Mantle heat exchangers for horizontal tank thermosyphon solar water heaters

This paper describes the characteristics of horizontal mantle heat exchangers for application in thermosyphon solar water heaters. A new correlation for heat transfer in horizontal mantle heat exchangers with bottom entry and exit ports was used to predict the overall heat transfer and stratification conditions in horizontal tanks with mantle heat exchangers. The model of a mantle heat exchanger tank was combined with the thermosyphon solar collector loop model in TRNSYS to develop a model of a thermosyphon solar water heater with collector loop heat exchanger. Predictions of stratification conditions in a horizontal mantle tank are compared with transient charging tests in a laboratory test rig. Predictions of daily energy gain in solar preheaters and in systems with in-tank auxiliary boosters are compared with extensive outdoor measurements and the model is found to give reliable results for both daily and long-term performance analysis.     

Experimental investigation on thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger

The thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger was investigated to show its applicability in China. The effect on the performance of the collector of using a heat exchanger between the collector and the tank was analyzed. A “heat exchanger penalty factor” for the system was determined and energy balance equation in the system was presented. Outdoor tests of thermal performance of the thermosyphon flat-plate solar water heater with a mantle heat exchanger were taken in Kunming, China. Experimental results show that mean daily efficiency of the thermosyphon flat plate solar water heater with a mantle heat exchanger with 10 mm gap can reach up to 50%, which is lower than that of a thermosyphon flat-plate solar water heater without heat exchanger, but higher than that of a all-glass evacuated tubular solar water heater.     

Experimental and numerical study of thermal stratification in a mantle tank of a solar domestic hot water system

The simulation and the optimisation of the mantle tank of solar domestic hot water systems needs dynamic simulation over long periods of time (e.g. 1 year). A model for such a mantle tank was developed by using the zonal approach. The dimensions of the zones are determined based on physical considerations. A mixing coefficient is identified to model the water flow in the mantle heat exchanger. Comparisons of the results of temperatures distribution of the model and of experiments show a difference <7% for three positions of the inlet water flow in the mantle heat exchanger.     

Examination of top and bottom inlet position in horizontal mantle heat exchanger solar thermosiphonic circuits

The present work deals with the examination of two different configurations of inlet mantle connections, on a solar thermosiphon of indirect heating type. In the first configuration, the heat transfer fluid enters the upper side of the mantle heat exchanger, while in the second it enters the lower side of the mantle. The outlet is always placed at the bottom of the mantle, oppositively to the inlet. Diurnal tests have been carried out by monitoring the inlet and outlet mantle temperatures, as well as thermosiphonic flow rates. Also, draw-offs measurements were performed, according to ISO 9459-2 and EN 12976-2 standards, at the end of each test day. The final achieved water temperatures of the first configuration are in general higher, than those of the second one, which means a better quality of the delivered energy. Also, the first configuration appears to pick up greater performance, under high solar energy gain (high insolation rate – low thermal losses).     

Heat transfer correlations for vertical mantle heat exchangers

This paper investigates heat transfer in vertical mantle heat exchangers for application in low flow solar domestic hot water systems. Two new heat transfer correlations for vertical mantle heat exchangers with top entry port and bottom exit ports are developed. The correlations are based on computational fluid dynamic modelling of whole vertical mantle tanks. The correlations are combined with a heat storage model in a simulation program that predicts the yearly thermal performance of low flow solar domestic hot water systems based on mantle tanks. The model predictions of energy gains and temperatures are compared with outdoor measurements and the model is found to give reliable results.     

Effectiveness of solar water-heating systems

In order to protect the solar water heaters against freezing and corrosion, the antifreeze solutions are circulated in the collector loop and then in the second loop this fluid heats the water in a heat exchanger. For this two-loop system, the performance can be described in terms of effectiveness similar to that for conventional heat exchangers. In this note an attempt is made to combine the collector and heat exchanger effectivenesses into an overall effectiveness, which is useful to optimize and design the solar water heating systems. The study has also been extended to obtain an overall effectiveness for the three-loop system in which a heat exchanger is placed between the collector and store.     

Indirect thermosiphon flat-plate solar collector performance based on twisted tube design heat exchanger filled with nanofluid

Stationary solar collector such as flat-plate collector is a thermal device, which traps solar energy and converts it into heat that can be used in industrial and domestic applications such as water heating. Flat-plate collector thermal performance enhancement is investigated in this research paper. Two cross-sectional geometries of the tube in the heat exchanger were investigated; a normal circular tube and a twisted tube were used in the experiment. The aim of the twisted tube exchanger is to enhance the performance of heat transfer of the tubes and to reduce the shell pressure drop; flat-plate solar collector is the used application to study the heat exchanger performance. Both twisted tubes heat exchanger and normal circular tubes heat exchanger were examined in the same location and conditions with the same solar collector, both were used in the heat exchanger to study their effect, with two different working fluids, which are distilled water and multiwalled carbon nanotube (MWCNT)/water nanofluid. The system shows an increase in the performance when twisted tubes were used in the system compared with the circular tubes in both distilled water and MWCNT/water nanofluid by 12.8% and 12.5%, respectively, with an improvement by 34% for twisted tubes with MWCNT compared with normal circular tubes with distilled water.     

Analysis of horizontal mantle heat exchangers in solar water heating systems

The characteristics of horizontal mantle heat exchangers are investigated for application in thermosyphon solar water heaters. An experimental model of a horizontal mantle heat exchanger was used to evaluate the flow patterns in the annular passageways and the heat transfer into the inner tank. Flow visualisation was used to investigate the flow structure, and the heat transfer was measured for isothermal inner tank conditions. A numerical model of the flow and heat transfer in the annular passageway was developed and used to evaluate the heat flux distribution over the surface of the inner tank. The numerical results indicate that configurations of mantle heat exchangers used in current solar water heater applications degrade thermal stratification in the inner tank. The effects of inlet flow rate, temperature and connecting port location are quantified.     

Review of solar dryers for agricultural and marine products

Drying for agricultural and marine products are one of the most attractive and cost-effective application of solar energy. Numerous types of solar dryers have been designed and developed in various parts of the world, yielding varying degrees of technical performance. Basically, there are four types of solar dryers; (1) direct solar dryers, (2) indirect solar dryers, (3) mixed-mode dryers and (4) hybrid solar dryers. This paper is a review of these types of solar dryers with aspect to the product being dried, technical and economical aspects. The technical directions in the development of solar-assisted drying systems for agricultural produce are compact collector design, high efficiency, integrated storage, and long-life drying system. Air-based solar collectors are not the only available systems. Water-based collectors can also be used whereby water to air heat exchanger can be used. The hot air for drying of agricultural produce can be forced to flow in the water to air heat exchanger. The hot water tank acts as heat storage of the solar drying system.     

Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle

A number of solar domestic hot water systems and many combined space and water heating systems have heat exchangers placed directly in the storage fluid to charge and/or discharge the tank. Operation of the heat exchanger produces a buoyancy-driven flow within the storage fluid. With a view toward controlling the flow field to increase heat transfer, a cylindrical baffle is inserted in a 350 l cylindrical storage tank. The baffle creates a 40 mm annular gap adjacent to the tank wall. A 10 m-long, 0.3 m² copper coil heat exchanger is placed in the gap. The effects of the baffle on the transient heat transfer, delivered water temperature, heat exchanger effectiveness, and temperature distribution within the storage fluid are presented during discharge of initially thermally stratified and fully mixed storage tanks. The baffle increases the storage side convective heat transfer to the heat exchanger by 20%. This increase is attributed to higher storage fluid velocities across the heat exchanger.     

Consumers’ influence on the thermal performance of small SDHW systems—Theoretical investigations

Usually when SDHW systems are tested and modeled the daily hot-water consumption and consumption pattern are constant during all days of the test period and simulation period. This is not realistic in ‘real life’. Numerical simulations with detailed simulation models have been carried out to investigate the influence on the thermal performance of different hot-water consumptions and consumption patterns. Two different small SDHW systems are taken into the calculation, a low-flow system where the heat storage consists of a mantle tank and a high flow system with an internal heat exchanger spiral in the heat storage. Two different domestic hot water (DHW)-load profiles have been taken into the calculation. One of the DHW-load profiles has three draw-offs at equal time, size and duration every day while the other DHW-load profile is a realistic profile for a Danish family where the hot-water consumption and consumption pattern vary from day to day and furthermore weekends and holidays are taken into account in this profile. Different volumes of the tanks in the two systems are taken into the calculation in order to determine how the thermal performance of the systems is influenced by the tank volume for different hot-water consumptions. Furthermore it is investigated how the thermal performance of the systems are influenced by mixing in the solar tank during draw-offs.     

Performance improvement by discharge from different levels in solar storage tanks

The thermal advantages by utilizing discharge from different levels in solar storage tanks are investigated, both for a small SDHW system and for a solar combisystem.The investigations showed that it is possible to increase the thermal performance of both types of systems by using two draw-off levels from the solar tanks instead of one draw-off level at a fixed position.The best position of the second draw-off level is in the middle or just above the middle of the tank. For the investigated small SDHW system with a realistic draw off hot water temperature of 40 °C and 45 °C and an auxiliary volume temperature of 50.5 °C the increase of the thermal performance by the second draw-off level is about 6%.For the investigated solar combisystem the increase in thermal performance by using one extra draw-off level, either for the domestic hot water heat exchanger or for the heating system, is about 3%, while an improvement of about 5% is possible by using a second draw-off level both for the domestic hot water heat exchanger and for the heating system.     

Heat exchanger penalties in double-loop solar water heating systems

In many solar water heating systems, it may prove desirable to use a double-loop system with a heat exchanger between the flat-plate collector and the water storage tank. This approach, using a second fluid which does not freeze in service and which does not lead to corrosion of metals, may be the most convenient way to avoid freezing or corrosion problems in the collector. Because of the heat exchanger, the collector is, however, forced to operate at a higher temperature with a corresponding performance penalty.

A heat exchanger factor has been developed, which makes it possible to determine the collection performance penalty in a straightforward manner. When the heat exchanger is of the counterflow type and is operated so that the mass flowrate-specific heat products of the two streams are equal, the expression becomes very simple, and lends itself to direct optimization of heat exchanger size. Several sample optimization calculations are shown.     

Experimental evaluation of solar thermosyphons with heat exchangers

The advantages of solar thermosyphons in terms of simplicity, reliability and cost have long been recognized. Recent studies have also shown their thermal performance to be comparable with that of equivalent active systems. When pump power is considered, the energy savings of domestic hot water thermosyphons can be significantly superior to active systems. In spite of these advantages, use of solar thermosyphons in the United States is almost negligible compared to their widespread use in other countries. A major limitation to the use of thermosyphons in the United States is lack of effective, reliable freeze protection. One technique for reliable, passive freeze protection is to use a heat exchanger in the storage tank and a nonfreezing fluid in the collector. Previous analytical work indicates that the performance penalty for these systems with practical-sized heat exchangers may be small enough to make these systems economically feasible. A full-scale, residential-size test facility has been constructed for testing this concept and validating the theoretical models. This paper describes results of testing comparing the performance of a horizontal tank with and without heat exchanger to a baseline case of a vertical tank without heat exchanger. An analytical expression for a “heat exchanger penalty factor” for these systems is derived and compared with the experimental results.     

Improved design of mantle tanks for small low flow SDHW systems

Side‐by‐side tests of two small low flow solar domestic hot water (SDHW) systems based on mantle tanks have been carried out under the same test conditions in a laboratory test facility. The systems are identical with exception of the mantle tanks. One of the mantle tanks has the mantle inlet port located at the top of the mantle and the other mantle tank has the mantle inlet port moved 0.175 m down from the top of the mantle. The thermal performance is almost the same for the two systems in the measuring period of 252 days. The solar fractions were 0.66 and 0.68 for the two systems. The tests showed also that the system with the low mantle inlet perform better than the system with the high mantle inlet in periods with low solar fractions, that is in less sunny periods. Further, calculations with a simulation model for low flow SDHW systems based on mantle tanks showed that mantle tanks currently marketed can be greatly improved by relatively simple design changes: increasing the height/diameter ratio, reducing the mantle height and increasing the insulation thickness on the sides of the tank. Copyright © 2006 John Wiley & Sons, Ltd.     

Evaporation performance of a new plate‐type heat exchanger with winding tubes on plate surfaces (estimation of performance of the heat exchanger)

A method for evaluating and predicting the performance of a newly developed plate‐type heat exchanger as an evaporator for water‐refrigerant systems such as chillers has been developed. The main component of the developed heat exchanger consists of plates packed together in a casing with winding tubes connected to both sides of the plates. Refrigerant flows inside the tubes, and water flows in the space between the plates. A herringbone‐like pattern is formed in this space by the cross sections of the winding tubes. The newly developed method estimates evaporation performance of the developed heat exchanger using new empirical correlations. There are correlations for heat transfer and pressure drop in winding‐tube banks on the water side, and correlation for the pressure drop on the refrigerant side. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 245–257, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20009     

Computational simulation of district solar heating systems with seasonal thermal energy storage

A computational simulation model for determining the thermal performance of large-scale community solar heating systems with interseasonal heat storage is described. Special attention has been paid to the mathematical formulation of the storage unit. It comprises an uninsulated stratified hot water tank excavated in rock. The storage capacity of the surrounding ground may also be utilized by vertical heat exchanger pipes. Comparisons of theoretical system performance predictions with recent experimental measurements from a full-scale prototype installation are presented and found to be in reasonable agreement. The simulation program is also used to evaluate the thermal performance of various district solar heating system configurations for northern cold climatic conditions (60°N).     

Natural convection heat exchangers in solar water heating systems: Theory and experiment

Natural convection heat exchangers can be used in solar hot water systems to replace the pump on the tank side of the exchanger. There is currently no experimentally verified way of designing this type of heat exchanger. An experimental apparatus to test natural convection heat exchangers was built and an extensive set of measured data obtained on two different exchangers sized for low-flow stratified tank system. Two theoretical models for the exchanger are presented: a finite-volume primitive variable numerical solution of the fundamental laminar equations of fluid motion and a laminar forced-convection-based solution method. Comparison of the model predictions with the experimental data showed good agreement when the modified Rayleigh number is less than about 400. The poor agreement under other conditions was attributed to turbulence and recirculation neither of which was accounted for in the models.     

Theoretical investigation of the performance of a novel loop heat pipe solar water heating system for use in Beijing,China

A novel loop heat pipe (LHP) solar water heating system for typical apartment buildings in Beijing was designed to enable effective collection of solar heat, distance transport, and efficient conversion of solar heat into hot water. Taking consideration of the heat balances occurring in various parts of the loop, such as the solar absorber, heat pipe loop, heat exchanger and storage tank, a computer model was developed to investigate the thermal performance of the system. With the specified system structure, the efficiency of the solar system was found to be a function of its operational characteristics - working temperature of the loop heat pipe, water flow rate across the heat exchanger, and external parameters, including ambient temperature, temperature of water across the exchanger and solar radiation. The relationship between the efficiency of the system and these parameters was established, analysed and discussed in detail. The study suggested that the loop heat pipe should be operated at around 72 °C and the water across the heat exchanger should be maintained at 5.1 l/min. Any variation in system structure, i.e., glazing cover and height difference between the absorber and heat exchanger, would lead to different system performance. The glazing covers could be made using either borosilicate or polycarbonate, but borosilicate is to be preferred as it performs better and achieves higher efficiency at higher temperature operation. The height difference between the absorber and heat exchanger in the design was 1.9 m which is an adequate distance causing no constraint to heat pipe heat transfer. These simulation results were validated with the primary testing results.