Search

Low temperature district networks (LTNs) supply both low temperature heat and cooling to multiple buildings. Heat input from renewables and waste heat can be fed into LTNs, and they are often connected to long term thermal storage such as borehole fields. LTNs which include new technologies and topologies need to be evaluated in order to enable their further development and application. The objective of this paper is to evaluate the energy performance of a recently constructed district energy system (DES) with a LTN using a multi-energy network optimisation model. The LTN connects buildings to a borehole field which can be regenerated with heat supply from hybrid solar panels and waste heat from building free-cooling. Monitoring data of heat and free-cooling demand was used as an input to the optimisation model and results were compared with monitoring data of electricity consumption of heat pumps and network pumps. Following validation of the initial model, scenarios with electric storage (batteries) and thermal storage were evaluated. Results from evaluation of scenarios provide insights on how the district energy self-sufficiency can be improved through installation of batteries or by improved use of existing thermal storage.

Renewable generation technologies (e.g. photovoltaic panels (PV)) are often installed in buildings and districts with an aim to decrease their carbon emissions and consumption of non-renewable energy. However, due to a mismatch between supply and demand at an hourly but also on a seasonal timescale; a large amount of electricity is exported to the grid rather than used to offset local demand. A solution to this is local storage of electricity for subsequent self-consumption. This could additionally provide districts with new business opportunities, financial stability, flexibility and reliability. In this paper the feasibility of hydrogen based electricity storage for a district is evaluated. The district energy system (DES) includes PV and hybrid photovoltaic panels (PVT). The proposed storage system consists of production of hydrogen using the renewable electricity generated within the district, hydrogen storage, and subsequent use in a fuel cell. Combination of battery storage along with hydrogen conversion and storage is also evaluated. A multi-energy optimization approach is used to model the DES. Results of the model are optimal battery capacity, electrolyzer capacity, hydrogen storage capacity, fuel cell capacity and energy flows through the system. The model is also used to compare different system design configurations. The results of this analysis show that both battery capacity and conversion of electricity to hydrogen enable the district to decrease its carbon emissions by approximately 22% when compared to the reference case with no energy storage.

Effective planning and control of district energy systems must account for numerous complexities and uncertainties, requiring advanced computational tools. This paper introduces the Ehub Modeling Tool, an open source software package for preliminary design optimization of district energy systems. Using automated code generation techniques that directly translate raw data descriptions of a given district into executable optimization code, the tool simplifies and accelerates the process of developing and executing district energy system optimizations, and visualizing/interpreting results. Example applications of the tool in research and education are described.

There is worldwide a trend towards urbanization, and cities try to improve their sustainability by saving natural resources and energy, and to mitigate the impact of climate change. Therefore growing cities need to optimize the energy demand of their new and existing buildings. There are a large number of urban scale phenomena that influence the energy demand of buildings. One of the most important influencing phenomena is the shortwave solar and longwave radiation exchange within an urban environment. It was demonstrated that the influence of neighbouring buildings has to be considered in detail to predict correctly the building energy demand. In this study, space cooling demand of stand-alone buildings is compared with buildings in an urban street canyon configuration. Detailed radiation models for the solar and longwave radiation are used to determine the radiation exchange between the buildings. The results show a strong influence of neighbouring buildings on the space cooling demands. This influence is strongly dependent on the shading device control strategy incorporated in the building energy management. Counterintuitively the space cooling demands are in some cases are higher for low values of solar irradiance, because the shading device control directs the devices to remain open during longer time periods when irradiance values are low, leading to higher total solar gains through the windows.

A hydrogen based cooking stove was developed and tested in respect to efficiency, functionality and hydrogen safety. Therefore the developed novel combustion unit technology is based on the catalytic oxidation of hydrogen on a platinum coated highly porous reticulated silicon carbide foam ceramics. Compared to other works [1], premixing of hydrogen and air is avoided, thus providing the basis for safe and reliable operation. With this catalytic combustion technology, no initial hydrogen ignition is required, implying a high application security. Separate air and hydrogen supplies make regulation of temperature and power efficient, simple and save. Catalytic hydrogen oxidation is performed already at room temperature; whereby at high hydrogen load, a small hydrogen slip is detected under cold start conditions, nevertheless in operation this becomes negligible. In order to overcome this issue, a hydrogen flow regulation, based on the hydrogen combustion temperature, was introduced. A heat exchanger is integrated into the exhaust line, in order to recover heat for the incoming air, required for the catalytic combustion. This leads to a stove efficiency of up to 80%, measured according to the DIN EN 30-2-1: 2005–08 test standard and greatly exceeds the postulated standard minimum efficiency of 35%. For comfortable and save use, the combustion unit is mounted under a glass ceramic coverage, as common for conventional electrical stoves.

In the past decades cities have been continuously growing. The microclimate in urban areas differs significantly from the microclimate in rural areas. The temperatures in urban areas are most of the time higher due to the urban heat island (UHI) effect and the wind speeds are lower due to wind sheltering. The local microclimate strongly influences the space cooling and heating demand of buildings and the human comfort and health in urban areas. The mechanisms causing the UHI effect have to be understood in detail to be able to improve the sustainability of (future) cities and the human comfort in urban areas. Wind driven ventilation is one of the most efficient ways of removing heat from urban areas. Knowledge of the heat fluxes caused by wind driven ventilation is important to be able to optimize the design of urban areas in terms of heat removal. The heat removal by wind and buoyancy can be increased by choosing optimal building geometries. In this study the urban heat fluxes are studied for six generic urban morphologies. Coupled CFD (computational fluid dynamics) and building energy simulations are conducted. This approach has the advantage of having highly spatially resolved temperature and flow fields and therefore being able of determining heat fluxes. Turbulent and convective heat fluxes are considered. The results show the importance of buoyancy for low wind speed cases and the strong influence of buildings upstream on the heat fluxes and temperatures further downstream.

We present a comprehensive review of modelling approaches and associated software tools that address district-level energy systems. Buildings play an important role in urban energy systems regarding both the demand and supply of energy. It is no longer sufficient to simulate building energy use assuming isolation from the microclimate and energy system in which they operate, or to model an urban energy system without consideration of the buildings that it serves. This review complements previous studies by focussing on models that address district-level interactions in energy systems, and by assessing the capabilities of the software tools available alongside the theory of the modelling approaches used. New models and tools that address these district-level interactions are reviewed and their competences assessed. These are divided into the following sections: district energy systems (including heat networks, multi-energy systems and low-temperature networks), renewable energy generation (including solar, bioenergy, wind and the related topic of seasonal storage), and the urban microclimate as it relates to energy demands. The scope and detail covered by twenty cross-disciplinary tools is summarised in a matrix; many other tools that focus on specific areas are also discussed. We end by summarising the current state of district-scale urban energy modelling as it relates to the built environment, along with our perspective on future challenges and research directions.

In the past decades the portion of the population living in urban areas has continuously increased. Due to the high building density, the microclimate in urban areas changes significantly compared to rural areas. The temperatures measured in urban areas are higher compared to the rural temperatures due to the urban heat island (UHI) effect. Furthermore, the longwave and solar radiation exchanges are influenced by shadowing, reflections between buildings and reduced sky view factors. The local urban microclimate has an influence on the energy demand of buildings and on human comfort and health in urban areas. In summer the human comfort in urban areas can decrease due to higher air and surface temperatures and lower wind speeds compared to rural areas. The local urban microclimate is difficult to predict because of the complex interaction of physical phenomena across a large range of time and length scales. Few guidelines exist for architects to mitigate UHI effects or its impacts. The aim of this paper is to model the urban microclimate with CFD and building energy simulations and to investigate in detail the influence of different urban building morphologies on the urban microclimate. This approach and the results of this study can be used to find measures to mitigate the UHI effect. The results show that building facade surface temperatures are mainly influenced by the distance between buildings. For urban morphologies with similar surface temperatures, the air temperatures can still strongly vary due to different wind flow patterns causing different rates of removing heat by wind.

This paper describes a method of integrating decentralized energy systems at neighbourhood scale. The method is based on the energy hub concept, which describes and manages the relation between input and output energy flows and thus can be used to optimise energy consumption. The original energy hub concept is further developed to include decentralized and local energy technologies such as photovoltaics, biomass, or small hydro power, together with district heating systems, building and district conversion and storage technologies at neighbourhood level. Additionally, input from a building simulation tool for evaluating time-dependent buildings energy demand is included in the method. The proposed approach can be used to evaluate and size urban energy systems according to their energy-autonomy, economic and ecological performance. The advantage is that the energy supply systems and local energy storage systems can be evaluated in a combined way at district scale. The suggested method allows to lower peaks in energy demands of neighbourhoods on the electrical grid and to reduce the overall consumption. The developed method is finally applied on a case study for which future energy scenarios of different implementation scale are suggested. The area is located in a village in the mountains and contains 29 buildings. Suggested scenarios include decentralized and local renewable energy sources and district and small heating networks. Results show a variation of 52–98 tons of CO 2 emissions, and an energy autonomy between 64% and 92%. In case of CO 2 emissions, the best solution is given for a combination of multiple supply technologies and small networks which shows 46% lower emissions than for a scenario with only PV and biomass based systems.

This paper aims to integrate decentralized energy systems in a village in Switzerland, which has the goal to phase out fossil fuels and rely on local renewable energy sources. To reach this ambitious target, a revision of the current energy system is required together with retrofitting of the building stock. The method applied is based on the energy hub concept, which can be used to optimize the energy consumption during planning and operation. To apply the energy hub concept at neighbourhood level, a three step approach is required, including modelling the energy demand of the buildings, the evaluation of the local renewable potential, and the management and optimization of demand and supply. Centralized and decentralized local renewable sources are investigated, namely photovoltaics, biomass, or small hydro power. Scenarios are evaluated based on their environmental performance and savings of CO 2 emissions. Results show that an energy sustainability (ratio of energy demand covered by renewables) of 83% and a CO 2 emissions reduction of 86% can be achieved. The results further suggest that up to 50% of the available energy potential from renewables cannot be utilized in this community when insufficient storage is provided in the energy system.

We present new formulations of the ‘energy hub’ model and evaluate their performance. The energy hub model consists of a mixed-integer linear programming problem that balances energy demand and supply between multiple energy carriers by determining the optimal conversion and storage schedule within certain constraints. The new formulations extend the model to account for performance constraints concerning system efficiencies, storage losses and operating limits. Each formulation allows a more accurate representation of real plant performance to be included in the optimisation, giving more accurate optimised schedules and carbon emissions totals. The first major innovation is a means of limiting the number of state changes (startups or shutdowns). This is achieved by specifying a minimum time for which the plant must operate once it is running. The second innovation is the use of stepwise approximations of efficiency curves, thus allowing part-load behaviour to be accurately simulated using a linear model. The third innovation adds a storage loss term that is a percentage of the current amount stored, rather than a fixed value. The new formulations are demonstrated in an example case, where the impact on the optimal schedule is observed. They are also analysed for each week of the heating season, and their impact on the time taken to find the optimal solution is also discussed. Overall changes in the predicted carbon emissions of up to 22% were found, highlighting the importance of accurate plant representation in energy hub models.

Longwave (infrared) radiation is exchanged between exterior building surfaces, the ground and the sky. This process affects exterior surface temperatures and hence building energy consumption. Current approaches to modelling this exchange in building and microclimate analysis tools are discussed. There are various shortcomings, the most significant concerning radiation exchange between building surfaces at different temperatures. This work implements improvements to the longwave exchange processes in the EnergyPlus simulation engine, allowing improved calculation of surface temperatures and thus of heating and cooling loads. The improvements cover ground temperature assumptions and the coupling of other building surfaces. The coupling of radiative transfer was achieved by means of a timelag; it is shown that for small timesteps this approach is adequate. The new approaches are compared to the existing implementation in EnergyPlus for arrangements including a street canyon and shading devices. The combined impact gave an increase in surface temperature of up to 6 °C, with an average change of 2 °C. This caused a decrease in annual heating load of 18% and an increase in cooling load of 19%. This confirms the importance of accurate longwave radiation modelling both for buildings in dense urban areas and for buildings with external shading devices.

Computational fluid dynamics (CFD) is often used to predict flow structures in urban areas for the determination of pollutant dispersion, human comfort or heat fluxes. During daytime building facades and ground surfaces are heated by solar radiation and thereby induce buoyancy, which changes the flow field around buildings significantly. The CFD models used to simulate buoyant flow fields in urban areas are not sufficiently validated. This study aims to validate CFD simulations for buoyant flows in urban street canyons by comparison with wind tunnel measurements. 2D steady RANS (Reynolds-Averaged Navier–Stokes) CFD simulations were conducted with different near-wall treatments. Velocity, turbulent kinetic energy and temperature profiles from CFD were compared with the measured flow fields (measurement technique: particle image velocimetry). Isothermal cases as well as cases with leeward wall or windward wall heating or all surfaces heated were considered. The results show that CFD can predict the general flow structures and the influence of buoyancy. The detailed flow field inside the street canyon is strongly dependent on the flow structure within the shear layer at the top of the street canyon. Therefore to get accurate results for the flow profiles inside the street canyon, the flow within the shear layer has to be predicted correctly.

Application limitations for solar heating are overcome by greatly improving heat storage. Adequate heat storage is achieved by: reducing time dependent thermal losses, reducing storage volume, and allowing easy adjustment of storage geometries. To this purpose much theoretical and practical work has been done at Empa, including a laboratory scale proof of concept of a closed sorption heat storage. This work has provided valuable insight into the potential as well as the challenges and limitations of this technology for the application as heat storage. The closed sorption heat storage concept is based on a continuous, but not full cycle, liquid state absorption heat pump. Heat is not directly stored; instead the potential to regain heat at a desired temperature from a low temperature thermal input is stored. The significant benefit in this is undoubtedly the time independent energy losses. Losses are encountered in the conversion processes during charging as well as discharging but not during storage time. For this reason there is great potential in the application of the closed sorption heat storage as a long term solar heat storage. In the scope of the EU funded project COMTES a prototype system based on the working pair sodium hydroxide and water is under construction. The system is dimensioned to cover space heating as well as domestic hot water in a single family house built to passive energy standards and stationed in Zurich. The two major challenges in the system design are: keeping the system volume favorable and keeping the parasitic electric energy consumption at a minimum.

The built environment represents a major share of global energy consumption. To effectively reduce the energy consumption of urban conglomerations, concepts to sufficiently integrate and manage energy from renewables are necessary. In this paper the energy-hub concept will be applied, which describes the relation between input and output energy flows and can be used to optimize the energy consumption during planning and operation. The concept will be used to evaluate a number of future energy scenarios for a village in Switzerland which has the goal of eliminating the consumption of fossil fuels. As a starting point the existing situation concerning the energy demand of the village with respect to different uses, the different energy carriers, their origin, their distribution and networks is captured and analyzed. In the next step the potential for different means of decentralized energy production is evaluated. Decentralized energy production includes building integrated or local renewable energy production by photovoltaics, biomass, or small hydro power. In the third step, different future energy scenarios for an energy sustainable community are defined. These different scenarios are distinguished by their scale of implementation. Finally an energy hub model of the village is developed and used to evaluate the different energy scenarios., Energy Procedia, 57, ISSN:1876-6102, 2013 ISES Solar World Congress

Closed sorption heat storage opens up a way to achieve seasonal thermal storage without loss during storage. Thermal energy is not stored as sensible heat or latent heat, but by the separation of substances. In this manner it is possible to store heat, or better said, the potential to regain heat. Such a system functioning as an absorption heat pump with intermediate storage is well suited for long term thermal energy storage. Nevertheless, the conversion losses in both the regenerating and heating process make it inferior to sensible thermal storage for short storage cycles. For this reason a hybrid system including a water tank as sensible thermal storage to cover the thermal demands of several days, and a closed sorption heat storage to cover longer periods of insufficient solar thermal input is proposed. The storage system energy density is directly proportional to the sorbate mass difference between regenerated and diluted sorbent. In order to reach high utilization, a large dilution in the sorbent must be reached during heating mode. Thus, the operating temperature parameters must be adjusted and regulated accordingly. In the scope of the EU funded project COMTES, a prototype system based on the working pair sodium hydroxide and water is under construction. The system is dimensioned to cover space heating as well as domestic hot water in a single family house in Zurich, built to passive energy standards. The prototype is starting operation in spring 2014 whereby the system is regenerated to be ready to cover the heating demands the following winter.

At day-times building facades and ground surfaces are heated by solar radiation. Due to the increased surface temperatures, buoyancy is induced which changes the flow field around buildings significantly. Wind tunnel measurements were conducted to study the influence of buoyancy on the flow in a scaled urban street canyon with heated surfaces. Particle image velocimetry was used to measure the flow field in a section of the street canyon. The two wall and the bottom surfaces of the street canyon were heated either individually or all together. A wide range of Froude numbers between 0.65 and 17.3 was covered with surfaces temperatures raised up to 70 °C–130 °C and freestream velocities between 0.68 m/s and 2.32 m/s. The velocity and turbulent kinetic energy (TKE) fields were analysed, and for some cases also the air temperatures inside the street canyon were measured. For most cases one main vortex is formed in the centre of the street canyon. This main vortex is strengthened, and the TKE inside the street canyon increased by, heating of (in order of importance) the ground, the leeward wall, and all three surfaces for low freestream velocities. For windward wall heating a second counter-rotating vortex is formed due to buoyancy and the flow direction close to the windward wall changes from a downward to an upward motion. The centres of the main and secondary vortex change their position for different windward wall temperatures with increasing freestream velocities. For low Froude numbers the air exchange rate is increased due to buoyancy.

An important part of the world's energy is used for space cooling and heating of buildings. Its minimization has great energy saving potential. An important part of the heat exchange between buildings and the ambient surrounding is due to convective and radiative heat flows. In this study detailed building energy simulation (BES) is used to analyse the effect of neighbouring buildings on these heat flows and their influence on the space cooling and heating demand of buildings. BESs were conducted for stand-alone buildings and buildings in street canyon. This study demonstrates the importance of accounting for the urban microclimate for the prediction of the energy demand of buildings. With the proposed model most of the thermal effects of the urban microclimate can be captured and quantified on street canyon scale. Due to multiple reflections more solar and thermal radiation is absorbed at the facades of buildings in street canyons than at facades of stand-alone buildings. These effects cause higher surface temperatures in street canyons leading to higher space cooling and lower space heating demands. Other reasons are the lower convective heat transfer coefficients in street canyons, the reduced removal of heat from street canyons and the urban heat island effect.

An important part of the world’s energy is used for space cooling of buildings. Minimisation of space cooling, especially in hot urban environments, has great energy-saving potential. An important part of the heat exchange between buildings and the ambient surrounding is due to convective and radiative heat flows. The impact of these heat flows on energy consumption for cooling is much more important in urban areas compared to rural areas. This study aims at quantifying the influence of the urban radiation balance, the urban heat island effect and urban convective heat transfer coefficients (CHTC) on the space cooling demands. CHTC correlations were determined using computational fluid dynamics (CFD) for different geometries including stand-alone buildings and street canyons of different lengths. Buoyancy was accounted for by considering differences between building surface and surrounding air temperature. It was found that the building geometry has a large impact on the CHTC correlations and that the effect of buoyancy cannot be neglected when wind speeds are low. These CHTC correlations were used for Building Energy Simulation (BES) predictions of the space cooling demand. Space cooling demands for a building in a street canyon differ up to a factor of 1.8 depending on the CHTC correlations used. Therefore, for accurate predictions of the space cooling demand, adequate CHTC correlations have to be used adjusted to the actual building configuration.

For computational fluid dynamics (CFD) simulations of large urban areas the air flow near surfaces is normally modelled using Wall Functions (WFs). This study aims at improving the accuracy of WFs, in terms of heat transfer predictions. Turbulent boundary layers at heated building surfaces in a street canyon were analysed with low-Reynolds number modelling (LRNM). Buoyancy was accounted for, due to its importance in street canyons e.g. by solar radiation. Two extreme types of normalized temperature profiles could be identified in the thermal boundary layer dependent on the Richardson (Ri) number. One extreme at low Ri number could be attributed to forced convective flow, which is adequately described by existing standard wall functions (SWFs), and the second extreme at Ri>1 to mixed convective flows, where WFs adapted for non-equilibrium flows (NEWFs) are appropriate. Based on these two extremes, an adaptive temperature wall function (AWF) was derived that varies between the two existing WFs dependent on the local Ri. This AWF can account for the co-occurrence of forced and mixed convective flow regimes at a single surface. CFD simulations of street canyon configurations with SWF showed errors in the wall heat fluxes up to 60%, compared to the LRNM results, and up to 30% for NEWFs. With the proposed AWF, these errors were reduced to less than 10% for the majority of the cases studied over the whole range of Ri numbers. We conclude that the proposed AWF allows for more accurate convective heat transfer analysis in urban CFD studies.

To explore the dynamic characteristics of the SOFC systems and to develop relevant control strategies, a previously developed steady state SOFC model is converted to a dynamic model. The model includes mass, momentum, thermal and electrochemical analysis, as well as the kinetic model of hydrocarbon reactions. Applying two control strategies i.e., cell constant fuel flow rate and constant fuel utilization during the transient time, the model is implemented to analyse the dynamic behaviour of a planar direct internal reforming (DIR) SOFC cell under several step-load changes. Transient response, resulting from an inlet temperature variation, is also investigated. The results show that the relaxation time is strongly related to the thermal behaviour of the cell and the applied control strategy. However, it is almost independent of the load variation magnitude.

This study presents the final results of a series of modelling steps which are undertaken for the performance assessment of the building cogeneration and polygeneration systems using solid oxide fuel cell (SOFC). Based on earlier work, generic SOFC cell stack and system models were developed and employed to analyze different SOFC systems configurations for optimal efficiencies, this SOFC system model is used to derive performance input data for transient whole-building and energy system simulation tools which contain simpler SOFC system models. These steps are shortly summarized here. Then the final step, the evaluation of building integrated co- and polygeneration SOFC systems in terms of primary energy demand and CO2 emissions, employing such tools, is presented here for a polygeneration system with typical heating and cooling loads, and electricity demand profiles, for different SOFC systems, including a comparison to current standard technologies.

By integrating the building structure as thermal energy storage into the building services concept, thermally activated building systems (TABS) have proven to be economically viable for the heating and cooling of buildings. Having already developed an integrated design method and various control concepts in the past, in the present paper the impact of different aspects of TABS regarding the energetic performance of such systems is analyzed. Based on a simulation case study for a typical Central European office building, the following conclusions can be drawn. The energy efficiency of TABS is significantly influenced by the hydronic circuit topology used. With separate zone return pipes energy savings of approximately 15–25 kW h/m2 a, or 20–30% of heating as well as cooling demand, can be achieved, compared to common zone return pipes, where energy losses occur due to mixing of return water. A strong impact on energy efficiency can also be observed for the control strategy. Thus, by intermittent operation of the system using pulse width modulation control (PWM), the electricity demand for the water circulation pumps can be reduced by more than 50% compared to continuous operation. Concerning cold generation for TABS, it is shown that free cooling with a wet cooling tower is most efficient, if the cold source is the outside air. Variants with mechanical chillers exhibit 30–50% higher electricity demands for cold generation and distribution, even though their runtimes are much shorter compared to the cooling tower runtimes. In conclusion, the results show that significant energy savings can be achieved using adapted system topologies and applying appropriate control solutions for TABS.

The Eawag's new headquarters “Forum Chriesbach” is an exemplary illustration of a ‘sustainable’ construction design for office buildings. With a unique combination of architectural and technical elements the building reaches a very low 88 kWh/m 2 overall primary energy consumption, which is significantly lower than the Swiss Passive House standard, Minergie-P. A monitoring and evaluation project shows that the building is heated mainly by using the sun and internal heat gains from lighting, electrical appliances and occupants, resulting in an extremely low space heating demand. Cooling is provided by natural night time ventilation and the earth-coupled air intake, which pre-cools supply air and provides free cooling for computer servers. However, values for embodied energy and electricity consumption remain significant, even with partial on-site electricity production using photovoltaics. TRNSYS computer simulations show the contributions of individual building services to the overall energy balance and indicate that the building is resilient towards changes in parameters such as climate or occupancy density. Measurements confirm comfortable room temperatures below 26 °C, even during an extremely hot summer period, and 20–23 °C in the winter season. An economic analysis reveals additional costs of only 5% compared to a conventionally constructed building and a payback-time of 13 years.

Thermally activated building systems (TABS) integrate the building structure as energy storage, and have proofed to be energy efficient and economic viable for the heating and cooling of buildings. Although TABS are increasingly used, in many cases control has remained an issue to be improved. In this paper, a method is outlined allowing for automated control of TABS in intermittent operation with pulse width modulation (PWM). This method represents one part of a TABS control solution with automatic switching between cooling and heating modes for variable comfort criteria which was published before. A first pulse width modulation control solution is derived based on a simple 1st order model of TABS. Then a second, even simpler solution is given that significantly reduces the tuning effort. Finally, the paper outlines a pulse width modulation control procedure and gives two application examples of the PWM control carried out in a laboratory test room.

Micro-cogeneration, also termed micro combined heat and power (MCHP) or residential cogeneration, is an emerging technology with the potential to provide energy efficiency and environmental benefits by reducing primary energy consumption and associated greenhouse gas emissions. The distributed generation nature of the technology also has the potential to reduce losses due to electrical transmission and distribution inefficiencies and to alleviate utility peak demand problems. Detailed MCHP models for whole-building simulation tools, developed in Annex 42 of the International Energy Agency (IEA) Energy Conservation in Buildings and Community Systems Programme, have been used to conduct a performance assessment study for a number of micro-cogeneration systems and residential buildings. Annual non-renewable primary energy (NRPE) demand and CO 2 -equivalent (CO 2 -eq) emissions were determined by simulation for different cogeneration technologies, namely natural gas-fuelled solid oxide (SOFC) and polymer electrolyte membrane fuel cells, Stirling and internal combustion engines. These were compared to the reference system with a gas boiler and electricity supply from the grid. A ground-coupled heat pump system was also analysed for comparison. The cogeneration units were integrated in single and multi-family houses of different energy standard levels. Two different electricity generation mixes were considered: European mix and combined cycle power plant (CCPP). For the MCHP devices, detailed dynamic component models as well as simplified performance map models were used, developed and calibrated with either results from laboratory experiments or with manufacturer data. The simulations were performed using the whole-building simulation programme TRNSYS, using domestic hot water and electric demand profiles specified in IEA Annex 42. Combinations of three demand levels were analyzed. In NRPE demand, for the European electricity mix, most MCHP systems offered reductions (up to 34%) in comparison with the gas boiler reference system and crediting the electricity exported to the grid. For the CCPP electricity generation mix, the largest NRPE reductions resulted for the ground-coupled heat pump systems (up to 29%). The maximum reduction with a cogeneration system was 14%. In terms of CO 2 -eq emissions, most cogeneration systems offered reductions for the European electricity mix (up to 22%). However, maximum reductions resulted for the heat pump system (23%). For the CCPP mix, maximum reductions by far again resulted for the heat pump systems (up to 29%). The maximum reduction for a cogeneration system was achieved with the ICE system in the single-family house (14%).

Exhaust cowls are used in conjunction with hybrid ventilation systems to efficiently convert wind energy into negative pressure and thus minimize the electrical energy required by the extract fan. Yet the fact that cowl performance is largely dictated by operating conditions imposes particularly stringent demands on modelling. This paper demonstrates, by way of a concrete example, the need for and potential benefits of a new methodological approach to the modelling of cowls. The study focuses on a specific modelling strategy, applied within a building simulation program, for a cowl used in a hybrid ventilation system. The method is progressively simplified to produce four variants, which chiefly vary according to their level of detail and, hence, the associated modelling effort. Wind pressure coefficients at facade, above roof and in the cowl are needed for all model variants. Some of the investigated variants rely on CFD computations of airflow around the building to determine these values. This study uses the example of a single-family house (SFH) to identify those criteria requiring particular attention in the performance of CFD numerical flow analyses. All four variants are examined on the basis of this example to determine which simplifications to the model are appropriate and permissible without unduly compromising the accuracy of the results.

SUMMARY A research project on the control and design of thermally activated building systems (TABS) has been started in May 2004. This paper presents one selected result after three years of work: A comprehensive TABS zone control strategy with a modular concept consisting of maximally four parts which allows creating four different control strategies. The (mandatory) first part of the comprehensive control strategy is an outside air temperature compensated supply water temperature control based on an unknown-but bounded approach to cope with uncertainties and variations in the heat gains during operation. The (optional) second part of the comprehensive control strategy is an algorithm for room temperature feedback control. With this part, room temperatures are controlled in an energyefficient way. The algorithm also corrects wrong parameter settings of the other control parts. The (optional) third part of the comprehensive control strategy is a pulse-width modulation (PWM) module for intermittent operation of the recirculation pump. The intermittent operation is used to save pump energy, but it is also applied to benefit from low-cost energy sources such as free-cooling during the night or electrical energy in low tariff phases. The (mandatory) fourth part of the comprehensive control strategy is a standard sequence controller to control the zone supply water temperature within the according setpoint range. In simulations, all four control strategies are applied to TABS and the simulation results are analyzed regarding comfort criteria and energy efficiency.

This paper presents application range and functionality of thermally activated building systems (tabs). Tabs are increasingly used for energy efficient and economical cooling and heating of commercial buildings. Thereby, the building structure is used as thermal storage allowing the use of renewable energy sources. Based on a simulation study for a typical office building the aspects of thermal comfort, maximum permissible heat gains in the room and the re-cooling of the building mass are analysed. It is shown that depending on the maximum permissible daily room temperature amplitude with tabs typical heat gain profiles with peak loads up to around 50 W/m2 floor area can be managed. However, the transitional (mid-season) periods with already high solar gains and still restricted comfort range, in most cases will be decisive for the dimensioning of tabs, thus limiting maximum loads to lower values. The results also show that processes on the room side are almost unaffected from the processes on the supply side. In the cooling case, this allows for the re-cooling period of the fabric being extended to 24 h a day with accordingly “high” supply temperatures and peak load reductions of up to 50%. The results given may serve as orientation guide whether a tabs may be applicable in a specific building, and provides relevant parameters for the dimensioning of tabs.

The reduction of greenhouse gas emissions in the building sector to a sustainable level will require tremendous efforts to increase both energy efficiency and the share of renewable energies. Apart from the lowering of energy demand through better insulation and fenestration, small combined heat and power (micro-cogeneration) systems may help improve the situation on the supply side by cutting both the non-renewable energy demand for residential buildings and peak loads in the electric grid. Though still on the brink of market entry, fuel cells are the focus of interest as the prime technology for such systems. In this study, a methodology for assessing the performance of such systems in terms of primary energy demand and the CO 2 emissions by transient computer simulations is established, and demonstrated for a natural gas driven solid oxide fuel cell (SOFC) and, to a lesser extend, a polymer electrolyte fuel cell (PEFC) home fuel cell cogeneration system. The systems were evaluated for different grid electricity generation mix types and compared to traditional gas boiler systems. The interaction with hot water storage and solar thermal collectors, and the impact of storage size and predictive control was analyzed. Typical heat and electricity demand load profiles for different types of residential buildings and occupancy were considered, and the sizing of the fuel cell system in relation to the heat demand of the building was analyzed. Primary energy savings decline for cases with lower heat demand and for cases with solar thermal systems, and peak for fuel cell systems sized in accordance with the heat demand of the building. Future assessments of fuel cell systems will need a refined methodology, and depend on realistic performance characteristics and models that accurately consider dynamic conditions.

Buildings complying with the Passive House standard are rapidly spreading across Germany, Austria and Switzerland. The underlying Passive House concept is based on a holistic approach, improving the building envelope to a degree that allows for substantial simplifications of the heating system. Passive Houses offer increased comfort at affordable costs while significantly reducing the energy consumption. The useful energy required for space heating has been reduced by ca. 80% compared with conventional (new) buildings. The overall primary energy consumption (including all services and electric appliances) has been reduced by more than 50%. Our paper introduces the Passive House standard and summarizes results of the EU project ‘Cost Efficient Passive Houses as European Standards’ (CEPHEUS) with respect to energy indices and comfort. Characteristics of the combined ventilation and heating system realized in many Passive Houses are presented in detail, including results of an expert working group at the Passive House Institute and on measurements and simulations conducted at EMPA. Chances and limitations of wood stoves as supplementary heat sources in Passive Houses are discussed shortly.

The European AIRLESS programme was started to identify the pollution sources in air-handling units, to characterise these sources using measurement protocols developed on purpose, to propose measures to avoid or eliminate this pollution, and to assess the effect of such measures on the energy use. This paper addresses the energy use in air-handling units and proposes several energy efficient ways to provide good indoor air and even efficient indoor environment conditioning. Cooling energy may be important in office buildings, even in temperate climate. An efficient and cheap cooling strategy is to combine a mechanical ventilation system designed for the minimum hygienic airflow rate with night passive cooling using natural ventilation.

COMIS simulates multizone air flow and pollutant transport. It has been developed in an international context at Lawrence Berkeley National Laboratory (LBNL) and within International Energy Agency (IEA) Annex 23. At the end of Annex 23, the Swiss Federal Laboratories for Materials Testing and Research (EMPA) took over the co-ordination of further developments, and—together with Centre Scientifique et Technique du Bâtiment (CSTB)—the maintenance of COMIS. In January 2001, the “COMIS v3.1 with IISiBat v2.4” package was released, and is available now from CSTB. In this paper, the main features of the COMIS simulation engine, and the latest improvements are presented. An example demonstrates the use of COMIS. Pollutant transport and impact evaluation with the new version are discussed. The coupling of COMIS with the thermal simulation program TRNSYS is discussed.

Rooms with high windows are likely to have comfort problems caused by cold air down draught, which are usually solved by placing heating appliances underneath the windows. In the city of Zug, Switzerland, a highly insulated educational building with a concrete core system for heating and cooling is planned. The purpose of our investigation was to find out whether any measures are necessary in this building to compensate the effects of down draught in the occupied zone. Special attention has been paid to the effect of passive measures like air flow obstacles or openings in the window sill. Experiments were carried out in a room climate laboratory on 1:1 scale to measure the down draught and the thermal comfort in the occupied zone. Several configurations of window sizes, insulating standards and outside temperatures were investigated. In addition, the results were compared to values obtained from analytical calculations and from literature. The conclusion was that the window frame, not the window pane, is the critical element and that there will be no unacceptable draught risk in the occupied zone, as long as the window frame is also highly insulating. Therefore, no additional measures to compensate the down draught were planned for the building in Zug. Furthermore, it was found that the down draught is not only dependent on the window temperature but also on the heat load in the room, i.e. a heat load increases the boundary layer thickness and decreases its peak velocity.

In Europe, hydronic concrete core systems are being increasingly used for room conditioning systems. The concrete slab acts as heat accumulator and permits dissipation of the load using, for instance, cooling towers. When using such systems the external climatic conditions limit the achievable water temperature. The convective loads, however, directly affect the room air temperature and reduce the storable part of the load. The ventilation system also has a large influence on the energy related operation of such a system. Moreover, the dimensions of the concrete slab and the layout geometry of the water pipes, especially the spacing, are important factors for the design of the system. A model is described which can be used to illustrate the transient two dimensional heat flow in such a construction. This method is suitable for simple hand calculations, but can also be integrated into existing building simulation programs without having to modify the program code. Thus, the complete system can be designed for practical applications to ensure optimum operation. In addition, this paper describes the interrelationship between heat storage capacity and pipe geometry. Finally, criteria are listed for suitable application of concrete slab cooling and further aspects are listed that need to be considered in connection with these systems.

Comfort evaluations cover air quality, thermal, visual and acoustic comfort. Today, only few computer programs allow for the integrated evaluation of several or all relevant parameters. Heat transport, ventilation as well as lighting in a room are influenced by each other. Therefore they should be integrally modelled. As a part of the IEA-ECBCS Annex 23 `Multizone Airflow Modelling', such a coupling has been realised by integrating the air flow and contaminant transport simulation code of Comis into the building and systems simulation code Trnsys . This paper gives a short description of the concept used for the coupling. Then, two application examples typical for a building design study situation are presented, the first being a multi-storey school building which was passively cooled at night due to natural stack airflow. In the second example the facade of the same building was retrofitted with a glazed outer facade. Ventilation was provided by naturally driven shaft ventilation through the facade spaces. For such cases as described in the examples, it may be necessary due to the complex interactions, to study many configurations to find optimum control strategies for the openings and the blinds with respect to overheating risk as well as to air quality. For the upper floors, the risk of overheating and low air quality may be difficult to minimize without extending the shaft above roof level.

The performance of mechanical ventilation systems has been checked in several innovative residential houses as part of either a Swiss research or a P&D (pilot and demonstration) project. This paper gives a list of performance criteria for the ongoing comparison and evaluation of these mechanical ventilation systems. For some criteria, target values are proposed. In the second part, this paper briefly describes four residential buildings with mechanical ventilation systems where such evaluations were performed, and highlights interesting design features and results from the measurements. It then focuses on discrepancies between the design goals and what has been encountered in reality during the evaluation campaign. Problem items were, among others, commissioning, occupant acceptance and window opening behaviour as well as sealing of ducts and heat exchangers.