Development of solar thermal technologies in China

Solar energy is receiving much more attention in building energy systems in recent years. Solar thermal utilization should be based on integration of solar collectors into buildings. The facades of buildings can be important solar collectors, and, therefore, become multifunctional. In addition, solar collectors can be used to enhance the appearance of the facade when considering their aesthetic compatibility. Currently, installation of collectors on the south tilted roofs, south walls, balconies or awnings of buildings are the feasible approaches for integration of solar collectors into buildings. The most well known solar energy demonstration projects in China are introduced in this paper, which cover different integrated approaches, and solar heating and cooling systems. In China's cities, the process of rebuilding apartment roofs from flat to inclined offers the ideal opportunity to carry out solar renovation in combination with roof-integrated collectors. It can be seen from the demonstration projects over the last twenty years, that, solar cooling systems were mainly used in public buildings for either absorption or adsorption. Besides, nearly all solar cooling systems are multifunctional. They have been used to supply heating and hot water in other seasons for the purpose of high solar fraction. In the 11th Five-year research project (duration 2006–2010), the government has encouraged solar energy researchers to study, develop, and break through the key technologies involved in the integration of solar thermal technologies with buildings.     

Experience on integration of solar thermal technologies with green buildings

The green buildings of Shanghai Research Institute of Building Science include an office building for the demonstration of public building and two residential buildings, which are for the demonstration of flat and villa, respectively. Here, a solar-powered integrated energy system including heating, air-conditioning, natural ventilation and hot water supply was designed and constructed for the office building. However, only solar hot-water systems were installed for the flat and villa. All the three solar thermal systems have continuously run for 2 years. Two different integrated approaches have been put into practice in the two green residential buildings. It is shown that, for new buildings, solar collectors can be mounted on balconies and awnings besides roofs, on condition that solar systems become part of the general building design. The solar-powered integrated energy system has the advantage of high utilization ratio with different functions according to different seasons. It is testified to be capable of taking on about 70% of the yearly building load regarding the involved space under the weather condition of Shanghai.     

Performance improvement of PV/T solar collectors with natural air flow operation

The electrical efficiency of a photovoltaic system drops as its operating temperature rises and PV cooling is necessary. The photovoltaic/thermal (PV/T) system is a relatively recent type of solar collector where a circulating fluid of lower temperature than PV module extracts heat from it, cooling the module to improve its output power while the solar pre-heated fluid provides sensible heat. In the present work, air cooling of a commercial PV module configured as PV/T air solar collector by natural flow is presented, where two low cost modification techniques to enhance heat transfer to air stream in the air channel are studied. The considered methods consist of thin metal sheet suspended at the middle or fins attached to the back wall of the air-channel to improve heat extraction from the module. A numerical model was developed and validated against the experimental data obtained from outdoor test campaigns for both glazed and unglazed PV/T prototype models studied. The validation results show good agreement between predicted values and measured data and thus could be used to study analytically the performance of these PV/T air collectors with respect to several design and operating parameters. The modified systems present better performance than the usual type and will contribute to better performance of integrated PV systems for natural ventilation applications in buildings, both space cooling and heating.     

FUTURE RESEARCH ACTIONS IN PASSIVE COOLING

The PASCOOL program was the most important European project on passive cooling of buildings. The project addressed topics included solar control, the combined effect of ventilation and thermal mass, thermal comfort during summer and the potential of natural cooling techniques. PASCOOL put in evidence also the axes towards which future research on passive cooling should be oriented. This research, giving the continuously increasing trend of energy consumption for cooling purposes, is absolutely necessary in order to take advantage of the complete potential that passive cooling can offer to buildings while maintaining the living standards, health and comfort of the occupants. This paper presents these future issues that comprise (a) research on the microclimatic scale in order to address the impact of outdoor environment on the cooling load of buildings, (b) investigation of comfort requirements under transient conditions during summer, (c) research on natural ventilation in urban environments and the impact of outdoor pollution on indoor air quality, (d) development of alternative cooling systems and techniques, (e) development of integrating design concepts optimising the use of solar heating, passive cooling and natural light in buildings.     

Review of passive systems and passive strategies for natural heating and cooling of buildings in Libya

By proper passive design concepts which essentially consist of collection, storage, distribution, and control of thermal energy flow, an energy saving of 2.35% of the world energy output is possible. The basic methods of heating and cooling of buildings are solar radiation, outgoing longwave radiation, water evaporation, and nocturnal radiation cooling. A Trombe-Michel wall consists of a large concrete mass, exposed to sunlight through large, south-facing windows; it is used for heating buildings. Solar absorption cooling and solar dehumidification and evaporative cooling are two approaches that utilize solar energy for the generation of the working fluid and the cooling of dwellings. Outgoing longwave radiation is the most practical way of cooling buildings in desert climates and is effective on roof surfaces, emitting the radiations from the surface of earth to the atmosphere and to outer space. Water evaporation in desert coolers is the usual method of cooling in arid regions. Nocturnal radiation both heats in winter and cools in summer, in suitable climates, and does so with no nonrenewable energy other than a negligible amount required to move the insulation twice a day. The study of 24 different locations in Libya divides the country into regions with distinct passive strategies. The northern region and the Mediterranean coast need passive heating. The buildings in this region should restrict conductive heat flow, prevent infiltration and promote solar heat gains. The southern region, a part of the Sahara desert, needs passive cooling. The buildings in this region need high thermal mass and should promote natural ventilation, restrict solar heat gains and encourage evaporative and radiant cooling. The difficulties encountered in passive solar design are the large exposed area required with suitable orientation for the collection of energy and the large space requirement for the storage of thermal energy. This paper reviews these passive systems and discusses suitable strategies to be adopted for Libya.     

Energy and economic assessment of desiccant cooling systems coupled with single glazed air and hybrid PV/thermal solar collectors for applications in hot and humid climate

This paper presents a detailed analysis of the energy and economic performance of desiccant cooling systems (DEC) equipped with both single glazed standard air and hybrid photovoltaic/thermal (PV/t) collectors for applications in hot and humid climates. The use of ‘solar cogeneration’ by means of PV/t hybrid collectors enables the simultaneous production of electricity and heat, which can be directly used by desiccant air handling units, thereby making it possible to achieve very energy savings. The present work shows the results of detailed simulations conducted for a set of desiccant cooling systems operating without any heat storage.System performance was investigated through hourly simulations for different systems and load combinations. Three configurations of DEC systems were considered: standard DEC, DEC with an integrated heat pump and DEC with an enthalpy wheel. Two kinds of building occupations were considered: office and lecture room. Moreover, three configurations of solar-assisted air handling units (AHU) equipped with desiccant wheels were considered and compared with standard AHUs, focusing on achievable primary energy savings.The relationship between the solar collector’s area and the specific primary energy consumption for different system configurations and building occupation patterns is described. For both occupation patterns, sensitivity analysis on system performance was performed for different solar collector areas. Also, this work presents an economic assessment of the systems. The cost of conserved energy and the payback time were calculated, with and without public incentives for solar cooling systems. It is worth noting that the use of photovoltaics, and thus the exploitation of related available incentives in many European countries, could positively influence the spread of solar air cooling technologies (SAC). An outcome of this work is that SAC systems equipped with PV/t collectors are shown to have better performance in terms of primary energy saving than conventional systems fed by vapour compression chillers and coupled with PV cells.All SAC systems present good figures for primary energy consumption. The best performances are seen in systems with integrated heat pumps and small solar collector areas. The economics of these SAC systems at current equipment costs and energy prices are acceptable. They become more interesting in the case of public incentives of up to 30% of the investment cost (Simple Payback Time from 5 to 10 years) and doubled energy prices.     

Quantifying the potential of automated dynamic solar shading in office buildings through integrated simulations of energy and daylight

The façade design is and should be considered a central issue in the design of energy-efficient buildings. That is why dynamic façade components are increasingly used to adapt to both internal and external impacts, and to cope with a reduction in energy consumption and an increase in occupant comfort. To gain a complete picture of any façade’s performance and subsequently carry out a reasonable benchmarking of various façade alternatives, the total energy consumption and indoor environment need to be considered simultaneously. We quantified the potential of dynamic solar shading façade components by using integrated simulations that took energy demand, the indoor air quality, the amount of daylight available, and visual comfort into consideration. Three types of façades were investigated (without solar shading, with fixed solar shading, and with dynamic solar shading), and we simulated them with various window heights and orientations. Their performance was evaluated on the basis of the building’s total energy demand, its energy demand for heating, cooling and lighting, and also its daylight factors. Simulation results comparing the three façade alternatives show potential for significant energy reduction, but greater differences and conflicting tendencies were revealed when the energy needed for heating, cooling and artificial lighting were considered separately. Moreover, the use of dynamic solar shading dramatically improved the amount of daylight available compared to fixed solar shading, which emphasises the need for dynamic and integrated simulations early in the design process to facilitate informed design decisions about the façade.     

DAYLIGHTING TECHNOLOGIES IN NON-DOMESTIC BUILDINGS

Natural daylight is an inexpensive and very efficient light source provided that the amount of daylight entering a building is controlled according to demand. In commercial buildings electricity for lighting can be cut by 50-75% using daylighting design techniques in combination with efficient artificial lighting. New lighting control technologies and advanced computer simulation tools lo optimize large buildings makes it possible to exploit these energy savings.

Daylight is a very efficient light source, providing more light for less input of thermal energy than any other artificial light source. Efficient shading systems are now emerging systems that can control the admission of daylight to the room according to requirement, and avoid overheating. Some of these systems, such as reflective light shelves, will also contribute to a better distribution of the daylight available by redistributing some daylight to the back of the room. However, more work is needed to develop and test such combined daylight and shading systems.

The use of daylight to reduce electric lighting must be seen as an integrated part of the overall energy optimization of the building. An efficient control of the use of daylight and artificial lighting will not only reduce electricity use for lighting. Additionally, the use of electricity for ventilation and cooling can be reduced also, because the internal heat gains provocating these electricity uses are reduced. This calls for an integrated design approach to the overall energy design of the building, involving the architect and the engineer from the very beginning of the design phase.

Visual comfort of office buildings receives increasing attention, partially because of the VDU's (Visual Display Units) of the computers, that are now almost standard equipment of every work place. The performance requirements for both daylighting systems and artificial lighting systems have been sharpened, and the attention to this fact is crucial in future development of lighting systems.     

Design of a solar collector for year-round climatization

Solar heating systems in buildings have increasingly been studied in the past two decades. In several applications the primary energy demand is now for both heating and cooling, and modern solar collectors should be designed to provide climatization during the whole year. Solar systems are seldom applied in Europe, and large buildings, such as office buildings and schools, continue to be built with mechanical ventilation systems.The study presented in this paper is part of a European XVII Thermie project entitled “Pilot project for photovoltaic, energetic and biohousing retrieval in a school”, whose aim was to install a photovoltaic plant and solar air collectors coupled with a sun breaker structure at a scientific high school in Umbertide, in central Italy.This paper describes the research and development activities concerning a solar air collector suited for winter heating and summer ventilation, which was installed at the high school. The collector physical and numerical modelling of heat transfer and fluid flow in winter operation is presented. The system performance has been estimated as a function of different parameters in order to provide a tool for the design process. Furthermore, the climate in the area has been simulated through the available experimental data, and the system behavior under these conditions is presented.The collectors were installed at the scientific high school in Umbertide in spring 2001. Summer ventilation cooling is under testing and an experimental test period is foreseen next winter to validate the design of the collectors and their performance.     

Energy and economic analysis of an integrated solar absorption cooling and heating system in different building types and climates

The use of solar energy in buildings is an important contribution for the reduction of fossil fuel consumption and harmful emissions to the environment. Solar thermal cooling systems are still in their infancy regarding practical applications, although the technology is sufficiently developed for a number of years. In many cases, their application has been conditioned by the lack of integration between cooling and heating systems. This study aims to evaluate the potential of integrated solar absorption cooling and heating systems for building applications. The TRNSYS software tool was used as a basis for assessment. Different building types were considered: residential, office and hotel. The TRNSYS models are able to run for a whole year (365 days), according to control rules (self-deciding whether to operate in heating or cooling modes), and with the possibility of combining cooling, heating and DHW applications. Three different locations and climates were considered: Berlin (Germany), Lisbon (Portugal), and Rome (Italy). Both energy and economic results are presented for all cases. The different local costs for energy (gas, electricity and water) were taken into account. Savings in CO₂ emissions were also assessed. An optimization of solar collector size and other system parameters was also analysed.     

A review for absorbtion and adsorbtion solar cooling systems in China

In the past decades, solar water collectors were installed for the main purpose of preheating domestic hot water or to cover a fraction of the space heating demand in China. However, solar cooling systems were constructed just for demonstration purposes. Since the building of the first solar-powered absorption cooling system in Shenzhen in 1987, there have been over 10 additional solar cooling demonstration projects constructed. In this paper, the most representative five projects including both absorption and adsorption cooling systems are introduced and summarized. From the demonstrations, solar absorption cooling systems have been shown to be more suitable for large building air-conditioning systems. Comparatively, solar adsorption cooling systems are more promising for small size air-conditioning systems. In order to attain high utilization ratio, it is highly recommended to design solar-powered integrated energy systems in public buildings. In addition, highly efficient heat pumps are considered as the most appropriate auxiliary heat sources for solar cooling systems, for the purpose of all-weather operation. In the 11th Five year research project (duration 2006–2010), solar cooling technologies will be further investigated to achieve a breaking through in the integration of solar cooling systems with buildings.     

Cooling-energy reduction in air-conditioned offices by using night ventilation

Night ventilation has been applied successfully to many passively-cooled or low-energy office buildings. This paper investigates the applicability of night ventilation in air-conditioned office buildings. A thermal and ventilation simulation model, together with suitable weather data were used to examine the potential for energy savings and/or improved internal comfort conditions by applying night ventilation cooling. It was found that natural ventilation strategies could save cooling energy in typical air-conditioned offices. However, the use of mechanical ventilation could lead to increased energy-consumption. If typical offices are modified to incorporate features assisting the application of night ventilation, then cooling energy could be saved when mechanical ventilation is used and further reduced in the case of natural ventilation. Such features would include exposed thermal mass or offices designed to ‘best practice’ guidelines, such as airtight construction and minimisation of internal and solar heat gains.     

THE APPLICATIONS OF RENEWABLE ENERGY TECHNOLOGIES IN BUILDINGS

Considerable opportunities exist, most particularly (though by no means exclusively) in the design of new buildings, to integrate renewable energy technologies and thus reduce or even replace conventional energy sources. These technologies can include passive and active solar heating and cooling, and photovoltaic power. Such strategies to improve the energetic performance of buildings and to reduce their environmental impact will always be complemented by conservation and energy efficiency measures.

Passive heating, natural cooling and daylighting represent a spectrum of strategies whose applicability is modified by region and building type, and whose contribution varies from the modest fraction by which most buildings already benefit, to that in well-designed new buildings where the solar energy contribution may represent more than half (and in suitable cases, when combined with other solar techniques, all) of the energy conventionally required to provide comfortable thermal and visual environments. Active solar heating systems have achieved considerable maturity, and developed commercial markets already exist in a number of countries. And building-integrated photovoltaic systems promise to make significant impact within the coming decade.

This paper outlines R&D programmes undertaken within the European Communities to improve our understanding of the science and engineering of these technologies, and to advance the design and industrial adaptation necessary to bring about widespread implementation of renewable energy technologies in European buildings.     

Review of windcatcher technologies

Mechanical cooling systems in buildings are the main producers of carbon dioxide emissions, which have negative impacts on environment and amplify global warming, particularly in hot climate. Due to the lack of energy supply, windcatchers can be utilized as a sustainable attempt for cooling and ventilation purposes. The objective of this paper is to review and provide a comprehensive literature on windcatcher system for space cooling and ventilation. The concepts were discussed according to the relevant parameters of windcatcher, i.e. windcatcher attributes, windcatcher configurations and windcatcher technologies. The pros and cons of this green architectural feature have also been highlighted and the future research need in this realm of study is proposed.     

Simulation of façade and envelope design options for a new institutional building

This paper presents a simulation case study of façade and envelope preliminary design options for the new Engineering building of Concordia University in Montreal. A major principle of the analysis was to create a high quality building envelope in order to optimally control solar gains, reduce heating and cooling energy demand and reduce electricity consumption for lighting, while at the same time maintain a comfortable and pleasant indoor environment. The stated approach of the design team was to aim for an energy-efficient building, employing innovative technologies and integrating concepts such as daylighting and natural ventilation. Detailed energy simulations were therefore performed from the early design stage, in order to present recommendations on the choice of façade, glazings, shading devices, lighting control options, and natural ventilation. Integrated thermal studies, a daylighting analysis and the impact of the above on HVAC system sizing were considered. Simulation results showed that, using an optimum combination of glazings, shading devices and controllable electric lighting systems, the energy savings in perimeter spaces can be substantial. Perimeter heating could be eliminated if a high performance envelope is used. The building is currently being commissioned.     

Feasibility study on applications of solar chimney and earth tube systems for BEAM/LEED assessment

Building Environment Assessment Method (BEAM) and Leadership in Energy and Environmental Design (LEED) aim to promote better environment performance of buildings in their life time. This study explores innovative solutions to achieve key requirement of Energy Use (EU) and Indoor Environmental Quality (IEQ) from BEAM/LEED assessments by Solar Chimney (SC) or Earth Tube (ET). EnergyPlus is used to perform the simulation of building's ventilation and energy usage under the typical Hong Kong's weather data. It was found that the SC performance is affected by the building's dimension and orientation and is also determined by solar availability and absorber surface temperature. In most simulation cases, SC provided sufficient natural ventilation, but it also increased cooling load to the space when ambient temperature was higher than indoor. The ET performance was affected by the pipe dimension and buried depth. It was found that a single ET could decrease cooling load to the space in the summer design day ‘July 21’ in Hong Kong, but it could not provide sufficient outdoor fresh air. The result also demonstrated that both SC and ET systems had capabilities to help achieve on‐site renewable energy requirements from BEAM/LEED because of their energy saving capacities on ventilation and thermal comfort. Because SC had higher capability to provide sufficient ventilation, SC could more significantly contribute on the sections pertinent to building ventilation in BEAM/LEED assessments. Because ET had higher capability to save cooling energy, ET could more significantly contribute on the sections pertinent to energy saving in BEAM/LEED assessments. Copyright © 2016 John Wiley & Sons, Ltd.     

Multi-objective optimization of integrated solar absorption cooling and heating systems for medium-sized office buildings

Integrated solar absorption cooling and heating (SACH) systems, which use solar energy to provide space heating, space cooling, and water heating, represent a promising substitute to reduce the earth's carbon emissions. SACH systems currently are designed based on engineering experience for the most part and few systematic methodologies are available to identify the key optimal parameters for SACH systems, such as the slope of the solar collectors, the area of the solar collectors, and the volume of the storage tanks. As a result, the established systems usually are not capable of yielding the greatest returns on investment. Motivated by the above facts, this study investigates a formal method for SACH system optimization by incorporating simultaneously a system's performance related to its economic, energy, and environmental aspects. The proposed method includes central composite design, regression, and multi-objective optimization. Central composite design (CCD) is used to select the significant experimental data generated by energy system simulation and life cycle analysis. Linear regression models are used to predict the functional relationship between system performance and the key system parameters using data sets. A multi-objective optimization model is then formulated and solved based on the Weighted-Tchebycheff metric approach. The proposed approach is applied to medium-sized office buildings located in Phoenix, Los Angeles, Atlanta, and Chicago; and the results suggest that the approach can provide a systematic mechanism to optimally design SACH systems.     

Solar absorption cooling in Spain: Perspectives and outcomes from the simulation of recent installations

In spite of the fact that Spain is one of the EU countries with the highest solar resource on annual basis, the huge seasonal variation in solar radiation availability and the relatively short period with heating demand, make it difficult to reach significant contributions of solar energy to the buildings heating energy demand. This compromises the economic viability of big solar collector areas per capita, and introduces technical difficulties for the dissipation of the excess solar energy available in the summer months. On the other hand, in a large part of the Spanish territory, in other to reach adequate comfort conditions in our buildings, the energy demand for cooling is more important or of the same order than the heating demand. Cooling energy demand is now experiencing a fast growing rate as this comfort requirement becomes internalized. Domestic air conditioning equipments based on vapour compression cycles are being used to reach comfort conditions in some of the rooms of buildings that were designed without taking into account cooling requirements. In spite of their so far small contribution to the total building sector energy demand, these equipments are already imposing important constraints on the environment and the electricity distribution system. Solar absorption cooling arises as an interesting alternative, which at the same time allows reaching a higher solar contribution to the heating demand. However, solar cooling installations present several peculiarities with respect to the more known DHW or even heating installations, which require to incorporate a more detailed approach and additional considerations in the design and performance evaluation processes. Besides, some limitations still persist in solar absorption systems, which could make them loose their market potential for the benefit of other solar cooling options. In this paper, we present some conclusions arising from the experience gained in detailed TRNSYS dynamical simulation of some of the first commercial solar heating and cooling installations recently implemented in Spain, and analyse their perspectives in comparison with other solar cooling options.     

Performance of a coupled cooling system with earth-to-air heat exchanger and solar chimney

Buildings represent nearly 40 percent of total energy use in the U.S. and about 50 percent of this energy is used for heating, ventilating, and cooling the space. Conventional heating and cooling systems are having a great impact on security of energy supply and greenhouse gas emissions. Unlike conventional approach, this paper investigates an innovative passive air conditioning system coupling earth-to-air heat exchangers (EAHEs) with solar collector enhanced solar chimneys. By simultaneously utilizing geothermal and solar energy, the system can achieve great energy savings within the building sector and reduce the peak electrical demand in the summer. Experiments were conducted in a test facility in summer to evaluate the performance of such a system. During the test period, the solar chimney drove up to 0.28 m³/s (1000 m³/h) outdoor air into the space. The EAHE provided a maximum 3308 W total cooling capacity during the day time. As a 100 percent outdoor air system, the coupled system maximum cooling capacity was 2582 W that almost covered the building design cooling load. The cooling capacities reached their peak during the day time when the solar radiation intensity was strong. The results show that the coupled system can maintain the indoor thermal environmental comfort conditions at a favorable range that complies with ASHRAE standard for thermal comfort. The findings in this research provide the foundation for design and application of the coupled system.