The effect of variations in convection coefficients on thermal energy storage in buildings Part II — Exterior massive walls and simulations

A theoretical study has been performed to determine the effect of variations in convection coefficients on the storage of thermal energy in structural materials in the exterior envelope of buildings. Detailed analytical and numerical analyses have been performed to study the fundamental aspects of the problem for simple geometries. Based on the detailed analyses, a thermal energy storage effectiveness parameter has been defined in terms of the changes in heating and cooling energy requirements of a single-zone building in response to the introduction of mass in its exterior walls. Calculations of the exterior wall effectiveness have been made to investigate the effect of variations in convection coefficients at the interior surface of external envelope materials, as well as the influence of dditional building parameters, such as internal loads, interior air temperature control strategy, and internal mass.To extent the results of the detailed analysis and to study the effects of variable convection coefficients on heating and cooling energy requirements in real buildings, simulations of two prototype residential buildings (in Mexico and the United States) have been performed using the building energy analysis computer program BLAST². Results indicate that the energy consumption of a typical uninsulated Mexican residence is quite sensitive to the variations in convection coefficients commonly occurring in buildings (a difference up to a factor of three over the range 0.5 ⩽ h ⩽ 10.0 W/m²K). Buildings energy consumption of a typical well-insulated U.S. residence is less sensitive to variations in convection coefficients, although for some climates the effects are still significant (up to a 40% increase over the range 0.5 ⩽ h ⩽ 10.0 W/m²K). Since the convection coefficients at interior building surfaces vary quite widely within this range, this work suggests that for some climates and building constructions, improved characterization of convection coefficients is needed to permit reliable calculation of the energy requirements of buildings incorporating large amounts of thermal mass.     

Energy performance optimization of radiant slab cooling using building simulation and field measurements

Few field studies of energy performance of radiant cooling systems have been undertaken. A recently constructed 17,500 m² building with a multi-floor radiant slab cooling system in the tower was investigated through simulation calibrated with measured building energy use and meteorological data. For the very cold, dry region where the building was located, it was found that a typical floor of the tower would have had 30% lower annual energy use with a conventional variable air volume system than with the as-built radiant cooling-variable air volume combination. This was due to (1) simultaneous heating and cooling by the existing radiant cooling and air systems, (2) the large amount of free cooling possible in this climate, and (3) suboptimal control settings. If these issues were remedied and combined with improved envelope and a dedicated outdoor air system with exhaust air heat recovery, a typical floor could achieve annual energy use 80% lower than a typical floor of the existing building HVAC system. This shows that radiant thermal control can make a significant contribution to energy-efficiency, but only if the building design and operating practices complement the strengths of the radiant system.     

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.     

The impact of indoor thermal conditions,system controls and building types on the building energy demand

It is possible to evaluate the energy demand as well as the parameters related to indoor thermal comfort through building energy simulation tools. Since energy demand for heating and cooling is directly affected by the required level of thermal comfort, the investigation of the mutual relationship between thermal comfort and energy demand (and therefore operating costs) is of the foremost importance both to define the benchmarks for energy service contracts and to calibrate the energy labelling according to European Directive 2002/92/CE. The connection between indoor thermal comfort conditions and energy demand for both heating and cooling has been analyzed in this work with reference to a set of validation tests (office buildings) derived from a European draft standard. Once a range of required acceptable indoor operative temperatures had been fixed in accordance with Fanger's theory (e.g. −0.5 < PMV < −0.5), the effective hourly comfort conditions and the energy consumptions were estimated through dynamic simulations. The same approach was then used to quantify the energy demand when the range of acceptable indoor operative temperatures was fixed in accordance with de Dear's adaptive comfort theory.     

Thermal comfort assessment and application of radiant cooling: A case study

A field assessment of thermal comfort was conducted at Mehran University of Engineering and Technology, situated in the subtropical region of Pakistan. The results show that people of the area were feeling thermally comfortable at effective temperature of 29.85 °C (operative temperature 29.3 °C). A comparison of this neutral effective temperature was made with the neutral effective temperature determined from adaptive models. It is found that the neutral effective temperature determined during this study closely match that of the adaptive model based on either indoor temperature or both indoor and outdoor temperatures. The results of thermal acceptability assessment show that more than 80% of occupants were satisfied at an effective temperature of 32.5 °C, which is 6.5 °C above the upper boundary of ASHRAE thermal comfort zone. Naturally ventilated classrooms and air-conditioned offices of the University were simulated using TRNSYS system simulation program for two cases, once when conventional air-conditioning is used for providing thermal comfort, and when comfort is achieved through radiant cooling. In the simulation, cooling tower was used to regenerate cooling water for the radiant cooling system. Energy consumption was estimated from simulation of both cases. The results show that it is possible to achieve thermal comfort for most of the time of the year through the use of radiant cooling without a risk of condensation of moisture from air on the radiant cooling surfaces. A comparison of the energy consumption estimates show that savings of 80% is possible in case thermal comfort is achieved through radiant cooling instead of conventional air-conditioning.     

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.     

Utilization of wind energy in space heating and cooling with hybrid HVAC systems and heat pumps

Approximately one-third of the primary energy resources are consumed in space heating, cooling, and air-conditioning with a very low exergetic efficiency. The depleting nature of primary energy resources, negative environmental impact of fossil fuels and low exergetic efficiencies obtained in conventional space heating and cooling are the main incentives for developing alternative heating, ventilating, and air-conditioning (HVAC) techniques which can employ low density and interrupted energy sources. In this respect, in spite of difficulties primarily encountered in coupling wind energy with conventional space heating and cooling equipment, wind energy seems to be an exciting alternative provided that synectic combinations are pursued and applied. In this paper, a new wind turbine coupled hybrid HVAC system is presented, which consists of an optimum combination of convective and radiant heating and cooling systems with in-space thermal energy storage. A design case for a single family home is presented. In this study a 6 kW(e) wind turbine drives a ground source heat pump (GSHP) which is coupled to a hybrid HVAC system to satisfy the thermal loads of a 100 m² home. In this example, sensible heating and cooling loads are satisfied by the high mass radiant floor which matches the daily peak demand and the available peak wind energy. Latent heating and cooling loads, along with ventilation requirements are satisfied by a forced-air system. Variable radiant and convective split type of control is implemented, and both systems are served by the same GSHP which also satisfies the domestic hot water (DHW) demand.     

Thermal mass activation by hollow core slab coupled with night ventilation to reduce summer cooling loads

This study deal with the analysis of the effectiveness of free cooling ventilation strategies coupled with thermal mass activation to reduce peak cooling loads.A numerical simulation of the temperatures distribution of an office placed in Milan, Italy, during the month of July, is conducted on a Simulink^® dynamical model.No air-conditioned system is present but two different free cooling systems are analysed and compared. Both systems act a primary ventilation during the day and a night ventilation during the non-occupancy period but the first is a traditional mixing ventilation system, the other is a thermal mass activation system, i.e. the outdoor ventilation air, before entering the room, flows through the ducts of the hollow core concrete ceiling slab.The performances of the two systems are investigated by means of time profile analyses of indoor operative temperatures and by means of frequency temperature distributions during the occupancy period.The cooling performances are measured by two different discomfort indexes: one represents the discomfort time percentage during occupation period, the other the discomfort weighted on the distance of calculated operative temperature from the acceptable temperature interval.This paper, in last analysis, tries to highlight the possibilities on cooling loads reduction and on thermal comfort increase in Mediterranean climate, connected to new strategies for thermal mass activation and night ventilation.     

Elaboration of global quality standards for natural and low energy cooling in French tropical island buildings

Electric load profiles of tropical islands in developed countries are characterized by morning, midday and evening peaks arising from an all year round high power demand in the commercial and residential sectors, due mostly to air-conditioning appliances and bad thermal conception of the building.To face this problem, the work presented in this paper has led to the realization of a global quality standard for energy saving and thermal comfort. This quality standard is obtained through optimized bioclimatic urban planning and architectural design, the use of passive cooling architectural components, natural ventilation and energy efficient systems. The methodology consisted of the study of typical dwellings with the use of a thermal and airflow software (CODYRUN). The simulations were carried out on the constituent components (roof, walls, windows) and on natural ventilation, in such a way as to estimate the influence of some technical solutions (insulation, horizontal shading, window dimension etc . . .) on each component in terms of thermal comfort and energetic performances.Throughout 1996, these technical solutions have been implemented in 280 new pilot dwelling projects.     

A full-scale experimental set-up for assessing the energy performance of radiant wall and active chilled beam for cooling buildings

Full-scale experiments under both steady-state and dynamic conditions have been performed to compare the energy performance of a radiant wall and an active chilled beam. From these experiments, it has been observed that the radiant wall is a more secure and efficient way of removing heat from the test room than the active chilled beam. The energy saving, which can be estimated to around 10%, is due to increased ventilation losses. The asymmetry between air and radiant temperature, the air temperature gradient and the possible short-circuit between inlet and outlet play an equally important role in decreasing the cooling need of the radiant wall compared to the active chilled beam. It has also been observed that the type and repartition of heat load have an influence on the cooling demand. Regarding the comfort level, both terminals met the general requirements, except at high solar heat gains: overheating has been observed due to the absence of solar shading and the limited cooling capacity of the terminals. No local discomfort has been observed although some segments of the thermal manikin were slightly colder.     

The effect of suspended ceilings on energy performance and thermal comfort

The objective of this study is to determine the potential energy savings and thermal comfort benefits of exposing concrete in the ceiling to the indoor air as an alternative to suspended ceiling. The performances were assessed through monitoring of room air and surface temperatures in an office building in operation, and simulation of different scenarios with a calibrated building simulation model. In this study, it is shown that ESP-r is capable of simulating an advanced controlled office building in operation with good agreement with the measurements. The results presented in this paper indicate that exposed concrete in the ceiling both reduces the number of hours with excessive temperatures considerably and create a better and more stable thermal environment during the working day. Also, exposed concrete increases the achievements of utilizing night free cooling significantly. However, by removing the suspended ceiling, only minor annual heating energy savings are achieved.     

Design and performance of solar powered absorption cooling systems in office buildings

The paper contributes to the system design of solar thermal absorption chillers. A full simulation model was developed for absorption cooling systems, combined with a stratified storage tank, steady-state or dynamic collector model and hourly resolved building loads. The model was validated with experimental data from various solar cooling plants.As the absorption chillers can be operated at reduced generator temperatures under partial load conditions, the control strategy has a strong influence on the solar thermal system design and performance. It could be shown that buildings with the same maximum cooling load, but very different load time series, require collector areas varying by more than a factor 2 to achieve the same solar fraction. Depending on control strategy, recooling temperature levels, location and cooling load time series, between 1.7 and 3.6 m² vacuum tube collectors per kW cooling load are required to cover 80% of the cooling load.The cost analysis shows that Southern European locations with higher cooling energy demand lead to significantly lower costs. For long operation hours, cooling costs are around 200 € MWh⁻¹ and about 280 € MWh⁻¹ for buildings with lower internal gains and shorter cooling periods. For a Southern German climate, the costs are more than double.     

Enabling the hybrid use of air conditioning: A prototype on sustainable housing in tropical regions

This paper demonstrates how the use of active or passive means only does not give the appropriate answers to a tropical design when considering housing. The author discusses about the idea of both modes of operation being used simultaneously or in parallel, and how this concept has been developed for one experimental building prototype in tropical areas of Brazil (Northeast region). Quantification is given through extensive parametric simulations which have been conducted using different environmental simulation programmes—TAS (EDSL, UK), ESP-r (ESRU, UK) and photovoltaic (PV)-Design PRO-G (Sandia Labs, USA). Thermal comfort levels along with energy use were assessed and compared, in terms of degree hours of overheating/under heating and cooling energy use. The prototype design has also taken into account the appropriate use of resources through sustainable design features: efficient use of energy, water and materials. The results have demonstrated that for regions such as the warm-humid tropics, the use of a mixed running strategy have optimized energy performance and provided better levels of thermal comfort in a much more effective way. For some cases, cooling energy savings up to 80% were feasible on a hybrid mode, where thermal comfort was improved up to 65%. It has also demonstrated the integration of energy efficiency and a PV grid-connected system, while enabling those daytime electrical needs to be accomplished by the photovoltaic component.     

A case study of ground source direct cooling system integrated with water storage tank system

Energy consumption for the commercial buildings has increasingly gained attentions, due to the significant electricity consumption and peak power demand. To solve this problem, this paper focuses on performance on the Ground Source Direct Cooling (GSDC) system integrated with a Water Storage Tank System (WSTS) in the summer, which directly utilizes the low-grade energy to supply high temperature water for the radiant floor cooling system and make full use of the electric rate difference between on-peak and off-peak periods. In summer, the indoor air temperature is controlled between 23 and 26 °C, resulting in a comfortable thermal environment. The total cooling capacities in 2014 and 2015 were 32.6 kWh/m² and 30.7 kWh/m², respectively. The annual energy consumptions for Electricity Unit Intensity (EUI) in 2014 and 2015 were 33.0 kWh/(m²·yr) and 32.1 kWh/(m²·yr), and the cooling energy consumptions only consumed 4.19 kWh/(m²·yr) and 4.55 kWh /(m²·yr), respectively. The annual operating cost of this cooling system only reaches 9 yuan/(m²·yr) through the analysis of 5 years’ operation. Compared to a conventional air cooled heat pump system, this cooling system has a larger initial cost, but its recovery period is less than 4.3 years, due to the extremely low operating cost. Overall, this GSDC system integrated with WSTS in the summer has remarkable advantages in thermal comfort and energy efficiency.     

An active and passive solar house

The majority of the world's population lives in rural and remote areas in the Third World countries. Many of these areas still do not have regular electric and water supplies. Having the increasing cost of conventional energy in mind, a prototype solar house has been developed in which all the energy requirements of the house are met with solar energy.

Concepts of photovoltaic electric power, thermal heating and passive cooling have been used. All the systems have been functioning satisfactorily. Although the cost of this type of house will be high, such houses can bring considerable improvement in the life-style of rural and remote dwellers in Third World countries.     

Towards global benchmarking for sustainable homes: an international comparison of the energy performance of housing

Over the past 15 years, house building standards across the western world have begun to address ecologically sustainable development (ESD) principles. Amongst the range of environmental sustainability issues arising from housing construction and occupation, the energy demand for heating and/or cooling to maintain thermal comfort has the longest history and is most widespread in policy and regulation. Since energy in our homes is mainly fossil-derived, a key issue is global climate change impacts. Since greenhouse gas emissions can be emitted in various locations across the globe with similar results, it follows that a given greenhouse gas emission arising from residential space heating and cooling has approximately equal impact, irrespective of the location of the building. These emissions are therefore an appropriate candidate for benchmarking internationally, yet there have been few attempts to undertake this activity. This paper reports on a study undertaken in Australia which compares the thermal energy performance of housing in the United States, Canada, UK and Australia. The comparison is based on energy ratings of over 50 house designs from the comparison countries. Each design was assessed as being current and verified as complying with rather than significantly exceeding local regulatory requirements. Issues in design of both the buildings and the modelling tool used are highlighted, and the results are presented. Conclusions are drawn on the reasons for wide variations in thermal energy performance, the implications for benchmarking, and the case for globally consistent housing environmental performance policies and regulation.     

A simple model to study ventilated facades energy performance

The aim of this work is to provide a simple and useful tool to study energy performance of different ventilated facades typology. To simulate the studied sample, a steady state energy balance has been applied to a control volume which basic equations have been solved by a finite element code with an iterative procedure. For each step Δx of the channel height, the different surface and air mass temperatures are calculated. The mass flow rate is evaluated as overall natural draught. Effects due to different cavity widths of the ventilation channel are investigated for winter and summer season with opened and closed ends. Comparisons show that it is possible to obtain a sensible solar cooling effect during summer, when the air cavity width of the chimney is wider than 7 cm. For wider cavities the cooling effect becomes stable. In winter, thermal insulation, provided by the closed ventilated facade is very important.     

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.     

Optimal design and operation of a thermal storage system for a chilled water plant serving pharmaceutical buildings

A group of buildings in the pharmaceutical industry located in Southern Germany is experiencing a trend of growing cooling loads to be met by the chilled water plant composed of 10 chillers of greatly varying cost effectiveness. With a capacity shortfall inevitable, the question arises whether to install an additional chiller or improve the utilization of the existing chillers, in particular those with low operating costs per unit cooling, through the addition of a chilled water thermal energy storage (TES) system. To provide decision support in this matter, an optimization environment was developed and validated that adopts mixed integer programming as the approach to optimizing the chiller dispatch for any load condition, while an overarching dynamic programming based optimization approach optimizes the charge/discharge strategy of the TES system. In this fashion, the chilled water plant optimization is decoupled but embedded in the TES control optimization. The approach was selected to allow for arbitrary constraints and optimization horizons, while ensuring a global optimum to the problem.     

Primary energy and comfort performance of ventilation assisted thermo-active building systems in continental climates

This article presents a simulation study comparing the primary energy and comfort performance of ventilation assisted thermo-active building systems (TABS) relative to a conventional all-air (VAV) system in a compact office building featuring good thermal envelope performance, heat recovery, and solar gain control for the continental climate of Omaha, Nebraska with pronounced heating and cooling periods. TABS heating is accomplished using a geothermal heat pump and TABS cooling using a geothermal heat exchanger without an additional vapor compression cycle required. It was found that the coordination of the TABS and VAV systems is crucial, i.e., supply air temperature and active layer temperature setpoints and reset schedules greatly affect the performance of the overall system. The small contribution of TABS in the heating case shows the need for the adaptation of the ventilation system configuration to the TABS system. Annual cooling energy demand for the ventilation assisted TABS is higher than for the pure VAV system, which is due to lower occupied period room operative temperatures and thus a higher comfort provided. While a 4% useful energy penalty for the combined TABS/VAV was recorded, the VAV case requires 20% more delivered energy than the TABS case because of the displacement of compressor driven coil loads with low-exergy cooling through the ground heat exchanger in the TABS case. A primary energy intensity of 189 kWh/m² a was recorded for the TABS case; in contrast, the conventional all-air (VAV) equipped building incurs a primary energy intensity of 229 kWh/m²a, which represents a penalty of 20%. Clear advantages of the TABS approach can be observed with respect to thermal comfort: during summer cooling periods, the mean radiant temperature of the TABS case is on average 2 K below that of the VAV case. Moreover, the VAV system is associated with a fairly constant predicted mean vote (PMV) value of 0.75, which is quite warm, while the TABS equipped system reveals an average of 0.56, which results in only 12% instead of 17% of people dissatisfied. Based on these results, ventilation assisted thermo-active cooling systems appear to be a very promising alternative to conventional all-air systems offering both significant primary energy as well as thermal comfort advantages provided the TABS is mated with low-exergy heating and cooling sources.