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Model predictive control (MPC) is a promising optimal control technique for activating building energy flexibility using its thermal mass. The performance of the MPC controller is directly related to the accuracy of the model prediction. Grey-box models, based on physical laws and calibrated on measurement data, are commonly used to represent the building thermal dynamics in MPC. Most research works use Linear Time-Invariant (LTI) grey-box models even though weather conditions vary significantly throughout the heating season. This is critical as inaccurate model prediction can lead to a lower performance of the MPC controller. This study introduces two adaptive MPC schemes to overcome this limitation of LTI models. The first one, called the Partially Adaptive MPC, only updates the effective window area of the prediction model. The second one, called the Fully Adaptive MPC, updates all the parameters of the grey-box model. The adaptive MPC performance is compared with MPC using LTI models in two different tests. The simulation-based results show that MPC based on LTI performs well if the control model is trained during a period with similar weather conditions as the period when the MPC will be applied. The Partially Adaptive MPC is unable to deliver satisfactory prediction performance due to the limited number of parameters that are updated. The Fully Adaptive MPC has the best performance compared to the other MPCs, especially as it avoids thermal comfort violations. • Control-oriented models are compared for MPC activating the building thermal mass. • Several linear time-invariant (LTI) and adaptive grey-box models are compared. • Only LTI models trained on data like the period of MPC operation are accurate. • Fully Adaptive MPC outperforms the LTI MPC and reduces thermal discomfort. [ABSTRACT FROM AUTHOR]

To generalize research on indoor air quality (IAQ) in bedrooms, the objective of this article is to characterize IAQ for the cold climate of Norway. The relationship between ventilation, indoor thermal environment and IAQ is revealed. 58 natural-ventilated bedrooms with different levels of renovation were investigated using detailed measurements during the heating season. Compared to other studies in Nordic climate, a larger fraction of the occupants used extensive window openings. A larger fraction of the bedrooms showed low CO 2 concentrations. Only few other studies also measured TVOC concentrations. Its average value was well correlated with CO 2. However, TVOC concentration is characterized by more peaks. Low CO 2 concentrations were found with extensive window openings. Nonetheless, extensive window openings did not systematically lead to low CO 2 concentrations, for instance the influence of curtains is shown. It is confirmed that the door opening can lead to relatively low CO 2 concentrations when the window is closed. The highest CO 2 concentrations are found with the window and door closed. The study confirms relatively low temperatures in bedrooms. Nevertheless, no clear correlation between the indoor temperature and the fraction of time with the window opened was observed. For instance, the lowest bedroom temperatures were obtained using window opening, but also with the window closed. Finally, the results suggest that it is possible to achieve a high IAQ with window ventilation when this is a deliberate strategy (if no filtering of the fresh air is needed), also in renovated buildings where the envelope airtightness has been improved. • Detailed field measurements of IAQ performed in natural-ventilated Norwegian bedrooms. • More window openings and lower CO 2 concentrations than in other cold climates were observed. • It is confirmed that many occupants in Norway have low bedroom temperatures. • Many occupants showed regular and consistent window opening habits, also in cold periods. • No difference in IAQ was measured between renovated and unrenovated houses. [ABSTRACT FROM AUTHOR]

In this work, a numerical model of the heat transfer through composite wood stove walls has been used to study the effect of material configurations on the heat storage and the heat release profile to the room, to arrive at improved thermal comfort performance. Cast iron and soapstone have been compared, showing that soapstone is a good material from a thermal comfort point of view, while cast iron (for the typical stove weights of today) exhibits a comparably higher peak and faster heat release. Applying a Phase Change Material (PCM) for such a cast iron stove changes this picture. It makes it possible to reduce peak heat release and achieve a more stable as well as an extended heat release period. 53% reduction in peak and 43% reduction in average heat release was demonstrated during the combustion cycle using Erythritol as PCM. However, the selection and use of a PCM must be optimized. Its capacity should be utilized to a maximum extent without overheating the material as it will cause irreversible damage to most PCM. [ABSTRACT FROM AUTHOR]

The necessity to drastically reduce the space-heating (SH) needs of residential buildings in Europe has pushed the emergence of building concepts based on a super-insulated building envelope, such as the passive house (PH) standard. In these envelopes, the SH distribution system can be simplified because it is theoretically not necessary anymore to place a heat emitter in each room, or in front of each window. There is lack of fundamental knowledge to support this simplification of the SH distribution system. The present contribution focuses on terraced houses heated using a reduced number of radiators. It aims at comparing the balance between energy efficiency, thermal comfort and user satisfaction using simplified SH distribution. For this purpose, two terraced houses built according to the Norwegian PH standard have been investigated using building simulations (using IDA-ICE), field measurements and occupant interviews. With a simplified distribution, one may suspect that occupants experience the thermal environment of rooms without heat emitter as too cold, typically bedrooms. On the contrary, the super-insulation and the high-efficiency heat recovery prevent significant temperature zoning to take place between rooms. Even though the SH distribution is simplified, occupants rather complain about the bedroom temperature which often experienced as too warm if they do not open windows. Unfortunately, this way to control indoor temperature has a strong adverse influence on the SH needs. The trade-off between bedrooms temperature and SH needs is here investigated for different control strategies using calibrated simulations. [ABSTRACT FROM AUTHOR]

In the development towards nearly zero energy buildings (nZEB) the load profiles for the heat demand and the heat use of the buildings are expected to be changed considerably compared to the situation today. This change will influence the design and the overall profitability of district heating (DH). Earlier heat use measurements have shown that the real heat use of LEB, for several reasons, quite often seems to be somewhat higher than the values from theoretical calculations. This means that knowledge on measured values of the total heat load profiles and the heat use from all types of LEB will be important for evaluating the economy and the competitiveness of DH in the future. The primary goal of the paper is to present measured load profiles and yearly heat use for three different blocks of flats connected to the DH-system in Trondheim, Norway. Official dimensioning outdoor temperature in Trondheim is -19°C. The presented load profiles show statistical hourly mean specific maximum values for the three buildings. Specific values for the yearly heat use and the outdoor temperature value when the heat demand goes down to zero, change point temperature (CPT) are also presented. System efficiency of the domestic hot water (DHW) system in the buildings is also given. The presented information is assumed to be useful in the planning of DH-systems for the future. [ABSTRACT FROM AUTHOR]

The necessity to drastically reduce the space-heating (SH) needs of residential buildings in Europe has prompted the emergence of building concepts based on a super-insulated building envelope, such as the passive house (PH) standard. In these envelopes, the SH distribution system can be simplified because it is theoretically not necessary anymore to place a heat emitter in each room, or in front of each window. There is lack of fundamental knowledge to support this simplification of the SH distribution system. The present contribution especially focuses on apartment buildings heated using a reduced number of radiators. It aims at comparing the balance between energy efficiency, thermal comfort and user satisfaction using simplified SH distribution. For this purpose, two flats built according to the Norwegian PH standard have been investigated using building simulations (using IDA-ICE), field measurements and occupant interviews. With a simplified distribution, one may suspect that occupants experience the thermal environment of rooms without heat emitter as too cold, typically bedrooms. On the contrary, the super-insulation and the high-efficiency heat recovery prevent significant temperature zoning to takes place between rooms. Even though the SH distribution is simplified, occupants rather complain about the bedroom temperature which is too warm if they do not open windows. Unfortunately, this way to control indoor temperature has a strong adverse influence on the space-heating needs, which is here investigated for different control strategies. Another limitation is the time needed to heat a bedroom only using internal door opening. It takes several hours to adjust to a higher set-point temperature; an aspect that can be critical if the bedroom temperature should be changed between daytime and nighttime. [ABSTRACT FROM AUTHOR]

The study presented in this paper originated from observations made regarding the thermal conditions during winter in highly insulated dwellings with mechanical ventilation with heat recovery (MVHR). Previous observations indicate an oversupply of heat to bedrooms and a successive extensive window ventilation, which leads to an increased space-heating demand. Detailed simulations were conducted to explain the causes for the observed thermal conditions and to elaborate improved solutions for heating and ventilation during winter. Various MVHR solutions and control strategies, as well as building design solutions, were investigated regarding their impact on the thermal conditions in bedrooms and on the space-heating demand. The results clearly illustrates that the supply-air temperature and the temperatures in the living room and bathroom have substantial effects on the thermal conditions in the bedrooms. A one-zone MVHR solution, with approximately the same the supply-air temperature to all rooms, has clear limitations regarding the provision of thermal comfort in bedrooms. The clear potential of a two-zone MVHR solution, where the supply-air temperature to the bedrooms is controlled independently from other rooms, was observed. With a two-zone MVHR solution, the thermal conditions in bedrooms can be improved and the space-heating demand can be reduced. [ABSTRACT FROM AUTHOR]

In highly-insulated buildings such as passive houses, the space-heating distribution subsystem can be simplified by reducing the number of heat emitters. In this context, the bi-directional flow through open doorways is known to be an efficient process to support the heat distribution between rooms. This process is therefore investigated using field measurements within a Norwegian passive house. The so-called large opening approximation proves to model fairly the mass flow rate, but also the convective heat transfer if the thermal stratification is accounted for. Furthermore, the discharge coefficient appears to be independent of the heater type and location in the room. [ABSTRACT FROM AUTHOR]

The density-driven bidirectional flow through an open doorway is of prime importance for ventilation and heat distribution between rooms in buildings. Although this flow has been extensively studied in the past, some important flow characteristics, such as unsteady flow phenomena, have not been documented in detail. Therefore, a high-resolution Large Eddy Simulation (LES) of the bulk flow through a doorway is performed. This LES can also serve as a reference solution to compare the accuracy of simpler evaluation methods, from the standard theoretical model calibrated using a discharge coefficient (C d) to CFD solving the Reynolds-Averaged Navier-Stokes (RANS) equations. Based on LES results, the bidirectional flow can generate turbulent mixing in the middle of the doorway. However, the effects remain limited to the close vicinity of the neutral plane. The bidirectional airstream in the doorway further develops into two non-isothermal jets that entrain a fraction of the airflow. Furthermore, the two jets create large unsteady flow structures when they expand in the adjoining rooms. The results show that unsteady RANS is a good alternative to the resource-intensive LES if the analysis of turbulent jets is not of interest. The standard theoretical model demonstrates that two-dimensional contraction is the dominant effect driving the C d value, while the viscous effects have a minor influence. Unlike previous studies, LES results show that viscous effects tend to increase the C d as they moderate the contraction effect. This paper also provides guidelines for the laboratory measurement of C d and its use in building performance simulation tools. • The density-driven bidirectional flow through a doorway is simulated using Large Eddy Simulation. • Unsteady flow phenomena such as interfacial mixing and re-entrainment are characterized. • URANS is unable to capture the large turbulent flow structures of the non-isothermal jets. • The viscous effects have a lower influence on the discharge coefficient than the flow contraction. [ABSTRACT FROM AUTHOR]

Abstract: The passive house (PH) standard was originally defined for Central Europe and has subsequently been applied to many cold climate countries. In these conditions, the relation between this standard and the air heating (AH) is not clear while both concepts are usually associated. Furthermore, the AH provides a way to simplify the space-heating distribution system. The present contribution investigates the feasibility of the AH concept in PH along with its challenges in terms of thermal dynamics: the magnitude of the AH temperature needed, the temperature difference between rooms, the impact of internal gains, the influence of thermal losses from ventilation ducts and the AH control. This is performed using detailed dynamic simulations (TRNSYS) on a typical detached house typology. Practically, four cold climate zones are considered as well as different insulation levels and construction materials. Results show limitations related to a centralized AH as well as provide guidelines for a consistent AH design in cold climates. In addition, a simple analytical method used for the design of German PH is tested and proved accurate enough to estimate the maximal AH temperature during the heating season. [Copyright &y& Elsevier]

A grey-box model is a combination of data-driven and physics-based approaches to modeling. For applications in buildings, grey-box models can be used as the control model in model predictive control (MPC) or to characterize the thermal properties of buildings. In a previous study using data generated from virtual experiments, the influence of data pre-treatment on the performance of grey-box models has been demonstrated. However, field measurement differs from data generated using building performance simulation (BPS). This is because the precision and accuracy, the location, and the dynamics of the sensors could be different. Consequently, this paper extends previous results and conclusions using a real test case of a highly-insulated residential building. The results confirm that data pre-processing has a minimal influence on the identified results (parameter values and simulation performance) for deterministic models. On the contrary, data pre-treatment influences the performance of stochastic models as follows. Firstly, large sampling time (T s) can cause the parameters to become non-physical and can sometimes reduce the one-day ahead prediction performance. With large T s , the anti-causal shift (ACS) proves to be beneficial to keep the parameters physically plausible while low-pass filtering can also contribute but to a lesser extent. With large T s , ACS does not guarantee a higher one-day ahead prediction performance for stochastic models, whereas pre-filtering generally has a positive impact. Secondly, for the stochastic model, the sensor dynamics should be modeled if the sensor has a noticeable time constant to guarantee the physical plausibility of the parameters. Thirdly, the dynamics of the hydronic radiator do not need to be modeled if the time constant in the temperature sensors is larger than the radiator. These findings provide practical guidelines for grey-box modeling of buildings with field measurement data. • Grey-box models of the building heat dynamics are investigated using a real test case. • Deterministic models are not much influenced by data pre-processing and sensor dynamics. • Stochastic models are significantly influenced by sampling time and anti-causal shift. • The sensor dynamics should be included inside the stochastic model. [ABSTRACT FROM AUTHOR]

Abstract: Wood stoves are attractive for the space-heating (SH) of passive houses. Nevertheless, there are still questions about their integration. Firstly, the power oversizing of the current stoves and their long operating time may lead to unacceptable overheating. Secondly, it is also unclear how one stove can ensure the thermal comfort in the entire building. The paper investigates these aspects using detailed dynamic simulations (TRNSYS) applied to a detached house in Belgium. An 8kW stove is assumed to be representative of the lowest available powers in the market. Results confirm that a large power modulation is important to prevent overheating. Opening the internal doors, a high building thermal mass and a heat emission dominated by radiation also reduce the overheating risk, but to a smaller extent. Besides, a single stove cannot enforce the thermal comfort during design weather conditions: a peak-load system is then needed. Using more standard conditions, a Typical Meteorological Year (TMY), the stove can mainly perform the SH but it then requires the internal doors inside the building to be opened. The temperature distribution between rooms is in fact dominated by the architectonic properties. Finally, the emission and distribution efficiency of the stove is also investigated. [Copyright &y& Elsevier]

• An economic model predictive control was tested for residential space heating. • Good predictive performance of the applied black-box state-space model. • A single thermal zone model provided acceptable temperature control. • The E-MPC successfully shifted consumption from high to low cost periods. Previous studies have identified significant demand response (DR) potentials in using economic model predictive control (E-MPC) of space heating to exploit the inherent thermal mass in residential buildings for short-term energy storage. However, the economically viable realisation of E-MPC in residential buildings requires an effort to minimise the need for additional equipment and labour-intensive modelling processes. This paper reports on an experiment where a novel E-MPC setup was used for thermostatically control of a hydronic radiator in a highly-insulated residential building located on the NTNU Campus in Trondheim, Norway. The E-MPC utilized data from a heating meter, two temperature sensors and an existing weather forecast web service to train a linear black-box model. The results showed that the precision of model trained on excitation data that was generated using setpoints of either 21 or 24 °C was sufficient to obtain good control of the indoor air temperature while shifting consumption from high to low price periods. The findings of the experiment indicate that a minimal E-MPC setup is able to realize the significant DR potential that lies in utilizing the inherent thermal mass in residential buildings. [ABSTRACT FROM AUTHOR]

[Display omitted] Grey-box models are data-driven models where the structure is defined by the physics while the parameters are calibrated using data. Low-order grey-box models of the building envelope are typically used for two main applications. Firstly, they are used as a control model in Model Predictive Control (MPC) where the thermal mass of the building is activated as storage (for instance in demand response). Secondly, they are used to characterize the thermal properties of the building envelope using on-site measurements. The influence of the data pre-treatment on the performance of grey-box models is hardly discussed in the literature. However, in real applications, information about data pre-processing by sensors or data acquisition systems is expected to be limited. Therefore, the influence of the sampling time, low-pass filters and anti-causal shift (also called data labeling) are analyzed for grey-box models in deterministic and stochastic innovation form. The influence on the optimizer performance is also investigated. The datasets are generated from virtual experiments using multi-zone building performance simulations of a residential building (in lightweight wooden construction) heated using different types of excitation signals. Results show that the parameters of deterministic grey-box models are significantly influenced by the training data while the data pre-treatment has a limited impact on the model and optimizer performance. Depending on the training data, the value taken by some parameters is not physically plausible. On the contrary, stochastic models are significantly influenced by the data pre-treatment, especially the sampling time, and less by the training data. The parameters can become non-physical for large sampling times. However, the anti-causal shift proves to be efficient to keep the parameters almost constant with increasing sampling times. Even though the parameter values of the deterministic model are less physically plausible, the simulation performance of deterministic models is higher than using the equivalent stochastic models. These results suggest that deterministic models seem better suited for MPC while stochastic models are better suited for the characterization of thermal properties (if suitable data pre-treatment is applied). [ABSTRACT FROM AUTHOR]

• Comparison of a modulating and an on-off air-source heat pump as well as direct electric heating. • Implementation of a detailed heat pump system model into a building performance simulation tool. • In-depth analysis of the controller for heat pump units. • Model complexity of the heat pump controller influences short-time behavior of the heat pump. • Domestic hot water prioritization of the heat pump can influence auxiliary heater operation during demand response. Building performance simulation (BPS) is a powerful tool for engineers working in building design and heating, ventilation and air-conditioning. Many case studies using BPS investigate the potential of demand response (DR) measures with heat pumps. However, the models are often simplified for the components of the heat pump system (i.e. heat pump, electric auxiliary heater and storage tank) and for their interactions. These simplifications may lead to significant differences in terms of DR performance so that more comprehensive models for a heat pump system may be necessary. The contribution of this work is twofold. Firstly, this work investigates the influence of the modeling complexity of the heat pump control on different key performance indicators for the energy efficiency, the DR potential and the heat pump operation. To this end, the performance of six different heat pump controls is compared. Secondly, it describes the implementation of a comprehensive control for a heat pump system in BPS tools. This control is not often documented in the BPS literature and is error-prone. Generic pseudo-codes are provided, whereas IDA ICE is taken as an example in the case study. A predictive rule-based control is implemented to study price-based DR of residential heating. It is shown that a realistic operation of the heat pump system can be achieved using the proposed modeling approach. The results prove that the modeling complexity of the system control has a significant impact on the performance indicators, meaning that this aspect should not be overlooked. For some performance indicators, e.g. the annual energy use for heating and average water tank temperature, it is shown that a proportional (P-) and proportional-integral (PI-) control can lead to similar results. If the heat pump operation is investigated in detail and a short-time resolution is required, the difference between P- and PI-controls and their tuning is important. As long as the heat pump operation and electrical power at short timescales are not of importance, the choice of controller (P or PI) is not crucial. However, the use of P-control significantly simplifies the modeling work compared to PI-control. If DR is performed for domestic hot water, it is also demonstrated that the prioritization of domestic hot water heating can indirectly influence the operation of auxiliary heaters for space-heating, significantly increasing the use of electricity. However, the electricity use is only slightly increased if DR control is only used for space heating. [ABSTRACT FROM AUTHOR]

• Solar photovoltaic-assisted ground-source heat pumps are studied in Nordic climate. • Ground regeneration with solar thermal energy improves long-term performance. • Designs are compared in terms of energy efficiency, cost, and ground heat transfer. • Photovoltaic-thermal leads to lower investment costs by reducing the borehole size. Ground source heat pumps (GSHP), as a renewable energy technology, are highly efficient systems suitable for meeting the heating needs of buildings, particularly in heating-dominated regions like Scandinavia. This study aims to better define the scope of application and sizing of GSHP combined with photovoltaic-thermal (PVT) collectors in Nordic conditions. The study compares seasonal performance factors (SPF), average ground temperatures, and investment costs for heating different numbers of apartment blocks in two distinct Nordic climates: Baltic climate and cold continental climate. To assess the long-term effects on the energy performance of GSHP + PVT systems, dynamic simulations using the TRNSYS software are conducted over a period of 50 years. The results demonstrate that PVT improves the long-term system performance, particularly in large and compact borehole (BH) fields, while in smaller BH fields, natural regeneration keeps playing a substantial role. For instance, PVT added to a GSHP system with large BH field can limit the ground temperature reduction after 50 years by 33 % compared to the configurations without PVT. In contrast, this reduction is only 13 % for systems with small BH fields. Moreover, the results show that GSHP + PVT can lead to a reduction of up to 95 % in the required area for installing the BH field while maintaining (almost) the same energy performance compared to a conventional GSHP. Notably, this study shows that the utilization of GSHP + PVT leads to a substantial reduction in BH length (up to 50 %) while maintaining a similar system SPF. It leads to a decrease in investment costs of up to 9.32 % and 22.92 % for the scenarios located in Norway and Sweden, respectively, compared to the case with PV instead of PVT. These findings suggest that PVT integration holds promise for advancing the GSHP adoption in high-latitude regions, especially in larger installations and colder climates, by mitigating investment costs while preserving the system SPF and ground temperature stability. [ABSTRACT FROM AUTHOR]

Highlights: [•] The article investigates the flexibility of an all-electric Net-ZEB using heat pump. [•] The article investigates the maximum onsite self-consumption in such Net-ZEBs. [•] The article studies the maximum grid relief that a Net-ZEB could offer to the grid. [•] The article presents different control strategies to realize these maximum limits. [•] The effect of these control strategies on the system performance is investigated. [ABSTRACT FROM AUTHOR]

Highlights • Comparison of modulating and on-off air-source heat pump as well as direct electric heating. • Demand response based on dynamic spot prices and CO 2eq. intensities of the electricity mix. • Sufficiently high daily fluctuations in spot prices and CO 2eq. intensity necessary to benefit from demand response measures. • Detailed modeling for both the heat pump system and the building. • Described model complexity necessary to gain insight in real behavior of the energy system. Abstract The building energy flexibility potential of a Norwegian single-family detached house is investigated using predictive rule-based control (PRBC) and building performance simulation (using IDA ICE). Norwegian timber buildings are lightweight and four different insulation levels are considered. Both on-off and modulating air-source heat pumps are analyzed and compared to direct electric heating which is the most common heating system for Norwegian residential buildings. A detailed model for both the heat pump system and the building is implemented, a level of detail not found in previous research on building energy flexibility. The three PRBC investigated have the following objectives: reduce energy costs for heating, reduce annual CO 2eq. emissions and reduce energy use for heating during peak hours. This last objective is probably the most strategic in the Norwegian context where cheap electricity is mainly produced by hydropower. The results show that the price-based control does not generate cost savings because lower electricity prices are outweighed by the increase in electricity use for heating. The implemented price-based control would create cost savings in electricity markets with higher daily fluctuations in electricity prices, such as Denmark. For the same reasons, the carbon-based control cannot reduce the yearly CO 2eq. emissions due to limited daily fluctuations in the average CO 2eq. intensity of the Norwegian electricity mix. On the contrary, the PRBC that reduces the energy use for heating during peak hours turns out to be very efficient, especially for direct electric heating. For air-source heat pumps, the control of the heat pump system is complex and reduces the performance of the three PRBC. Therefore, results suggest that a heat pump system should be modeled with enough detail for a proper assessment of the building energy flexibility. First, by varying temperature set-points there is a clear interaction between the prioritization of domestic hot water and the control of auxiliary heaters which increases energy use significantly. Second, the hysteresis of the heat pump control and the minimum cycle duration prevent the heat pump from stopping immediately after the PRBC requires it. Finally, the paper shows that the influence of thermal zoning, investigated here by cold bedrooms with closed doors, has a limited impact on the building energy flexibility potential and the risk of opening bedroom windows. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of how the design of a photovoltaic (PV) system influences the greenhouse gas emission balance in a net zero emission building (nZEB). In a zero emission building, the emissions associated both with the energy required in the operation of the building (operational emissions) and the energy used to produce the building materials (embodied emissions) are offset by renewable energy generated on-site (avoided emissions). The analysis is applied to a nZEB concept for a single-family building, developed by the Norwegian Research Centre on Zero Emission Buildings. Previous analyses have shown that the installation of a PV system accounts for a significant share of the embodied emissions of a nZEB. The objective of this paper is to assess how the PV system design choices influence the embodied and avoided emissions, in order to determine how the environmental impact can be minimised. Four different PV technologies (Si-mono, poly-Si and CIS, and high-efficiency Si-mono) in four different system designs for flat roofs are evaluated using two different grid emission factors. The installations are compared by means of net avoided emissions, greenhouse gas payback time (GPBT), greenhouse gas return on investment (GROI), and finally the net emission balance of the building. The results show that the system with the largest area of high-efficiency Si-mono modules achieves the best lifetime emission balance, but that the greenhouse gas return on investment is highest for the optimally oriented CIS modules. [ABSTRACT FROM AUTHOR]