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

Ja takk, begge deler! En kvalitativ studie av hvilke erfaringer samfunnsfaglærere har med bruken av digitale og analoge lærebøker

Denne masteroppgaven i samfunnsfagdidaktikk har tatt utgangspunkt i problemstillingen: Hvilke erfaringer har lærere i samfunnsfag med bruk av digitale og analoge lærebøker? Formålet med denne studien har vært å undersøke hvordan et utvalg samfunnsfaglærere sier om hvordan de bruker den digitale og analoge læreboka i faget, hvilke muligheter og utfordringer de opplever og eventuelt andre faktorer som er med på å påvirke deres bruk av læreboka. Med ny læreplan så har det kommet nye lærebøker. Bruk av lærebøker er et relevant tema i skolen. Samfunnsfag har blant annet fått et spesielt ansvar for digitale ferdigheter, og under koronapandemien har både dette og digitale lærebøker vært i fokus. Oppgaven tar utgangspunkt i et teoretisk rammeverk om lærebokas endring i skolen, muligheter og utfordringer med læreboka, samt hvordan læreboka og andre læremidler blir brukt i skolen knyttet til samfunnsfag og andre fag. I denne studien har jeg intervjuet fire lærere som underviser i samfunnsfag – tre fra ungdomsskolen og en fra videregående skole. Tre av lærerne har en digital lærebok, mens en lærer har både digital og analog lærebok tilgjengelig i samfunnsfag. Funnene fra analysen viser først og fremst at det er ulik bruk av den digitale læreboka hos informantene, selv om alle har den tilgjengelig i faget. Videre så viser det seg at det lærerne ønsker seg aller mest er en kombinasjon av læremidler i sin samfunnsfagundervisning. Både en analog lærebok med en digital lærebok eller digital ressurs ved siden av. I tillegg til læreboka bruker mange av informantene andre digitale ressurser i faget. Valget av lærebøker påvirkes av skoleledelsen, lærerkollegiet og økonomi. Det som styrker den analoge læreboka er at den er lett tilgjengelig, noe konkret å forholde seg til og den er strukturert. Svakhetene ligger i at den blir fort utdatert. Den digitale læreboka har muligheter for å oppdateres jevnligere, inneholder tidsriktig informasjon, kan tilpasse og variere undervisningen. Utfordringene med en digital lærebok er at det skaper forstyrrelser hos elevene, fordi den er på datamaskin eller nettbrett. Videre at lærernes tolkning og gjennomføring av Kunnskapsløftet 2020 er forskjellig. Denne studien er basert på kun fire lærere i samfunnsfag, så viser det seg at det er forskjeller og ulikheter, som trolig kan speile hvordan samfunnsfaglærere i hele landet forstår læreplanene og hvordan de underviser med lærebøker. Hensikten med studien kan medføre en økt bevissthet hos samfunnsfaglærere ved bruk av digitale og analoge lærebøker

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

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

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