Theoretical and experimental analysis of the thermal behaviour of a green roof system installed in two residential buildings in Athens,Greece

Measurements of the thermal behaviour of two residential buildings equipped with a green roof system have been performed in Athens, Greece. Experimental data have been used to calibrate detailed simulation tools and the specific energy and environmental performance of the planted roofs system has been estimated in detail. Simulations have been performed for free‐floating and thermostatically controlled conditions. The expected energy benefits as well as the possible improvements of the indoor thermal comfort have been assessed. It is found that green roofs have a limited contribution to the heating demand of insulated buildings operating under the Mediterranean climate. On the contrary, the green roof system is found to contribute highly to reduce the cooling load of thermostatically controlled buildings. For the considered residential buildings, a cooling load decrease of about 11% has been calculated. In parallel, it is found that green roofs contribute to improve thermal comfort in free‐floating buildings during the summer period. The expected maximum decrease of the indoor air and roof surface temperatures is close to 0.6°C. Such a decrease contributes to reduce by 0.1 the summer absolute Predicted Mean Vote Comfort Index levels in the building. Copyright © 2009 John Wiley & Sons, Ltd.     

An analysis of future building energy use in subtropical Hong Kong

Principal component analysis of prevailing weather conditions in subtropical Hong Kong was conducted, and a new climatic index Z (as a function of the dry-bulb temperature, wet-bulb temperature and global solar radiation) determined for past (1979–2008, measurements made at local meteorological station) and future (2009–2100, predictions from general circulation models) years. Multi-year (1979–2008) building energy simulations were carried out for a generic office building. It was found that Z exhibited monthly and seasonal variations similar to the simulated cooling/heating loads and building energy use. Regression models were developed to correlate the simulated monthly building cooling loads and total energy use with the corresponding Z. Error analysis indicated that annual building energy use from the regression models were very close to the simulated values; the difference was about 1%. Difference in individual monthly cooling load and energy use, however, could be up to 4%. It was also found that both the DOE-simulated results during 1979–2008 and the regression-predicted data during 2009–2100 indicated an increasing trend in annual cooling load and energy use and a gradual reduction in the already insignificant heating requirement in cooling-dominated office buildings in subtropical climates.     

The effects of changes in the climate on the energy demands of buildings

It is generally accepted that climate changes will have a major effect on our lives. However, buildings will also be faced with climate changes, and these changes will have an impact on indoor comfort, energy demands and the efficiency of building services, especially on those supporting free cooling and free heating. In order to predict the expected changes in a building's thermal response during its lifetime, it is necessary to look at the climate changes predicted for the future. In our study, the climate changes were considered by using simplified mathematical models combined with available test reference years to establish ‘corrected test reference years’. A transient simulation tool, TRNSYS, was used to simulate the indoor climate and the useful energy demand for the heating and cooling of different buildings with different free‐cooling techniques. In order to predict the expected changes in a building's thermal response, the meteorological parameters for the moderate, continental climate region of Slovenia were taken into account. The study shows that during a building's lifetime, significant changes in useful energy demands can be expected—a decrease in the useful energy demand for heating of between 23 and 40% and an up‐to‐38‐times increase in the useful energy needed for mechanical cooling. In buildings without mechanical cooling, the efficiency of the different free‐cooling techniques should be increased by between 100 and 200% to ensure the same living comfort. The results presented in the study confirm that it is necessary to evaluate the consequences of global climate changes from the point of view of energy use in buildings, their construction and the buildings' service installations. Copyright © 2008 John Wiley & Sons, Ltd.     

Analysis of energy saving optimization of campus buildings based on energy simulation

The energy consumption of campus buildings has specific characteristics, because of the concentrated distribution of people’s working time and locations that change in line with distinct seasonal features. The traditional energy system design and operation for campus buildings is only based on the constant room temperature, such as 25°C in summer and 18°C in winter in China, not taking into consideration the real heating or cooling load characteristics of campus buildings with different functions during the whole day and whole year, which usually results in a lot of energy waste. This paper proposes to set different set-point temperatures in different operation stages of public and residential campus buildings to reduce the heating and cooling design load for energy station and total campus energy consumption for annual operation. Taking a campus under construction in Tianjin, China as an example, two kinds of single building models were established as the typical public building and residential building models on the campus. Besides, the models were simulated at both set-point room temperature and constant room temperature respectively. The comparison of the simulation results showed that the single building energy saving method of the peak load clipping could be used for further analysis of the annual energy consumption of campus building groups. The results proved that the strategy of set-point temperature optimization could efficiently reduce the design load and energy consumption of campus building groups.     

夏季大空间屋顶节能喷淋器的实验研究

针对大空间建筑模型进行喷淋效果实验,通过实验得到以下结论:由于Low-E玻璃材料在夏季具有较强的温室效应,未喷淋时,屋顶内表面平均温度为56.8℃,与环境平均温差为21.9℃,增加了空调的能耗。彩钢板屋顶具有较好的保温性能,未喷淋时,屋顶内表面平均温度为40.6℃,模型内部空间平均温度为38.9℃,该温度远低于Low-E玻璃模型温度,能耗比Low-E玻璃模型小。喷淋时,Low-E内层温度比外层温度低约5.4℃;彩钢板内外层温度在喷淋时两者相差不大,仅为0.5℃,喷淋对Low-E玻璃屋顶空调节能意义明显。     

Cooling load reduction in office buildings of hot-arid climate,combining phase change materials and night purge ventilation

Night purge ventilation is a well-known passive technique for conserving cooling energy by storing night coolth in the thermal mass of the building fabric. We study the effect of phase change materials (PCM) as a light thermal mass, on the cooling load of a typical office building with HVAC system and night purge ventilation in hot-arid climate. In this paper the proper conditions to start night ventilation and the ventilation rate by fans is determined. Additionally, the effect of melting point temperature of the PCMs on the cooling load of the building is investigated. PCMs with proper melting temperature were applied to various building elements, and to the whole model. It was revealed that, application of PCMs will significantly contribute in reducing the cooling load, except for the floor on the ground, which resulted in an increase of the load.     

Design parameters and control strategies for a combined passive heating and cooling system in Louisville,KY

ABSTRACT

Conventional passive solar systems can significantly reduce a building's heating load. However, the integration of passive heating and cooling systems in the same building and the benefits of actively controlling passive systems has largely been unexplored. The objective of this study was to determine the relative performance of a passive solar heating and sky cooling system operating with a range of control strategies, with the goal of minimising the overall annual energy use for space conditioning. A combined system (CS) and a separate system (SS) were simulated with thermal networks using MATLAB, with weather data for Louisville, KY. The control strategies simulated included: Seasonal, Ambient, Room and Matrix. The highest fraction of energy supplied by ambient sources for the SS was 0.707 with Matrix control, while for the CS, the highest fraction (0.704) was with Matrix temperature control with switchable attributes for heating and cooling.     

System performance and economic analysis of solar-assisted cooling/heating system

The long-term system simulation and economic analysis of solar-assisted cooling/heating system (SACH-2) was carried out in order to find an economical design. The solar heat driven ejector cooling system (ECS) is used to provide part of the cooling load to reduce the energy consumption of the air conditioner installed as the base-load cooler. A standard SACH-2 system for cooling load 3.5 kW (1 RT) and daily cooling time 10 h is used for case study. The cooling performance is assumed only in summer seasons from May to October. In winter season from November to April, only heat is supplied. Two installation locations (Taipei and Tainan) were examined.It was found from the cooling performance simulation that in order to save 50% energy of the air conditioner, the required solar collector area is 40 m² in Taipei and 31 m² in Tainan, for COP_j = 0.2. If the solar collector area is designed as 20 m², the solar ejector cooling system will supply about 17–26% cooling load in Taipei in summer season and about 21–27% cooling load in Tainan. Simulation for long-term performance including cooling in summer (May–October) and hot water supply in winter (November–April) was carried out to determine the monthly-average energy savings. The corresponding daily hot water supply (with 40 °C temperature rise of water) for 20 m² solar collector area is 616–858 L/day in Tainan and 304–533 L/day in Taipei.The economic analysis shows that the payback time of SACH-2 decreases with increasing cooling capacity. The payback time is 4.8 years in Tainan and 6.2 years in Taipei when the cooling capacity >10 RT. If the ECS is treated as an additional device used as a protective equipment to avoid overheating of solar collectors and to convert the excess solar heat in summer into cooling to reduce the energy consumption of air conditioner, the payback time is less than 3 years for cooling capacity larger than 3 RT.     

The benefits of combining utility-controlled demand response with residential zoned cooling

This paper evaluates the effectiveness of combining direct load control with a residential zoned-cooling technology in meeting the objectives of reducing peak demand and maintaining home comfort level. In contrast, the traditional approach has been for utilities to smooth summer peak cooling loads, by controlling the cooling load of the whole house. While accounting for weather, dwelling characteristics and demographics, with detailed field data, we are able to develop empirical models to evaluate the benefits of utility control of cooling loads for a residential zoned cooling system during summer peak-demand periods and to compare with non-zoned systems. A zoned house allows for an upper floor cooling interruption without affecting the comfort on the main floor. An upper floor interruption for a full 4 h during the day leads to an average peak air conditioning change of ?0.52 kW, approximately 1.6 times the reduction from the curtailment of cooling by cycling the air conditioning serving the whole house.     

Theoretical study on the performance of an integrated ground-source heat pump system in a whole year

Being environmental friendly and with the potential of energy-efficiency, ground-source heat pump (GSHP) systems are widely used. However, in southern China, there exists large difference between cooling load in summer and heating load in winter. Thus the increase of soil temperature gradually year-by-year will decrease the COP of the GSHP system. In this paper, the configuration of a vertical dual-function geothermal heat exchanger (GHE) used in an integrated soil cold storage and ground-source heat pump (ISCS&GSHP) system, which charged cold energy to the soil at night and produced chilled water at daytime in summer, and supplied hot water for heating in winter, is presented. This is then followed by reporting the development of the mathematical model for the GHE considering the impact of the coupled heat conduction and groundwater advection on the heat transfer between the GHE and its surrounding soil. The GHE model developed was then integrated with a water-source heat pump and a building energy simulation program together for a whole ISCS&GSHP system. Then the operation performance of the ISCS&GSHP system used for a demonstration building is studied. These simulation results indicated the system transferred 71.505% of the original power consumption at daytime to that at nighttime for the demonstration building. And the net energy exchange in the soil after one-year operation was only 2.28% of the total cold energy charged. Thus we can see the feasibility of the ISCS&GSHP system technically.     

Numerical assessing energy performance for building envelopes with phase change material

In hot climate, phase change material (PCM) can be incorporated into building envelopes to reduce heat gain through the building envelopes and therefore reduce its cooling demand. In this study, the energy performance of building envelopes integrated with PCM has been explored using a popular dynamic building performance simulation package, EnergyPlus, and the energy saving mechanism of PCM was investigated. The simulation results reflected that PCM could effectively help to reduce the building's annual energy consumption by 20.9% for Guangzhou, China. In addition, for the Guangzhou city, 27°C transition temperature, smaller thermal conductivity of roof, and higher amount of PCM can all help to improve the building's energy performance. Additionally, it is suggested that in real building development/retrofit projects, the selection of PCM needs to be based on both their thermal properties and the local climatic conditions of the building.     

Cooling potential of ventilated PV façade and solar air heaters combined with a desiccant cooling machine

Thermal energy collected from a PV-solar air heating system is being used to provide cooling for the Mataro Library, near Barcelona. The system is designed to utilise surplus heat available from the ventilated PV facade and PV shed elements during the summer season to provide building cooling. A desiccant cooling machine was installed on the library roof with an additional solar air collector and connected to the existing ventilated PV façade and PV sheds. The desiccant cooling cycle is a novel open heat driven system that can be used to condition the air supplied to the building interior. Cooling power is supplied to the room space within the building by evaporative cooling of the fresh air supply, and the solar heat from the PV-solar air heating system provides the necessary regeneration air temperature for the desiccant machine. This paper describes the system and gives the main technical details. The cooling performance of the solar powered desiccant cooling system is evaluated by the detailed modelling of the complete cooling process. It is shown that air temperature level of the PV-solar air heating system of 70 °C or more can be efficiently used to regenerate the sorption wheel in the desiccant cooling machine. A solar fraction of 75% can be achieved by such an innovative system and the average COP of the cooling machine over the summer season is approximate 0.518.     

Performance analysis & energy benefits of a desiccant based solar assisted trigeneration system in a building

In this paper, performance details and operational benefits of a large scale solar trigeneration system that provides for solar assisted desiccant cooling, heating and hot water generation installed in a teaching institute building have been reported. A two-rotor desiccant system designed for handling 12 000 m3/hr of air was integrated into existing plant to provide a net reduction in energy consumption over the pre-existing heating ventilation and air-conditioning and domestic hot water systems. The system is controlled and monitored by a building management system which has been used to investigate and analyse the typical system behaviour. Heat from solar energy contributed consistently to reduce gas usage for water heating and on an annual basis showed a reduction of 21% of consumed energy. The solar energy contribution for space heating varied over winter months and during some months it was observed to contribute more than 50% of the total energy requirements for space heating. Under suitable ambient conditions, approximately 35% of total building cooling load was met by the solar driven desiccant cooling system. Continuous monitoring has also helped understand some of the operational issues of the system.     

Energy performance of building envelopes in different climate zones in China

Energy performance of office building envelope designs in the five major climate zones – severe cold, cold, hot summer and cold winter, mild, and hot summer and warm winter – in China was investigated. A major city within each climate zone was selected. These were Harbin, Beijing, Shanghai, Kunming and Hong Kong. Generic building envelopes were developed based on data gathered from building surveys, local energy codes and the ASHRAE Standard. The overall thermal transfer value (OTTV) method and the heating degree-days technique were adopted in the analysis. Cooling and heating requirements due to heat gain/loss through the building envelopes were determined based on the respective OTTV parameters and building load coefficients. Different shape coefficients were also considered. For a typical floor, chiller load due to heat gain through the building envelope varied from 1.0 kW h/m² in Kunming to 23.5 kW h/m² in Beijing, and the heating load ranged from 2.7 kW h/m² in Hong Kong to 124.3 kW h/m² in Harbin.     

Green roofs as passive system for energy savings in buildings during the cooling period: use of rubber crumbs as drainage layer

The building sector is responsible for most of today’s energy and materials consumption. Construction systems such as green roofs can improve the energy performance of buildings, but meanwhile, they themselves should be more sustainable. This research focuses on the study of the benefits, in terms of energy consumption, of an extensive green roof (without insulation layer) in comparison to a conventional flat roof solution (with insulation) under Mediterranean continental climate. Moreover, in order to improve the sustainability of this system, the use of recycled rubber instead of traditional stone materials as drainage material is evaluated as well. For this purpose, the electrical energy consumption of the cooling system and thermal behaviour of three identical experimental cubicles, with only differences on the roof composition, was evaluated during summer period. The results show that a simple extensive green roof 9 cm thickness provides a reduction of 5 % in case of rubber crumbs and 14 % in case of pozzolana, in terms of electrical energy consumption, than a conventional flat roof with an insulation layer of 3 cm polyurethane, even when only the 20 % of the surface is covered by plants. Furthermore, small differences in thermal behaviour were observed when replacing volcanic gravel by recycled rubber crumbs as drainage material.     

Performance of antisolar insulated roof system

Rooms with concrete slab roofs directly exposed to the sun become unbearably hot during summer and very cold during winter. Huge amounts of energy are required to keep them comfortable. Application of thermal insulation on roofs significantly reduces energy required for heating and cooling. The effectiveness of roof insulations may be further enhanced if a layer of antisolar coating is applied on top of the insulation. The antisolar coating reflects most of the incident sunlight and prevents the roof from heating up. This reduces the daily cycles of thermal expansion and contraction which cause cracks in the roof slabs for the rainwater to leak through. The antisolar coating prolongs the useful life of the building structure as well as the life of the insulation that evaporates with heat. The method of application of the antisolar coating has been specially developed to eliminate thermal bridges formed between the edges of the tiles. This report presents the results of an experiment conducted at the Attock Refinery Limited (ARL) Rawalpindi to assess the performance of the antisolar insulated roof system. Record of the room temperature before and after the installation of the system shows a significant reduction in the indoor temperature. The room occupants, who used to experience a very high thermal stress after 10:30 am in spite of the 1.5-ton air conditioner operating in the room, felt much relieved after the installation. They had to turn back the thermostat of the air conditioner and even had to switch it off occasionally. A detailed thermal analysis of the room shows that cost of an antisolar system is paid back in less than a year in the form of savings of energy required for air-conditioning in summer and for gas heating in winter. In addition, the system prevents the addition of 150 kg per year of green house gases to the atmosphere for each square meter of the area covered by the system. It also provides a quieter environment by reducing the operational duration of the air-conditioning and gas heating appliances.     

Computer simulation of solar cooled buildings in Khartoum

The transient state heat transfer formulation has been used to determine the air conditioning cooling load for two selected one room buildings in Khartoum: one made from brickwork and the other from wood. This formulation has been achieved by applying an explicit finite difference numerical techniques and adopting thermal network of electrical analogy to solve the transient conduction heat transfer equations. A computer program was developed to analyse the thermal network and to determine the cooling load for the wall, roof, window, ventilation and the total cooling load for each building for a typical hot summer day in Khartoum.A solar cooling system comprising mainly of a flat plate solar collector, a Lithium Bromide-Water absorption air conditioner, a storage and auxiliary tanks was integrated in the building and the energy equation for each component was obtained. A general simulation program for the solar cooled buildings has been developed and it was found that about 65% of the total cooling load demanded by the brick building could be supplied by energy from the solar collector, the other 35% portion had to be met by the auxiliary tank compared with 70% of the total cooling load demanded by the wood building which could be supplied by energy from the solar collector.     

Recapitulation on latent heat hybrid buildings

In the existent paper, the performance of thermal storage hybrid buildings exploiting the latent heat of phase change materials (PCMs) for thermal refrigeration and heating of the contemporary period has been investigated. The conventional buildings consume a large amount of electricity, primarily for the heating and cooling applications. Electricity generation primarily relies on coal-based thermal power plants. The emissions from these establishments pose a serious threat to the environment. Moreover, conventional heating/cooling units rely on exorbitant energy cost. The usage of any kind of thermal storage system is an efficacious way of stockpiling thermal energy and utilizing it when needed. This paper gives a comprehensive overview of the available thermal storage units incorporating PCMs. The various segments of the buildings, viz, ceiling, window, wall, and floor have been analyzed in details. The results are quite promising in terms of load reduction and overall energy saving. Indoor surface temperature reduction of up to 7^oC has been achieved. The energy saving of up to 40% can be realized by employing PCM. A comprehensive list of the PCMs is also tried to build up for end users according to their temperature requirement.     

Pilot study on building-integrated PV: Technical assessment and economic analysis

Building-integrated photovoltaics (BIPV) is an innovative green solution that incorporated energy generation into the building façade with modification on the building material or architectural structure. It is a clean and reliable solution that conserves the aesthetical value of the architecture and has the potential to enhance the building's energy efficiency. Malaysia's tropical location has a high solar energy potential to be exploited, and BIPV is a very innovative aspect of technology to employ the available energy. Heriot-Watt University Malaysia (HWUM) has a unique roof design that could be utilized as an application of the BIPV system to generate electricity, reducing the carbon footprint of the facility. Eight BIPV systems of different PV technologies and module types and with capacities of 411.8 to 1085.6 kW were proposed for the building. The environmental plugin software has been integrated with a building geometry modelling tool to visualize and estimate the energy potential from the roof surface in a 3D modelling software. Additionally, detailed system simulations are conducted using PVSyst software, where results and performance parameters are analysed. The roof surface is shown to provide great energy potential and studied scenarios generated between 548 and 1451 MWh yearly with PR range from 78% to 85%. C-Si scenarios offer the best economical profitability with payback period of 4.4 to 6.3 years. The recommended scenario has a size of 1085.5 kW and utilizes thin-film CdTe PV modules. The system generates 1415 MWh annually with a performance ratio of 84.9%, which saves 62.8% of the electricity bill and has an estimated cost of 901 000 USD. Installation of the proposed system should preserve the aesthetical value of the building's roof, satisfy BIPV rules, and most importantly, conserves energy, making the building greener.