Indoor thermal environment and energy saving for urban residential buildings in China

The purpose of this survey is to investigate the actual conditions of the residential indoor thermal environment in urban areas in China for evaluating thermal comfort and predicting the energy conservation feasibility for space heating and cooling.The apartment homes under investigation were located in the urban areas of nine major cities. The questionnaire survey revealed building characteristics, the types of space heating and cooling system in use, aspects of life style, during winter and summer seasons, and so on. The measurement showed that winter indoor temperatures in Harbin, Urumqi, Beijing and Xi’an remain at a relatively stable level near 20 °C due to the central heating system installed. However in the other cities lacking central heating systems, indoor temperatures fluctuated as a function of the change of outdoor temperature. On the other hand, summer indoor evening temperatures in Shanghai, Changsha, Chongqing and Hong Kong were higher than the comfort zone of ASHRAE. Therefore it is expected that energy use for space heating and cooling in the southern China will increase in the near future because of occupants’ requirement for comfortable indoor environment. Based on the results yielded by this study, in Beijing the calculation of space heating and cooling loads indicated that the energy used to heat indoor spaces can be halved by installing thermal insulation and properly sealing the building.     

Indoor thermal environment of urban residential buildings in China: winter investigation in five major cities

The purposes of this investigation are to look into the actual conditions of urban residential indoor environment in China during the winter season, and to discuss the thermal comfort as well as to understand the possibility of space heating energy conservation. Investigations of indoor thermal environment were carried out in Harbin, Beijing, Xi’an, Shanghai and Hong Kong of China. The results showed that the indoor thermal condition in heating usage zone is good, such as Harbin, Beijing and Xi’an. The indoor thermal comfort is strongly affected by the outdoor climate in non-heating usage zone, such as Shanghai and Hong Kong.     

Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions

The impact from using cool roof coatings on the cooling and heating loads and the indoor thermal comfort conditions of residential buildings for various climatic conditions is estimated. The energy cooling loads and peak cooling demands are estimated for different values of roof solar reflectance and roof U-value. The results show that increasing the roof solar reflectance reduces cooling loads by 18–93% and peak cooling demand in air-conditioned buildings by 11–27%. The indoor thermal comfort conditions were improved by decreasing the hours of discomfort by 9–100% and the maximum temperatures in non air-conditioned residential buildings by 1.2–3.3 °C. These reductions were found to be more important for poorly or non-insulated buildings. For the locations studied, the heating penalty (0.2–17 kWh/m² year) was less important than the cooling load reduction (9–48 kWh/m² year). The application of cool roof coatings is an effective, minimal cost and easy to use technique that contributes to the energy efficiency and the thermal comfort of buildings.     

New concepts and approach for developing energy efficient buildings: Ideal specific heat for building internal thermal mass

The shortcomings or limitations of the traditional approach to developing energy efficient buildings are that they can not determine: (1) the ideal thermophysical properties of building envelope material, where “ideal” means that such material can use ambient air temperature variation and/or solar radiation efficiently to keep the indoor air temperature in the thermal comfort range with no additional space heating or cooling; (2) the best natural ventilation strategy; (3) the minimal additional energy consumption for space heating in winter or air-conditioning in summer. To overcome these problems, some new concepts for developing energy efficient buildings are put forward in this paper. They are the ideal thermophysical properties of the building envelope material, the ideal natural ventilation rate, and a minimal additional space heating or cooling energy consumption. A new approach for determining these properties is also developed. In contrast to the traditional approach (the thermophysical properties of building envelope material are known and constant so that the relating equations describing the indoor air temperature tend to be linear differential equations), the new approach solves the inverse problem (thermophysical properties, etc. of a buildings are unknown), whose solution can be a function instead of a value. As a first step, the ideal specific heat of the building envelope material for internal thermal mass is analyzed for buildings located in various cities in different climatic regions of China, such as Beijing, Shanghai, Harbin, Urumchi, Lhasa, Kunming and Guangzhou. We found that the ideal specific heat is composed of a basic value and an excessive one which is of δ function for the cases studied. Some limitations that would need further study are introduced in the end of the paper.     

Impact of climate change on residential building envelope cooling loads in subtropical climates

Future trends of cooling load due to heat gain through the building envelopes in the residential sector in subtropical Hong Kong under different emissions scenarios in the 21st century were investigated. Predicted monthly weather data from five general circulation models were gathered and analysed. An increasing trend of building envelope cooling load was observed. The average annual cooling load during the 2009-2100 period would be 6.1% and 9.8% more than that during 1979-2008 for low and medium forcing, respectively. If only the last 30 years (2071-2100) were considered, the percentage increase would be much larger at 12.3% and 21.6%. Four mitigation or energy conservation measures - raising the indoor temperature, thermal insulation, double glazing and tinted glass - were considered. Among them, raising the indoor temperature has the best mitigation potential because there is a growing awareness and recognition of adaptive thermal comfort and it can be readily applied to both existing and new buildings at no extra cost.     

Building signatures: A holistic approach to the evaluation of heating and cooling concepts

A sustainable and environmentally responsible building concept aims at a high workplace comfort, a significantly reduced heating and cooling demand, a high-efficient plant system, and the use of renewable energy sources to condition the built environment. This paper presents a comprehensive analysis of the heating and cooling concepts of 11 low-energy buildings in terms of energy use, efficiency and occupant thermal comfort. All buildings investigated employ environmental energy sources and sinks – such as the ground, ground water, rainwater and the ambient air – in combination with thermo-active building systems. A limited primary energy use of about 100 kWh_prim/(m²_neta) as a target for the complete building service technology (HVAC and lighting) was postulated for all buildings presented. With respect to this premise, a comprehensive long-term monitoring in high time resolution was carried out for two to five years, with an accompanying commissioning of the building performance. Measurements include the useful heating and cooling energy use, auxiliary energy use for the hydraulic system, as well as end and primary energy use, occupant thermal comfort and local meteorological conditions. A new methodology is proposed for a holistic approach to the evaluation of heating and cooling concepts, which not only considers the occupants thermal comfort, but also the useful energy consumption and the efficiency of the generation, distribution and delivery of heating and cooling energy.     

An intelligent approach to assessing the effect of building occupancy on building cooling load prediction

Building cooling load prediction is one of the key factors in the success of energy-saving measures. Many computational models available in the industry have been developed from either forward or inverse modeling approaches. However, these models usually require extensive computer resources and lengthy computation. This paper discusses the use of the multi-layer perceptron (MLP) model, one of the artificial neural network (ANN) models widely adopted in engineering applications, to estimate the cooling load of a building. The training samples used include weather data obtained from the Hong Kong Observatory and building-related data acquired from an existing prestigious commercial building in Hong Kong that houses a mega complex and operates 24 h a day. The paper also discusses the practical difficulties encountered in acquiring building-related data. In contrast to other studies that use ANN models to predict building cooling load, this paper includes the building occupancy rate as one of the input parameters used to determine building cooling load. The results demonstrate that the building occupancy rate plays a critical role in building cooling load prediction and significantly improves predictive accuracy.     

Raising evaporative cooling potentials using combined cooled ceiling and MPCM slurry storage

Evaporative cooling is able to generate the cooling medium at a temperature approaching to the ambient wet bulb temperature. In this paper, a low-energy air-conditioning strategy is proposed, which is a combination of cooled ceiling (CC), microencapsulated phase change material (MPCM) slurry storage and evaporative cooling technologies. The assessment of evaporative cooling availability and utilization is done for five representative climatic cities, including Hong Kong, Shanghai, Beijing, Lanzhou and Urumqi in China, and the energy saving potential of the proposed air-conditioning system is analyzed by using a well validated building simulation code. The results indicate that the new system offers energy saving potential up to 80% under northwestern Chinese climate and up to 10% under southeastern Chinese climate. The optimal design method of the slurry storage tank is also proposed based on the slurry cooling storage behaviors and cooling demand variations of the ceiling panels.     

Optimum insulation thicknesses for building walls with respect to cooling and heating degree-hours in the warmest zone of Turkey

Thermal insulation is one of the most effective energy conservation measures for cooling and heating in buildings. Therefore, determining and selecting the optimum thickness of insulation is the main subject of many engineering investigations. In this study, the determination of optimum insulation thickness on external walls of buildings is comparatively analyzed based on annual heating and cooling loads. The transmission loads, calculated by using measured long-term meteorological data for selected cities, are fed into an economic model (P₁−P₂ method) in order to determine the optimum insulation thickness. The degree-hours method that is the simplest and most intuitive way of estimating the annual energy consumption of a building is used in this study. The results show that the use of insulation in building walls with respect to cooling degree-hours is more significant for energy savings compared to heating degree-hours in Turkey's warmest zone. The optimum insulation thickness varies between 3.2 and 3.8 cm; the energy savings varies between 8.47 and 12.19 $/m²; and the payback period varies between 3.39 and 3.81 years depending on the cooling degree-hours. On the other hand, for heating load, insulation thickness varies between 1.6 and 2.7 cm, energy savings varies between 2.2 and 6.6 $/m², and payback periods vary between 4.15 and 5.47 years.     

Performance evaluation of a PV ventilated window applying to office building of Hong Kong

PV ventilated glazing technology for application in warm climate provides energy saving opportunities through the reduction in air-conditioning load, the daylight utilization, and the green electric power generation. In a working environment, the use of semi-transparent a-Si glazing is deemed better than the one with non-transparent c-Si solar cells. This paper reports an evaluation of its integrated performance using a small office room in Hong Kong as an example. An energy model of a PV ventilated window system is first introduced. Based on this together with the TMY weather data of Hong Kong and the daylight simulation capability of the EnergyPlus program, the overall performance analysis have been executed for different window orientations. It was found that a solar cell transmittance in the range of 0.45–0.55 could achieve the best electricity saving.     

Application of advanced glazing to high-rise residential buildings in Hong Kong

The energy saving that can be achieved by applying advanced glazing to a typical high-rise residential building in Hong Kong was evaluated using the simulation software EnergyPlus. It was found that application of low-e glazing would lead to a reduction in cooling electricity use by up to 4.2%. The saving due to application of low-e reversible glazing would be up to 1.9%; double-clear glazing up to 3.7%; and clear plus low-e glazing up to 6.6%. The achievable saving would depend on orientation of building wings, and type and location of rooms. The analysis suggests that for the high-rise public housing blocks in Hong Kong, the use of expensive advanced glazing would not be economically viable from the point of view of saving in cooling energy cost.     

Benchmarking Hong Kong and China energy codes for residential buildings

Mandatory energy codes to curb energy use of residential buildings have been formally launched in China for more than two decades but little has been publicized in literature. Similar codes are not available for residential buildings in Hong Kong, but most residential buildings in Hong Kong, especially public housing estates, are HK-BEAM certified to demonstrate their compliance with regulatory and basic design requirements. Given HK-BEAM is internationally recognized and there are doubts about the effectiveness of the China codes, how the energy efficiency of the HK-BEAM certified buildings compare with buildings in compliance with the China codes is of interest to most building designers and policy makers. This paper describes how the energy efficiency of a case study building in compliance with the China codes compare with the one in compliance with HK-BEAM. The energy simulation by HTB2 and BECRES reveal that the case study building in compliance with the China codes is 51.1% better in energy use. In the study, the relative impact of each compliance criterion on energy use and cooling load has been quantified by sensitivity analysis. The sensitivity values indicate that energy use is most sensitive to air-conditioning operation hours, indoor design temperature, coefficient of performance (COP) of the room air-conditioners (RAC) units, and the envelop characteristics. The results of this study indicate that a HK-BEAM certified building cannot satisfy the China codes requirements. This provides good reference to the policy makers, the building owners, and to the China and Hong Kong Governments when considering reciprocal recognition of building energy codes.     

Selection of micro turbines to meet electrical and thermal energy needs of residential buildings in Iran

Micro gas turbines are considered to meet the electrical, domestic hot water, heating and cooling energy needs of a residential building located in Tehran, Ahvaz and Hamedan. The building is 10 stories high and has a total of 8000 m² floor area with the peak demands of electricity of 32.96 kW, DHW of 0.926 kW, heating load of 1590 kW and the cooling load of 2028 kW, when the building is located in Tehran. With these demands, 30 micro turbines of 30 kW (nominal power) are needed to meet all the energy needs of the building. The excess electricity generated by the micro turbines is to be used in a heat pump, and the energy in the exhaust gases is to be used to meet other thermal energy needs of the building. With proper energy conservation measures and the use of ceiling fans in each room, the peak heating and cooling demands of the building were reduced to 225 kW and 760 kW, respectively. With these measures, two micro gas turbines of 30 kW nominal capacity, or one of 40 kW, could meet all the electrical, DHW, heating and a great portion of the cooling needs of the building. The remaining cooling needs of the building during the hot hours of summer could be met by an additional absorption refrigeration, utilizing natural gas as its energy source. It is recommended that with energy conservation measures, the heating and cooling loads of buildings be reduced as much as possible, and micro gas turbines be employed to meet the electrical demands and a portion of heating and cooling needs. The remaining thermal energy needs are to be met through the use of natural gas. Only with these measures, the on-site combined heat and power (OS-CHP) is a viable option for residential buildings in Iran.     

Solar energy systems installed on Chinese-style buildings

A building-integrated solar energy system is proposed, with the panels installed such that the overall morphology resembles that of a traditional Chinese building, i.e., roofing (eaves) at each storey, in addition to that on top of the building. The panels include photovoltaic cells and solar thermal collectors, thus producing electric power as well as heating. The particular morphology provides a number of advantages, in terms of solar energy collection and shading, and their matching to temporal and locational variations in energy demand. These are in addition to the advantages of solar energy generally. Solar heating and photovoltaic power generation were calculated for a number of locations. These were compared with the space heating and air conditioning demands, respectively. The requirement for supplementary energy was calculated. Equivalent calculations for similar buildings without solar panels allowed the saving in non-solar energy to be estimated. Calculations were made for Beijing in winter, as an example of high space heating demand, for Hong Kong in summer, as an example of high air conditioning demand, and for Shanghai, as an intermediate example. These showed potential savings of up to 15% in space heating, and up to 55% in air conditioning energy demand.     

Application of switchable glazing to high-rise residential buildings in Hong Kong

For typical high-rise residential building in Hong Kong, we evaluated the impact of using switchable glazing on energy use for space cooling. By using software EnergyPlus, it was found that application of switchable glazing would lead to a reduction in annual cooling electricity consumption by up to 6.6% where the actual amount depends on existence of overhangs, orientation of building wings, types and locations of rooms. Energy saving by this magnitude, however, cannot justify use of switchable glazing in residential buildings in Hong Kong, mainly because of the high glazing cost.     

Performance analysis on a hybrid air-conditioning system of a green building

This paper presents the performance analysis on a hybrid air-conditioning system according to the hybrid building energy system of the green building demonstration project in Shanghai, in which a 150 m² solar collector is used to power two 10 kW adsorption chillers, a vapor compression heat pump is used to cool air in the evaporating end while the condensing heating at about 80 °C is fully used to regenerate a liquid desiccant dehumidification system. In the hybrid system, the sensible cooling to the air is treated mainly by solar adsorption cooling and vapor compression cooling, whereas the latent heat is treated by the liquid desiccant dehumidification system with regeneration from the condensing heat of the heat pump. The results show that the performance of this system is 44.5% higher than conventional vapor compression system at a latent load of 30% and this improving can be achieved by 73.8% at a 42% latent load. The optimal ratio of adsorption refrigerating power to total cooling load for this kind of hybrid systems is also studied in this paper.     

Greenhouse gas emissions in building construction: A case study of One Peking in Hong Kong

The construction of buildings has a very important impact on the environment, and the process of manufacturing and transporting of building materials, and installing and constructing of buildings consumes great energy and emits large quantity of greenhouse gas (GHG). The present paper defines four sources of GHG emissions in building construction, which are: manufacture and transportation of building materials; energy consumption of construction equipment; energy consumption for processing resources; and disposal of construction waste, and then establishes the calculation method of GHG emissions. This paper presents a case study of GHG emissions in building construction in Hong Kong. The results show that 82–87% of the total GHG emissions are from the embodied GHG emissions of building materials, 6–8% are from the transportation of building materials, and 6–9% are due to the energy consumption of construction equipment. The results also indicate that embodied GHG emissions of concrete and reinforced steel account for 94–95% of those of all building materials, and thus the use of recycled building materials, especially reinforced steel, would decrease the GHG emissions by a considerable amount.     

Impacts of the shading-type building-integrated photovoltaic claddings on electricity generation and cooling load component through shaded windows

The shading-type building-integrated photovoltaic (BIPV) claddings can act as power generators as well as external shading devices of a building, which reduce the energy consumption of the building. However, there is little information about energy impacts of different tilt angles of the shading-type BIPV claddings. By considering the typical meteorological conditions of Hong Kong, the energy performance of the shading-type BIPV claddings, in terms of the electricity generation and the cooling load reduction, is analyzed in this paper. The optimum tilt angle of PV modules for maximum electricity generation is found to be 20° instead of local latitude. Combining electricity generation and cooling load reduction, it can be concluded that the optimum tilt angles for the first type of the shading-type BIPV claddings vary from 30° to 50°, while the optimum tilt angle for the second type is 0°.     

An integrated model for the design of air-cooled chiller plants for commercial buildings

Cooling load calculation is the first step in designing the air-conditioning system of a building. The calculated cooling capacity with appropriate buffer is then used to select the number and size of chillers in the system. N + 1 is a common formula used by designers to size the chiller plants in Hong Kong buildings, where N is the actual number of chillers required and 1 is a redundant chiller provided to ensure reliability. This paper reviews the problem of excess capacity and discusses the risk exposure of chiller systems without redundant chillers. The cooling load profiles of the chiller plants of four medium-sized commercial buildings in Hong Kong are reviewed. The risk exposure of chiller systems without redundant chillers can be minimized by applying risk-based preventive maintenance. The just-in-demand design reduces capital cost of the building and frees up funds for continuous energy measurement and improving the energy efficiency of chiller plant systems. This paper presents a model for designing chiller plants that improves the energy efficiency of the plant in a cost effective and thoughtful manner. It is designed with consideration of the life cycle of the plant and real-time continuous commissioning, monitoring, measurement, comparison and execution for better energy management.     

Impact of adaptive thermal comfort criteria on building energy use and cooling equipment size using a multi-objective optimization scheme

Recently adaptive thermal-comfort criteria have been introduced in the international indoor-climate standards to reduce the heating/cooling energy requirements. In 2008, the Finnish Society of Indoor Air Quality (FiSIAQ) developed the national adaptive thermal-comfort criteria of Finland. The current study evaluates the impact of the Finnish Criteria on energy performance in an office building. Two fully mechanically air-conditioned single offices are taken as representative zones. A simulation-based optimization scheme (a combination of IDA-ICE 4.0 and a multi-objective genetic-algorithm from MATLAB-2008a) is employed to determine the minimum primary energy use and the minimum room cooling-equipment size required for different thermal comfort levels. The applicability of implementing energy-saving measures such as night ventilation, night set-back temperature, day lighting as well as optimal building envelope and optimal HVAC settings are addressed by investigating 24 design variables. The results show that, on average, an additional 10 kWh/(m² a) primary energy demand and a larger 10 W/m² room cooling-equipment size are required to improve the thermal comfort from medium (S2) to high-quality (S1) class; higher thermal comfort levels limit the use of night ventilation and water radiator night-set back options. Compared with the ISO EN 7730-2005 standard, the Finnish criterion could slightly decrease the heating/cooling equipment size. However, it significantly increases both the heating and cooling energy demand; the results show 32.8% increase in the primary energy demand. It is concluded that the Finnish criterion-2008 is strict and does not allow for energy-efficient solutions in standard office buildings.