Field Measurements – Heat Pump Systems in NZEB

The IEA Heat Pump Programme (HPP) Annex 40 "Heat Pump Concepts for Nearly Zero Energy Buildings" deals with the application of heat pumps as core component of the HVAC system for Nearly or Net Zero energy buildings (NZEB). Annex 40 has been structured in four tasks which comprise the following activities: Task 1 – State-of-the-art analysis The Task 1 is to give an overview on NZEB on the national level of the participating countries. In more detail, the political framework in terms of NZEB (e.g. building codes, legislation, definitions of NZEB), the state of market introduction, and applied technologies both on the building envelope and the building HVAC system shall be characterised. The compiled technical concepts shall be analysed regarding the heat pump application. Moreover, technologies shall be classified in a technology matrix and evaluated regarding specific advantages of single technologies for dedicated applications such as new buildings, retrofit, office, residential, etc. Technologies shall also be considered regarding different aspects of the definitions, e.g. characteristics regarding load match and grid interaction, the necessity of a grid connection, or the capability to integrate local storage. Task 2 – Assessment of system technology Task 2 is dedicated to identify promising concepts for the further development of system configurations and in-depth analysis of technologies and system configurations suitable for different applications in NZEB. Concepts shall be optimised by simulations regarding design, integration options, and control, but also regarding further aspects like self-consumption of energy, load match, and grid interaction, which shall be considered in Task 4. Evaluation is made based on energy performance and cost. Task 3 – Technology development and field monitoring Task 3 is dedicated to technology developments on the component and system level as well to gather field experiences of system solutions in field monitoring projects. Marketable and prototype technologies could be lab-tested or investigated in field monitoring. Task 3 is accomplished in parallel to Task 2. Task 4 – Integration of NZEB into the energy system Task 4 is also to be accomplished in parallel and deals with the integration of NZEB into the energy system. An NZEB should be designed in a way not to produce additional stress for the grid. In this respect, load match profiles and grid interaction shall be investigated in order to maximize selfconsumption and minimize grid interaction. Thus, local storage options shall be evaluated. On the other hand, the ability of NZEB to react to signals from the grid (“smart grid”), i.e. demand response technology options shall be examined. Heat pumps are also a unique system in this respect due to the option of a transformation of electrical surplus to storable heating or cooling energy, as well in simultaneity, and due to the connection to source and sink systems which may be used as short-term storage like the ground. Control issues play an important role for these investigations, as well. This report gives the results for the field monitoring project (Task 3) for NORWAY. The Norwegian activities in IEA HPP Annex 40 are organized and carried out by SINTEF Building and Infrastructure (http://www.sintef.no/home/building-and-infrastructure), while COWI AS (www.cowi.no) and NTNU (http://www.ntnu.edu) are subcontracting partners. The project is funded by the governmental organization Enova SF (www.enova.no) and the Norwegian Research Centre on Zero Emission Buildings, ZEB (www.zeb.no).publishedVersio

Heat recovery ventilation design limitations due to LHC for different ventilation strategies in ZEB

Today's buildings are becoming more insulated and airtight to reduce transmission heat losses. Energy use for ventilation can represent up to half of these buildings' total energy use. Heat recovery in ventilation and demand-controlled ventilation (DCV) are energy-efficient measures to reduce ventilation energy use, especially when combined. However, this study revealed that the often-overlooked longitudinal heat conduction (LHC) in aluminium rotary heat exchangers might yield less efficient heat exchangers, particularly for intended high-efficiency heat recovery at low ventilation rates in DCV. This study presents a theoretical method to assess the effect of LHC on the amount of energy used to heat ventilation air for several ventilation strategies. The method is demonstrated in a case study for a virtual office building in a cold climate (Oslo, Norway). When neglecting the LHC effect, the energy used to heat the supplied air using DCV with a rotary heat exchanger is about three times smaller than when considering LHC. Unlike earlier studies, we find that DCV may consume more ventilation heating energy than constant air volume (CAV) ventilation when the selected wheel is deep and oversized due to LHC. This study highlights the need to design rotary heat exchangers carefully in order to account for the LHC effect, particularly when targeting zero emission buildings (ZEB).publishedVersio

Global sensitivity analysis and optimal design of heat recovery ventilation for zero emission buildings

Energy-efficient building services are necessary to realise zero-emission buildings while maintaining adequate indoor environmental quality. As the share of ventilation heating needs grow in well-insulated and airtight buildings, heat recovery in mechanical ventilation systems is increasingly common. Ventilation heat recovery is one of the most efficient and viable means to reduce ventilation heat losses and save energy. Highly efficient heat exchangers are being developed or applied to maximise the energy-saving potential of heat recovery ventilation. Nevertheless, the effects of practical operating conditions and the constraints of heat recovery – such as variations in ventilation rates, frost protection, and the prevention of an overheated air supply over a long-term period, which may significantly influence realistic recovery rates – have been less considered in efforts to maximise the energy savings. It is unclear which design parameters for heat recovery devices have the greatest effect on the annual energy savings from ventilation. This study proposes annual efficiency and annual net energy saving models for heat recovery ventilation that consider ventilation rate variations, the longitudinal heat conduction effect and operating controls. We use a global sensitivity analysis to quantify the contributions of various design input parameters to the variation in annual recovery efficiency and annual net energy savings. We identify the most influential parameters and their significant interaction effects for the annual energy performance of heat recovery ventilation. More attention should be paid to these most influential parameters during the design process. Furthermore, the optimal designs for rotary heat exchangers (as identified by a pattern-search optimisation algorithm) can improve annual net energy savings in demand-controlled ventilation by 33–48%, depending on the building areas. In combination with the reference year analysis presented in this study, heat recovery and demand-controlled ventilation can help to meet the need for highly efficient ventilation systems and zero-emission buildings.publishedVersio

Measurements of indoor air quality in four Norwegian schools

Children spend a minimum of six hours per day in Norwegian schools. Their exposure to different indoor air quality it is known to affect their performance. It is very common to use demand-controlled ventilation (DCV) in schools as is estimated to save about SO% of the conventionally used energy for ventilation. CO2 and temperature are the preferred control parameters. Usually, it was expected that these human-centric controls resulted in high indoor air quality as occupants are the largest source of contaminants. This study presents measurements for two months to up to one year in the supply and room air in the four classrooms whose ventilation is CO2-based DCV. Using low-cost sensors formaldehyde, PM1, PM2.s, relative humidity CO2 and temperature were monitored. Even when the CO2 concentration lied below 1000 ppm 1) the concentration of formaldehyde surpassed the recommended WHO thresholds in 30 % of the time and 2) RH is below 20 % during 56 % of the time.publishedVersio

Performance evaluation and control scenarios for targeted heat injection and extraction in an existing geothermal borehole field in Norway

This work presents the calibration, validation, and analysis of a borehole thermal energy storage (BTES) in building performance simulation using operational data from an existing borehole field in Kalnes, Norway. The data used in this study comprises the results of a thermal response test as well as operational data from the borehole field, which is used to cover the heating and cooling load of a hospital complex. The first part of this study focuses on the calibration and validation of the borehole field model using the IDA ICE software. Then, the validated model is used to explore the impact of different operational strategies in which the charging/discharging of the three sections of the borehole field are prioritized in different orders and compared to the most recent operational strategy. This analysis is carried out on a single year and a 20-year perspective to evaluate long-term temperature trends. This investigation aims to evaluate the current and future impacts of the existing operational strategy and whether it could be improved given that the real-life system struggles with below zero brine temperature at the end of the heating season. The simulation results show that the outgoing brine temperature in each borehole section is strongly dependent on the mass flow rate used in the BTES but that the temperature in an individual section had little impact on the neighboring one. When a section was prioritized by the control logic for heat extraction or injection (both in terms of order and mass flow), there was a notable increase or decrease in the outgoing brine temperature, indicating that it was possible to have targeted heat injection/extraction. In the 20-year operational horizon, the simulation results predicted a gradual warming trend of the outgoing brine temperature of approximately 1 °C due to the additional heat injected in all three sections and regardless of the scenario. The study shows that the most recently implemented operational strategy, in which all sections are charged and discharged simultaneously, is the most viable operation scenario for the borehole thermal energy storage at Kalnes, thus confirming previous findings in literature. Since none of the sections had a superior storage or heat regeneration capacity, prioritizing sections would only lead to more significant temperature swings in the ground. On the other hand, the current operation strategy leads to an overall higher temperature in the ground and reduces the risk of low outgoing brine temperaturespublishedVersio

A multi-objective optimisation framework to design membrane-based energy recovery ventilation for low carbon buildings

Membrane energy exchangers (MEEs) are increasingly being studied and utilized to contribute to realising energy-efficient building services and providing satisfactory indoor environments. The performance of MEEs has been extensively studied in terms of heat and mass transfer and pressure drop (PD). However, a model for optimizing the performance of membrane energy exchangers in residential ventilation, which takes into account the influential factors, is lacking in order to support the design of membrane energy exchangers. The purpose of this study was to establish a framework for the multi-objective optimisation design of membrane energy exchanger performance. This framework was demonstrated by considering the competing objectives of maximising thermal recovery effectiveness and minimising pressure drop. One of the constraints used for optimising membrane energy exchangers was the total membrane area, which strongly influences the investment cost of the exchanger. Another constraint was the moisture recovery intensity of the membrane energy exchangers, which affects indoor humidity levels. Pareto optimal solutions were obtained by solving the developed multi-objective optimisation framework using the genetic algorithm in MATLAB. Using multi-objective optimisation, the pressure drop of the MEE was reduced by 41% while the thermal recovery effectiveness remained unchanged. The resulting pressure drops as low as 5 Pa, enables the application of membrane energy exchangers in natural and hybrid ventilation. Factors influencing the Pareto optimal solutions including moisture recovery effectiveness, total membrane area and operating airflows have been investigated. A better understanding of optimal membrane energy exchanger designs considering thermal recovered energy and fan power resulted from this study.publishedVersio

Holistic methodology to reduce energy use and improve indoor air quality for demand-controlled ventilation

Ventilation control logics are usually based on the control indicators of occupancy. However, strategies including control of contaminants not linked to occupancy are requested and more feasible with the introduction in the market of low-cost sensors (LCS). In this work, a methodology for the improvement of demand-controlled ventilation (DCV) using measurements of IAQ parameters with LCS, correlation analysis, and co-simulation EnergyPlus/CONTAM is presented. Its goal was reduced annual energy use and the fraction of time with room air concentration of IAQ parameters outside thresholds. The ventilation control sequences of supply airflow rates and recirculation of return air focused on the significant parameters chosen by cross-correlation functions in the de-trended measurements. The results revealed that the methodology successfully developed control sequences that simultaneously reduced annual energy use and the number of hours outside the recommended IAQ guidelines compared to the baselines. In cold cities with excellent outdoor air quality, recirculation could reduce energy use and increase the RH in winter. Further simulations demonstrated that the use of recirculation had a protective effect on the indoor concentrations of PM2.5, assuming low outdoor air quality. However, when using recirculation, it is essential to control the IAQ to avoid excessive pollutants, RH, and temperatures.Holistic methodology to reduce energy use and improve indoor air quality for demand-controlled ventilationpublishedVersio

State-of-the-Art for the use of Phase-Change Materials in Tanks Coupled with Heat Pumps

With the goal of increasing heat storage in the same accumulation volumes, phase-change materials are considered. There are different substances with different phase-change temperatures that can be used for storing heating or cooling implemented in heat pump systems for applications of space heating and cooling, ventilation or domestic hot water production. Reducing the size of the buffer tanks used with heat pumps, avoiding the oversizing of heat pumps or detaching thermal energy production and consumption are among the benefits that could result from the combination of heat pumps and latent heat thermal energy storage. In addition, this form of thermal energy storage allows enhancing the use of renewable energy sources as heat sources for heat pump systems. Most previous review works focus mainly on the different materials available that can be used as phase-change materials. Conversely, this review encloses, classifies and describes the results of different works found in the literature that studied individual solutions to enhance the performance of systems combining heat pumps and latent heat thermal energy storage.acceptedVersio

Assessing the indoor air quality and their predictor variable in 21 home offices during the Covid-19 pandemic in Norway

In this study, concentrations of pollutants: formaldehyde, carbon dioxide (CO2), and total volatile organic compounds (TVOC) and parameters: indoor room temperature and relative humidity (RH) were measured in 21 home offices for at least one week in winter in Trondheim, Norway. Eleven of these were measured again for the same duration in summer. Potentially explanatory variables of these parameters were collected, including building and renovation year, house type, building location, trickle vent status, occupancy, wood stove, floor material, pets, RH, and air temperature. The association between indoor air pollutants and their potential predictor variables was analyzed using generalized estimation equations to determine the significant parameters to control pollutants. Significantly seasonal differences in concentrations were observed for CO2 and formaldehyde, while no significant seasonal difference was observed for TVOC. For TVOC and formaldehyde, trickle vent, RH, and air temperature were among the most important predictor variables. Although higher concentrations of CO2 were measured in cases where the trickle vent was closed, the most important predictor variables for CO2 were season, RH, and indoor air temperature. The formaldehyde concentrations were higher outside working hours but mostly below health thresholds recommendations; for CO2, 11 of the measured cases had indoor concentrations exceeding 1000 ppm in 10% of the measured time. For TVOC, the concentrations were above the recommended values by WHO in 73% of the cases. RH was generally low in winter. The temperature was generally kept over the recommended level of 22–24 ◦C during working hours.publishedVersio