Thermal performance of phase change material energy storage floor for active solar water-heating system

The conventional active solar water-heating floor system contains a big water tank to store energy in the day time for heating at night, which takes much building space and is very heavy. In order to reduce the water tank volume or even cancel the tank, a novel structure of an integrated water pipe floor heating system using shape-stabilized phase change materials (SSPCM) for thermal energy storage was developed and experimentally studied in this paper. The thermal performances of the floors with and without the SSPCM were compared under the intermittent heating condition. The results show that the Energy Storage Ratio (ESR) of the SSPCM floor is much higher than that of the non-SSPCM floor; the SSPCM floor heating system can provide stable heat flux and prevent a large attenuation of the floor surface temperature. Also, the SSPCM floor heating system dampens the indoor temperature swing by about 50% and increases the minimum indoor air temperature by 2°C–3°C under experimental conditions. The SSPCM floor heating system has a potential of making use of the daytime solar energy for heating at night efficiently.     

Performance of a hybrid heating system with thermal storage using shape-stabilized phase-change material plates

Performance of a hybrid heating-system, combined with thermal storage using shape-stabilized phase-change material (SSPCM) plates, is investigated numerically. A direct gain passive solar house in Beijing is considered, which includes SSPCM plates as the inner linings of walls and the ceiling. Unsteady simulation is performed using a verified enthalpy model, with a time period covering the winter heating-season. Additional heat supply is employed during load hours at late night and early morning (23:00–07:00 in Beijing) or during the whole day necessary to keep the minimum indoor air temperature above 18 °C. The results indicate the thermal storage effect of SSPCM plates, which improves the indoor thermal comfort level and saves about 47% of normal-and-peak-hour energy use and 12% of total energy consumption in winter in Beijing.     

Performance investigation on a thermal energy storage integrated solar collector system using nanofluid

The effect of Fe nanofluid on the performance enhancement on solar water heater integrated with thermal energy storage system is investigated experimentally. A 0.5% wt fraction of Fe nanoparticle was synthesized with the mixture of water/propylene‐glycol base fluid. The experimental implementation utilized 40‐nm‐size Fe nanoparticle, 15 ° collector tilt angle, and 1.5 kg/min mass flow rate heat‐transfer fluid circulation. The system efficiency reached 59.5% and 50.5% for with and without nanofluid. The water tank temperature was increased by 13 °C during night mode. The average water tank temperature at night mode was 47.5 °C, while the average ambient temperature was 26 °C. The Fe nanofluid improved the system working duration during night mode by an average of 5 h. The techno‐economic analysis results showed a yearly estimated cost savings of 28.5% using the Fe nanofluids as heat transfer fluid. The embodied energy emission rate, collector size, and weight can be reduced by 9.5% using nanofluids. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermal analysis of a direct-gain room with shape-stabilized PCM plates

The thermal performance of a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) plates has been analysed using an enthalpy model. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM (melting temperature, heat of fusion and thermal conductivity), inner surface convective heat transfer coefficient, location and thickness of the SSPCM plate, wall structure (external thermal insulation and wallboard material) etc. The results show that: (1) for the present conditions, the optimal melting temperature is about 20 °C and the heat of fusion should not be less than 90 kJ kg⁻¹; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation of the exterior wall obviously influences the operating effect and period of the SSPCM plates and the indoor temperature in winter; (5) the SSPCM plates create a heavyweight response to lightweight constructions with an increase of the minimum room temperature at night by up to 3 °C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime.     

Energy performance of a hybrid space-cooling system in an office building using SSPCM thermal storage and night ventilation

Thermal performance of a hybrid space-cooling system with night ventilation and thermal storage using shape-stabilized phase change material (SSPCM) is investigated numerically. A south-facing room of an office building in Beijing is analyzed, which includes SSPCM plates as the inner linings of walls and the ceiling. Natural cool energy is charged to SSPCM plates by night ventilation with air change per hour (ACH) of 40 h⁻¹ and is discharged to room environment during daytime. Additional cool-supply is provided by an active system during office hours (8:00-18:00) necessary to keep the maximum indoor air temperature below 28 °C. Unsteady simulation is carried out using a verified enthalpy model, with a time period covering the whole summer season. The results indicate that the thermal-storage effect of SSPCM plates combined with night ventilation could improve the indoor thermal-comfort level and save 76% of daytime cooling energy consumption (compared with the case without SSPCM and night ventilation) in summer in Beijing. The electrical COPs of night ventilation (the reduced cooling energy divided by fan power) are 7.5 and 6.5 for cases with and without SSPCM, respectively.     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

Analysis of solar aided heat pump systems with seasonal thermal energy storage in surface tanks

Annual periodic performance of a solar assisted ground-coupled heat pump space heating system with seasonal energy storage in a hemispherical surface tank is investigated using analytical and computational methods. The system investigated employs solar energy collection and dumping into a seasonal surface tank throughout the whole year with extraction of thermal energy from the tank for space heating during the winter season. A computational model is presented in this study for the prediction of the annual periodic transient behaviour of the system under investigation. The present computational model is based on a hybrid analytical–numerical procedure which facilitates determination of the annual variation of water temperature in the surface tank, the amounts of solar thermal energy collected during each month and the annual periodic performance of the solar aided space heating system.     

The performance of a solar assisted heat pump water heating system

Analytical and experimental studies were performed on a solar assisted heat pump water heating system, where unglazed, flat plate solar collectors acted as an evaporator for the refrigerant R-134a. The system was designed and fabricated locally, and operated under meteorological conditions of Singapore. The results obtained from simulation are used for the optimum design of the system and enable determination of compressor work, solar fraction and auxiliary energy required for a particular application. To ensure proper matching between the collector/evaporator load and compressor capacity, a variable speed compressor was used. Due to high ambient temperature in Singapore, evaporator can be operated at a higher temperature, without exceeding the desired design pressure limit of the compressor, resulting in an improved thermal performance of the system. Results show that, when water temperature in the condenser tank increases with time, the condensing temperature, also, increases, and the corresponding COP and collector efficiency values decline. Average values of COP ranged from about 4 to 9 and solar collector efficiency was found to vary between 40% and 75% for water temperatures in the condenser tank varying between 30°C and 50°C. A simulation model has been developed to analyse the thermal performance of the system. A series of numerical experiments have been performed to identify important variables. These results are compared with experimental values and a good agreement between predicted and experimental results has been found. Results indicate that the performance of the system is influenced significantly by collector area, speed of the compressor, and solar irradiation. An economic analysis indicates a minimum payback period of about two years for the system.     

太阳能地面采暖系统蓄热水箱容积分析

通过分析太阳能采暖系统所需蓄热鼍与建筑热负荷、太阳能集热量日变化规律之间的关系,得出太阳能采暖系统所需蓄热水箱容积的理论算式.根据拉萨、银川、西宁、西安等地的太阳辐射强度及建筑热负荷的日变化规律,模拟得出系统所需蓄热量变化规律;并对各种蓄热温差下对应的蓄热水箱容积进行了模拟分析,结果表明:太阳能采暖系统所需蓄热量随太阳集热器的集热量与建筑热负荷之间的差值增大而增加;蓄热水箱容积随蓄热温差增大而减小,当蓄热水温达到80℃时,在各种地面采暖系统取水温度下,单位集热器面积所需蓄热水箱容积趋于相等.     

Solar multi-mode heating system based on latent heat thermal energy storage and its application

为克服太阳能间断性和不稳定性的缺点进而实现太阳能集热与采暖的能量供需调节和全天候连续供热,提出了基于相变储热的太阳能多模式采暖方法（太阳能集热直接采暖、太阳能集热采暖+相变储热、太阳能相变储热采暖）,并在西藏林芝市某建筑搭建了太阳能与相变储热相结合的采暖系统,该系统可根据太阳能集热温度和外界供热需求实现太阳能多模式采暖的自动控制和自动运行。实验研究表明：在西藏地区采用真空管太阳能集热器可以和中低温相变储热器很好地结合,白天储热器在储热过程中平均储热功率为10.63 kW,储热量达到92.67 kW·h,相变平台明显;晚上储热器在放热过程中供热量达85.23 kW·h,放热功率和放热温度平稳,储放热效率达92%,其储热密度是传统水箱的3.6倍,可连续供热时间长达10 h,从而实现了基于相变储热的太阳能全天候连续供热,相关研究结果对我国西藏地区实施太阳能采暖具有一定的指导作用。     

Experimental study and exergetic analysis of a CPC-type solar water heater system using higher-temperature circulation in winter

The common solar water heater system can meet low temperature requirements, but exhibits very low efficiency in attaining higher water temperatures (55–95 °C). In the current paper, a compound parabolic concentrator (CPC)-type solar water heater system experiment rig with a U-pipe was set up, and its performance in meeting higher temperature requirements was investigated. The experiments were conducted in December at Hefei (31°53′ N, 117°15′ E), in the eastern region of China. The system showed steady performance in winter, with overall thermal efficiency always above 43%. The water in the tank was heated from 26.9 °C to 55, 65, 75, 85, and 95 °C. Through the experimental study and exergetic analysis of the solar water heater system, results of the five experiments showed thermal efficiency of above 49.0% (attaining 95 °C water temperature) and exergetic efficiency of above 4.62% (attaining 55 °C water temperature). Based on these results, the CPC-type solar water heater system with a U-pipe shows superior thermal performance in attaining higher temperatures and has potential applications in space heating, heat-powered cooling, seawater desalination, industrial heating, and so on.     

太阳能地板辐射系统用于夜间采暖时系统性能的实验研究

利用太阳能集热器制得低温热水作为地板辐射采暖系统的热源,是一种清洁、节能、舒适的采暖方式。在南京地区搭建了太阳能地板辐射采暖系统实验台,系统运行策略为白天集热、夜晚采暖,通过实验得到了集热器集热效率、地板进出水温度、室内不同朝向围护结构温度、不同高度的空气温度等参数,最后对系统的性能进行了概括和总结。     

Performance of a multi-functional direct-expansion solar assisted heat pump system

This paper reports on the long-term performance of a direct-expansion solar assisted heat pump (DX-SAHP) system for domestic use, which can offer space heating in winter, air conditioning in summer and hot water during the whole year. The system employs a bare flat-plate collector array with a surface area of 10.5 m², a variable speed compressor, a storage tank with a total volume of 1 m³ and radiant floor heating unit. The performance under different operation modes is presented and analyzed in detail. For space-heating-only mode, the daily-averaged heat pump COP varied from 2.6 to 3.3, while the system COP ranged from 2.1 to 2.7. For water-heating-only mode, the DX-SAHP system could supply 200 l or 1000 l hot water daily, with the final temperature of about 50 °C, under various weather conditions in Shanghai, China. For space-cooling-only mode, the compressor operates only at night to take advantage of a utility’s off-peak electrical rates by chilling water in the thermal storage tank for the daytime air-conditioning. It shows that, the multi-functional DX-SAHP system could guarantee a long-term operation under very different weather conditions and relatively low running cost for a whole year.     

Analysis of the opportunities and challenges of solar water heating system (SWHS) in India: Estimates from the energy audit surveys & review

A solar water heating system (SWHS) is a device that makes available the thermal energy of the incident solar radiation for use in various water heating applications. SWHS largely depends on the performance of the collector's efficiency at capturing the incident solar radiation and transferring it to the water. With today's SWHS, water can be heated up to temperatures of 60–80 °C. Heated water is collected in a tank insulated to prevent heat loss. Circulation of water from the tank through the collectors and back to the tank continues automatically due to the thermosiphon principle. The hot water generated finds many end-use applications in domestic, commercial, and industrial sectors. India has the highest energy intensities in Asia. Very little investment and priority are being given to increase of the efficiency. On the other hand, the India has a high potential for developing energy production from renewable energy sources (RES): solar, water, wind and biomass. However, these potentials are not studied and exploited enough and the present situation for their utilization is not so good. Although energy is a critical foundation for economic growth and social progress of any country, there are many constraints for RES development in all of them (political, technological, financial, legislative, educational, etc.). Obviously, defining development strategies and new support measures is necessary since renewable energy sources can make an important contribution to the regional energy supply and security. The main purpose of this paper is to explore the solar water heating system (opportunities) in India.     

A novel thermal water pump for circulating water in a solar water heating system

This research purpose was to perform a parametric study of a novel thermal water pump well fitted in a simulated solar water heating system (SWHS). The SWHS was composed of a heating tank (HT), a hot water storage tank (ST) and an overhead tank (OT). The HT together with a specially designed valve act as a novel thermal water pump that gets power from hot water vapor and air pressure produced by a built-in electric heater in order to transfer heat from the HT to ST. The general operation of this pump has four stages for each cycle: heating, water circulating, vapor circulating and water supplying. The discharge water heads were varied with an increment of 0.25 m from 0.75 to 3 m. According to the experiment, it was found that the pump could operate at an average HT temperature of about 80–95 °C leading to 70–80 °C ST temperatures and 20–35 pumping cycles and consumed 17 MJ energy input during 9-h period. The overall thermal efficiency of the SWHS was 33–42% and the mean pump efficiency was about 0.005–0.011% depending upon the discharge heads.     

ICS solar systems with horizontal (E–W) and vertical (N–S) cylindrical water storage tank

The use of a horizontal cylindrical water storage tank contributes to pressure resistant, low height and efficient ICS solar systems. These systems can satisfactorily achieve water heating when the cylindrical storage tank is combined with stationary CPC or involute type curved reflectors. The diameter of the cylindrical storage tank determines the length of the reflectors, the system depth and the ratio of the stored water per aperture area. In these solar systems the storage tank can be partially thermally insulated to suppress thermal losses from it to the ambience. We constructed four experimental models with truncated symmetric CPC reflectors, two with 90° and other two with 60° of acceptance angle, half of them without and half with a 1/4 thermally insulated storage tank cylindrical surface. In addition, we constructed two ICS systems with involute reflectors, with acceptance angle 180°, one without and the other one with a 1/4 thermally insulated storage tank. The six ICS systems were tested under the same weather conditions and without water drain, to determine their stored water temperature variation, mean daily efficiency and thermal losses during night. The results showed that CPC reflectors contribute to efficient operation of systems day and night, while involute reflectors mainly to the water heat preservation during night.     

Experimental study of the burning-cave hot water soil heating system in solar greenhouse

In the northern China areas, the traditional heating methods are widely used in solar greenhouse, for example: electric heating, hot air heating, hot water heating, burning-cave heating etc. If copying the assuring building indoor environment of constant heating ways into solar greenhouse, it will further increase building energy consumption, thus improving the efficiency of energy utilization, establishing appropriate growing environment, and realizing the agricultural waste recycling are important ways of consistent with the Chinese conditions, construction of sustainable development, improving the efficiency of the greenhouse production. To solve the problem of traditional heating method for high heating energy consumption, the inharmonious between greenhouse air temperature and soil temperature, uneven soil temperature, the research build the burning cave hot water soil heating system of solar greenhouse experimental platform in accordance with principle of energy cascade utilization. This experiment platform will transfer burning cave internal heat into soil heating system. The soil is evenly heated by system. Through testing the actual operation effect of the burning cave hot water soil heating system of new solar greenhouse, electric heating system, no taking any heating measures system, burning cave hot water soil heating system of solar greenhouse can improve the soil average temperature 5 ∼ 6 °C. This research provides experimental basis for practical applications and promotion.     

Investigation of thermal performance of a passive solar building with floor radiant heating

This paper presents a theoretical study of an integrated radiant floor heating–direct gain passive solar system. Thermal mass is utilized both for storage of auxiliary heating energy and direct solar gains incident on the floor. An explicit finite difference model is developed to accurately model nonlinear effects and auxiliary heating control. The numerical simulation model is employed to study the performance of a passive solar outdoor test-room with different amounts of thermal mass under various control strategies with constant and sinusoidal room thermostat setpoints. A satisfactory thermal mass is determined based on energy savings, reduction of room temperature swings, and prevention of floor surface overheating. Control of auxiliary heating based on a room effective (operative) temperature is shown to result in improved thermal comfort and higher utilization of passive solar gains as compared to room air temperature control.     

Outdoor test method to determine the thermal behavior of solar domestic water heating systems

The dynamics of the market, the generation of new promotion programs, fiscal incentives and many other factors are to be considered for the massive application of solar domestic water heating systems (SDWHS) mainly of the compact thermosiphon type, makes it necessary to choose simple and inexpensive procedure tests that permit to know their characteristic thermal behaviors without an official standard being necessary. Moreover, it allows the comparison among systems and offers enough and reliable information to consumers and manufacturers.In most developing countries, an official national standard for SDWHS is not available, therefore it is necessary to adopt an international test procedure in which the cost and time of implementation is very important. In this work, a simple and inexpensive test method to determine the thermal behavior of SDWHS is proposed.Even though these procedure tests do not have an official standard structure they permit, by comparing different solar systems under identical solar, ambient, and initial conditions, the experimental determination of: (a) the maximum available volume of water for solar heating; (b) water temperature increment and available thermal energy at the end of the day; (c) temperature profiles (stratification) and the average temperature in the storage tank after it is homogenized; (d) the average global thermal efficiency; (e) water temperature decrement and energy lost overnight; and (f) the relationship between hot water volume and solar collector area as function of the average heating temperature. An additional proposed test permits to know the heat losses caused by the reverse flow in the collector loop. These tests will be carried out independently of the configuration between the solar collector and the storage tank, the way the fluid circulates and the type of thermal exchange. The results of this procedure test can be very useful, firstly, for the local solar manufacturers’ equipment in order to design and optimize its products comparing their systems against a reference system under identical test conditions and secondly, by the consumers in order to select the most suitable system. The resulting experimental data for a particular thermosiphon system is presented and discussed.     

电加热相变材料蓄热地板采暖的热性能模拟

为消除电采暖引起的电网峰谷差并降低采暖运行费用，该文提出了一种带有相变材料潜热贮能板的地板电采暖系统，并建立了分析此系统热性能的地板和房间理论模型，对给定的电加热相变蓄热地板采暖房间，模拟了室内空气温度和地面温度的变化，藉此分析了我国几个气候地区冬季该系统的应用效果，结果证明此采暖方式在使房间热负荷不大的建筑和气候条件下，基本能满足人的热舒适性要求，有较好的应用前景。