an innovative approach by incorporating an autonomous solar tracker into a flat plate solar collector, strategically optimizing its orientation towards the sun. The solar tracker, a low-cost and fully automatic prototype, plays a pivotal role in maximizing incident solar rays, thereby enhancing the overall energy absorption efficiency of the system at the same time increasing the quality of dried tea leaves.
 Methodology: The investigation delves into the realm of indirect solar drying methodologies, with a specific focus on the Sun-Tracking Solar Dryer (STSD). This sophisticated apparatus, engineered through the integration of light-dependent resistors, a wiper DC motor with gearbox, and a microcontroller, was developed within the confines of the Agricultural and Food Engineering department at IIT Kharagpur. Subsequent to its development, the study entails a comparative analysis wherein the position of the absorber in the dryer, as ascertained by the developed prototype, is juxtaposed with the ideal position requisite for optimal solar light incidence. Moreover, an examination of the dried tea leaves produced by the prototype is conducted, involving a comprehensive comparison with those dried using the conventional open sun drying method.
 Results: The instantiated prototype demonstrated commendable efficacy in aligning with the ideal position, exhibiting an average deviation of 3.25 degrees. Facilitated by this prototype, the attainment of the targeted moisture content in tea leaves was expedited, culminating 2 hours earlier than the conventional open sun drying method. The resultant dried product manifested a water activity of 0.531 and a DPPH radical scavenging activity of 73.53%, both registering a reduction in comparison to the open sun drying counterpart. Concurrently, the total phenolic content exhibited an increment of 11.48% when contrasted with the open sun drying process.
 Conclusion: The affordably developed prototype demonstrated exceptional capabilities in sun tracking, showcasing the promising potential for augmenting the efficiency of conventional flat plate solar collectors. Additionally, its performance in preserving the quality attributes of dried tea leaves was commendable.

Experimental assessment of novel designed solar hot water storage collector incorporating an array of partitioned ducts absorber

A Novel design of a solar water collector consisting of an array of adjacent partitioned ducts representing the collector absorber is investigated experimentally. Each duct is divided into two flow passages by a partitioned plate made from insulation material. The heated water flows over the partitioned plate to the storage tank, while the cold water flows from the storage tank to the duct down the portioned plate. The designed partitioned ducts solar collector (PDSC) performance is compared to conventional flat plate solar water collectors (FPSC) with the same dimensions and materials. The study is performed under hot weather conditions of upper Egypt for collectors’ tilt angles of; 17°, 27°, and 37°. The findings reveal that PDSC has higher storage water temperature, energy storage, energy efficiency, and lower energy losses. The PDSC has average daily efficiencies of 57.5%, 55.2%, and 47.5%, with enhancement percentages of 31.2%, 32.4%, and 38.6%, relative to the FPSC, at the studied tilt angles, respectively. The PDSC has average exergy efficiencies of 4.47%, 3.71%, and 2.89%, with enhancement percentages of 36.3%, 36.5%, and 41% relative to the FPSC, at the studied tilt angles, respectively. The maximum required thermosyphon heads of the PDSC are 0.8, 0.7, and 0.1, and the corresponding values of the FPSC are 1.7, 1.65, and 0.9 mm at the studied tilt angles, respectively.

Evaluation and optimization of the performance and efficiency of a hybrid flat plate solar collector integrated with phase change material and heat sink

This study presents an experimental real case study on the performance optimization of a flat plate solar collector (FPSC) by integrating a layer of phase change material (PCM) and a heat sink. The system was tested under the semi-arid climatic conditions of Abha, Saudi Arabia. Based on preliminary experimental testing on conventional FPSCs, the novel composite structure of the PCM material was developed by mixing polyethylene glycol (PEG) and magnesium hydroxide (Mg (OH)2) in an absolute ethanol solution. The PEG/Mg (OH)2 PCM was installed beneath the pipe and had a lower supercooling value, a higher latent heat value, and thermal stable characteristic, thus increasing the thermal performance of the FPSC. The results showed that using a PCM ensured that the collector continued to deliver hot water for a longer period of time. Furthermore, a heat sink was attached to the PCM layer for proper temperature distribution. The results demonstrated that the FPSC equipped with a PCM and heat sink produced hot water for a longer period of time in the evening while having a lower output temperature in the morning. Also, the average daily FPSC thermal efficiency was enhanced by 6–8% by adding only PEG/Mg (OH)2 PCM as compared with conventional FPSCs and those with PCMs and heat sinks.

Performance analysis of solar drying system with sunlight transparent thermally insulating aerogels

Solar drying can solve the problems of product quality caused by open sun drying and high energy consumption caused by conventional energy drying. However, due to the low solar heat collection efficiency of conventional flat-plate solar collectors, the maximum drying temperature of the existing solar drying system is typically <70 °C, making them difficult to be used in cold regions. Herein, we proposed a new solar drying system with high solar heat collection efficiency, high hot air outlet temperature, and high drying efficiency based on silica aerogels that are transparent to sunlight and have ultra-low thermal conductivity. The results show that the optimum thickness of the silica aerogel insulation layer is 10 mm. At ambient temperatures of −10 to 30 °C, the air outlet temperatures of the designed solar collector are 52–97 °C, increasing the air outlet temperature by 23 °C and 28 °C, respectively, compared to conventional solar collectors. The proposed solar drying system incorporating a 10-mm-thick silica aerogel insulation layer can meet the drying requirements of most agricultural products without an additional heat source and shorten the drying time by 58%.

Experimental and numerical study to develop TRNSYS model for an active flat plate solar collector with an internally serpentine tube receiver

Flat solar collectors are extensively utilized in various domestic and industrial applications to reduce energy consumption. In this study, an-active flat plate solar collector (FPSC) with an internal absorber tube receiver was fabricated and tested in Al-Samawa city, Iraq (latitude 31.19°N and longitude 45.17°E). The ambient temperature and incident solar radiation at the experimental location were reached 39 °C and 840 W/m2, respectively. In this study, the number of riser tubes connected to headers that are covered with a glass sheet in a conventional FPSC were replaced with a single serpentine-shaped collector tube covered with a plastic sheet. The proposed solar collector used a smooth copper tube with internal and exterior diameters of 9.5 and 12 mm, respectively, and a total length of 1000 mm. A TRNSYS model of a flat plate collector integrated with an absorber tube was developed, simulated, and validated using the experimental data. Temperature and flow rate data were obtained concurrently throughout the experiments to evaluate the performance of the fabricated solar collector. The temperature at the solar collector input stayed relatively constant at 37.7 °C, and the water flow rate remained constant at 0.75 L/min. The results indicated that the temperature at the solar collector output ranged from 52 to 61 °C, with an average of 58 °C. The efficiency of the proposed solar collector ranges from approximately 45% to 67%, with an average of 58%. Overall, the simulation results of the TRNSYS model are in excellent agreement with experimental data. The average discrepancy between the tests and simulations for temperature differential and collector efficiency is approximately 1%.

Experimental investigation on heat transfer performance of solar collector with baffles and semicircular loops fins under varied air mass flow rates

Insertion of fins and turbulators is one of the effective approaches to augment the restricted heat transfer characteristics of an air-based flat plate solar collector and the creation of holes/perforations in baffles and fins further increase the degree of improvement. Therefore, the prime objective of the present study is the experimental assessment of temperature distribution, heat transfer coefficients and thermal efficiency of a solar air heater integrated with baffles and fins in the form of hollow semicircular loops. The plain absorber plate was modified with the continuous insertion of semicircular loops of 0.03 m radius, constructed with wire of 0.002 m thick in a staggered manner. The experiments were conducted for measurement of solar intensity and collector temperature at air mass flow rates ranging from 0.006 to 0.02 kg/s. Results indicated the temperature of the air to be lower than loops and absorber plate but surpassed the glass plate temperature. The average temperature rise of air in finned collector was ranging from 14.08 to 22.78 °C, remarkably higher than conventional flat plate solar collector. Air mass flow rate of 0.02 kg/s presented 109%, 80.72% and 62.56% augmented average heat transfer coefficient than 0.006, 0.01 and 0.015 kg/s, respectively and 22.03% enhanced thermal efficiency than 0.006 kg/s. Further, the finned collector at 0.02 kg/s yielded 19.73–34.3% superior average thermal efficiency and 37.38–66.05% escalated heat transfer coefficient over the smooth absorber plate solar collector. The solar collector with hollow fins and baffles will be suitable for domestic and industrial applications like agricultural products drying, greenhouse, and space heating.

First test field performance of highly efficient flat plate solar collectors with transparent insulation and low-cost overheating protection

The present work demonstrates prototypes of highly efficient flat plate solar thermal collectors prototypes based on transparent insulation materials (TIM) technology for efficiency improvement and an overheating protection system. The design and optimization of the collectors have been numerically carried out using a previously developed simulation tool based on an in-house software platform (NEST) capable of simulating all the entities constituting the system as a whole and using efficient coupling between the elements. Three design variants for the demonstration have been previously tested under laboratory conditions. These collectors are then mounted on the roof of a hospital building. Their performance is comparatively tested along with a conventional flat plate solar collector, under actual meteorological conditions and during long periods. The energy collected is about 2.5 and 1.4 times higher than standard collectors in winter and spring. Thus, due to the long-term exposure of the collectors, aspects such as reliability, durability, energy performance, and correct functioning of the protection system have been analyzed to improve the detected shortcomings for the future generations of the present design.

An Experimental Study on the Performance of a Hybrid Photovoltaic/Thermal Solar System

Considered as the widespread renewable energy, solar energy is used to produce the electricity and heat for carbon peaking and carbon neutralization. However, the photo-electric conversion efficiency will decrease with the increase of cells temperature. To solve this problem, a water-type hybrid photovoltaic/thermal (PV/T) solar system has been designed and tested in Hong Kong. The outdoor experiment proceeded all days in 6 months with three different operating modes. The data of selected 33 days were used to analyze the performance of the system. The results showed that the thermal efficiencies were 33.6%, 52%, and 48.3% at zero reduced temperature, corresponding to three modes. Besides, the overall efficiencies were 43.2%, 60.9%, and 55.2% accordingly, which are higher than those of the conventional flat-plate solar collector. The average water temperature in the storage tank could reach 39.9°C, and the system could produce 0.07 kWh electric power after one-day operation, when the collector with 96 kg/m2 M/Ac installed vertically combining with external wall for each family. It is realizable to utilize the developed building integrated water-type photovoltaic/thermal (BIPVT) solar system for residentialintensive high-rise buildings in Hong Kong, satisfying the needs of hot water, and eventually reducing energy consumption of the buildings.

Thermal performance analysis of large-scale flat plate solar collectors and regional applicability in China

The thermal performance of flat-plate solar collectors (FPSCs) depends not only on environmental and operational parameters but also on its dimensions. In this study, the thermal performance improvement mechanism of FPSCs is studied focusing on the impact of collector size. Numerical simulation models for both large-scale flat-plate solar collectors (LSFPSCs), and conventional FPSCs in parallel, are introduced. The relationship between thermal performance and collector dimensions is studied for the LSFPSCs. Furthermore, the effect of the environmental and operational parameters on the thermal performance of the two collector types is investigated. Moreover, the applicability of LSFPSCs in China is analyzed with respect to available operating times, useful energy, and heat loss. The results indicate that increasing the collector dimensions can improve the thermal performance of FPSCs effectively, and the LSFPSCs perform better than conventional FPSCs in parallel. Compared to the conventional FPSCs, the collector efficiency of the LSFPSCs is higher-especially for low solar irradiance, low ambient temperatures, and high mass-flow rates. In addition, the LSFPSCs excel in solar-energy rich areas, the available daily operating time in Lhasa is 9.6 h, which is the longest operating time among the studied cities, and the proportion of useful energy is about 55 %.

Experimental comparative analysis of a flat plate solar collector with and without PCM

In this work, a comparative experimental analysis of a conventional flat plate solar collector (FPSC) and an identical prototype with thermal storage system by PCM is presented. Thermal performance of the devices was studied from experimental and mathematical approaches. For experimental approach, both collectors were operated and analyzed simultaneously under the same real outdoor conditions. An experimental set-up monitored solar radiation, water inlet and outlet temperature, ambient temperature, wind speed, and temperatures of main components as glass cover, absorber plate and PCM. Heat transfer rates losses, internal PCM behavior as liquid fraction of PCM and accumulated energy were estimated and discussed using a mathematical approach. In this method, experimental results were evaluated to assess the performance of both collectors under the same operating conditions. In addition, different experimental cases were considered in order to improve the thermal performance of the prototype. Two different PCMs, a contact thermal conduction enhancement, and inclination of the collectors were analyzed. Experimental and analytical results indicate that the contact conduction between the absorber and PCM, and the selection of the PCM itself are key factors to increase the collector performance respect to conventional FPSC, while tilting the collector presented no significant improvement in latitude of the experimental tests. The prototype with PCM increased the night outlet temperature in 2 °C and maximum accumulated energy increased 630 Wh respect to the collector without PCM. The proposed improvements for collector with PCM rose the maximum liquid fraction of PCM from 0.3 to 0.9.

Performance study of FPSC integrated with twisted tape inserts

Abstract An experimental study has been carried out to enhance the performance of the Flat Plate Solar Collector (FPSC) using passive techniques. For this purpose, we have used a concept of twisted tapes such as single twisted tapes. The presence of twisted tape inside the pipes creates a swirl flow (secondary flow) which provides better mixing and higher heat transfer coefficients. A lot of researches have been done on twisted tape modifications to get excellent results. This study uses a twisted tape inside the copper pipe to examine the outcome on the heat transfer coefficient, heat transfer rate and enhancement of heat transfer performances. A physical representation of the experimental setup has been designed, built and instrumented for calculating those parameters. Then using the values and the formula, the efficiency of the Flat plate solar collector with and without twisted tape is found out. The efficiency of a flat plate solar collector integrated with twisted tape is 66%. The efficiency value of flat plate solar collectors without twisted tape is 48%. This showed the eminent improvement in twisted tape inserts than in usual solar collectors. It shows a greater good of using twisted tape inserts to increase the heat transfer. There has been a serious study on the kind of twist ratio to be used to enhance the transfer rate. In this project we have used the twist ratio of 2. Modifications on the geometry of twisted tape will lead to give better performances than the conventional flat plate solar collector.

“Performance comparison of innovative spiral shaped solar collector design with conventional flat plate solar collector”

Flat plate solar collector is an essential device, which facilitate direct application of solar energy for water heating in household and industrial sector. Existing Flat plate solar collectors (FPSC) suffers from stagnant and comparatively low efficiency. Further research to improve efficiency of FPSC by inculcating innovative design is necessary. To fill the gap, the presented work is an experimental investigation of an innovative design and fabrication approach which deals with analysis of flat plate solar collector efficiency. Design modifications of solar collector always offer an important alternative to achieve significant effect on thermal efficiency. In this work single spiral shaped collector tube as compare to number of riser tubes connected with headers in conventional type flat plate solar collector has been developed. Keeping all other parameters similar to conventional design, it has been observed very encouraging outcomes in efficiencies of solar collector. Under forced mode of testing, enhancement in thermal efficiency achieved is, 21.94% compared to conventional flat plate collector design. Enhancement in exergy efficiency is 6.73%. Overall material saving is about ∼30% and cost of manufacturing and maintenance can be significantly reduced at equivalent performance of conventional collector.

Design and numerical analysis of a hybrid geothermal PCM flat plate solar collector dryer for developing countries

Drying techniques reduce post-harvest food deterioration. However, conventional drying consumes 20–25% of the energy used in food processing industries. This work presents a detailed design and comprehensive numerical analysis of a hybrid geothermal-PCM flat plate solar collector, which is in effect a self-sustaining renewable energy food drying system. The applicability of the hybrid geothermal-solar dryer for developing countries with a significant underground geothermal temperature gradient is investigated. The performed numerical analysis promises high efficiency and effectivity of this near-zero hybrid energy technology. The proposed conceptual design assessed using the related mathematical model to assess the functional parameters of the hybrid geothermal PCM flat plate solar collector dryer. Numerical simulation showed that the efficiency of hybrid geothermal PA-FPSC is 20.5% higher than conventional flat plate solar collector when the mass flow rate is at 0.02 kg/s. Therefore, successful integration of this technology to the existing food processing industries of the developing countries could be a step towards the prevention of irreversible climate change achieving SDG 12 (sustainable development goals).

A comparative investigation of solar-assisted heat pumps with solar thermal collectors for a hot water supply system

The concept of a hybrid solar collector was newly proposed, which has a solar thermal receiver part and air-source receiver part that can operate as a fin-tube exchanger with fans when there is no solar radiation. This is the distinguishing feature of the hybrid solar collector. The solar-assisted heat pump system applying the hybrid solar collectors was also introduced. To analyze the proposed solar-assisted heat pump system, a simple thermodynamic model considering the electric power consumption for fans and circulation pumps was developed. The model results for the air-source heat gain were validated from the corresponding experimental data with the maximum error of 8.8%. Based on the parametric study, the increase of the collector area and the absorption coefficient led to the enhancement of the system performance of the solar-assisted heat pump with hybrid solar collectors. In addition, a comparative investigation was carried out for three different indirect solar-assisted heat pumps. Regarding of annual operation hour, the serial-type solar-assisted heat pump was worst at 3509 h over a year. On the other hand, the solar-assisted heat pump with hybrid solar collectors had the superior performance of 7036 h. If the efficiency of the hybrid solar collector was as superior as the conventional flat plate solar collector, the system COP of the suggested solar-assisted heat pump system in the winter season could be improved by 5.1% compared with that of the parallel-type solar-assisted heat pump system. Consequently, it clearly showed the feasibility of the suggested solar-assisted heat pump system as a standalone system for hot water supply due to the merits of significantly extending the annual operation hour, the simple system configuration, and the economic aspect with lower capital cost.

Heat Transfer Optimization in Air Flat Plate Solar Collectors Integrated with Baffles

This paper presents an experimental analysis for comparisons of conventional flat plate solar collectors and collectors integrated with different numbers of baffles. Heat transfer between absorber plate and drying fluid (air) has been one of the major challenges in the design and operations of the indirect solar dryer systems. In this experiment, efficiency of air flat plate solar collector integrated with 2, 3, 4 and 8 baffles was studied and compared with the ordinary collector. The results showed that integrating solar collector with baffles significantly increased the efficiency of the system. It was noted that collector with 2, 3, 4 and 8 baffles had a mean efficiency of 29.2%, 31.3%, 33.1% and 33.7% respectively while with no baffles was 28.9%. The analysis showed that when there were less than four baffles in the collector, heat transfer was dominant over pressure drop and hence high efficiency. However, when the number of baffles exceeded four, the effect associated with an increase in pressure drop highly observed compared to heat transfer coefficient, thus resulted to insignificant increase in efficiency. Therefore, the optimum number of four baffles was commended for the designed model for optimum efficiency.

Fabrication of Flat Plate Solar Geyser with Flat Grooved Heat Exchanger Having Special Exit System

The main objective of this paper is to introduce the concept of novel flat plate solar geyser with integrated heat exchanger and open loop passive system.The heat exchanger acts both as collector for solar radiations and as a heat exchanger its self for cold water beneath it. Contrary to the conventional flat plate solar collectors, water is in direct contact with the collector or flat grooved heat exchanger. A safety control box is installed to minimize hydraulic pressure of cold water reservoir on the flat grooved heat exchanger as contrary to tube a plate cannot sustain high pressures. The heat exchanger has circular grooves which adds in sustaining hydraulic pressure, increase heat exchanger exposed area to the sun and also cause turbulence in flowing cold water to increase heat rate. A special exit system for hot water is used having a float tube which enable user to withdraw hot water without pressure of cold water from cold water reservoir.