THERMAL ANALYSIS OF A ONE-FAMILY, ECO-BUILDING WITH TWO TYPES OF SINGLE SOLAR AIR COLLECTORS

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1 THERMAL ANALYSIS OF A ONE-FAMILY, ECO-BUILDING WITH TWO TYPES OF SINGLE SOLAR AIR COLLECTORS BARTNICZAK Jakub HEIM Dariusz HENSEN Jan mgr inż. Jakub Bartniczak absolwent Wydziału Budownictwa, Architektury i Inżynierii Środowiska, Politechniki Łódzkiej, kierunek dyplomowania: Budownictwo Ekologiczne. Tematyka zainteresowań: symulacje komputerowe zjawisk cieplnowilgotnościowych zachodzących w budynku, budownictwo ekologiczne i energooszczędne, odnawialne źródła energii. jabartniczak@hotmail.com prof. dr ir. Jan Hensen, professor of building performance simulation and research director of the Center for Building & Systems TNO TU/e, Technische Universiteit Eindhoven, Netherlands. sen mgr inż. Dariusz Heim, asystent w Katedrze Fizyki Budowli i Materiałów Budowlanych Politechniki Łódzkiej. Tematyka zainteresowań: modelowanie i komputerowa symulacja procesów transportu masy i energii w budynkach oraz procesów cieplnowilgotnościowych zachodzących w jego elementach darkheim@p.lodz.pl ANALIZY CIEPLNE JEDNORODZINNEGO BUDYNKU EKOLOGICZNEGO ZINTEGROWANEGO ZE SŁONECZNYMI KOLEKTORAMI POWIETRZNYMI Głównym celem pasywnych i aktywnych systemów słonecznych jest nie tylko skuteczne pochłanianie i magazynowanie energii promieniowania słonecznego ale również właściwy jej rozdział we wnętrzu budynku. W pracy przedstawiono wyniki analiz cieplnych dla jednorodzinnego budynku ekologicznego Bartek-8 zintegrowanym z dwoma, różniącymi się wielkością i budową, rodzajami słonecznych kolektorów powietrznych. Pierwszy kolektor, stanowił zaprojektowany od strony południowo-zachodniej ogród zimowy o kubaturze 33,1m 3. Drugim kolektorem był płaski element dachowy o około 3-krotnie mniejszej kubaturze równej 1,1m 3. Oba kolektory, połączone z pomieszczeniami mieszkalnymi budynku, miały za zadanie podgrzać powietrze wentylacyjne do ustalonej temperatury wewnętrznej równej 2 C. Do analiz wybrano okres dwóch tygodni, podczas którego warunki brzegowe po stronie zewnętrznej zmieniały się zgodnie z rzeczywistymi

2 danymi klimatycznymi Warszawy. W pracy przedstawiono historie temperatur powietrza w kolektorze oraz w pomieszczeniu dla wybranego okresu dwóch tygodni. Pokazano wpływ zastosowania kolektora powietrznego na warunki cieplne panujące w pomieszczeniu oraz różnice w pracy instalacji grzewczej. Zastosowanie kolektorów zmniejszyło zapotrzebowanie na energię cieplną do ogrzewania i liczbę godzin pracy instalacji grzewczej w tzw. okresie przejściowym od marca do końca sezonu grzewczego. Na koniec wyznaczono współczynnik wydajności grzejnej kolektorów jako stosunek ciepła uzyskanego z energii promieniowania słonecznego do sumy ciepła uzyskanego z promieniowania słonecznego oraz ciepła dostarczonego przez dodatkowy układ grzewczy. ABSTRACT The main purpose of passive or active solar system applications is not only high efficient collecting and storing of solar energy but effective distribution the heat around the building spaces as well. The main goal of this work is to analyse and compare two types of solar air collector integrated in an ecological, low-energy, one family building. The first one, is a large volume sunspace located on the south-west of the building and the second, is a small volume and flat solar panel on the roof. Both collectors are connected with particular rooms by a ventilation system. The components of this system include air flow gaps for natural ventilation and various devices for mechanical ventilation. Some components are controlled on the basis of the zone s air temperature. Numerical analyses have been conducted for selected two-week periods using hourly Polish weather data. The results show that during the selected, coldest winter period, the air temperature inside the collectors does not exceed 2 C and there is no additional heat flow from the collectors to the rooms. Effective heat gains appear just from March to the end of the heating period. During this period, an additional fan is on and excess energy is transferred to the rooms not only by conduction but by convection as well. The presented results illustrate the differences in heating energy demand and thermal performance of the building. The utilisation of solar energy can reduce heating energy demand and decrease the number of heating hours. Finally the efficiency coefficient of the system was estimated as a function between solar energy gains and heating energy requirements. 1. INTRODUCTION Solar air collectors can be used for many applications at low and moderate temperatures e.g. space heating and drying. A solar air heater is a special kind of heat exchanger that absorbs and transforms solar radiant energy to heat, which is then carried away by air [1]. It occupies an important place among solar heating systems because of the minimal use of materials. Furthermore, direct use of air as the working substance reduces the number of components required in the system. The primary disadvantage of solar air heaters is the need for handling relatively large volumes of air with low thermal capacity as the working fluid. Many commercial buildings and houses suffer from high heat losses caused by high ventilation and infiltration rates. Although all that fresh air is usually good for indoor air quality, heating it can be very expensive. Considerable reduction of heating ventilation air

3 can be achieved by applying some advanced technique such as ventilation preheating or heat recuperation [2]. Solar air collectors represent the first of the systems mentioned above and can be easily integrated with a building. It is a rather passive than active solar system and it requires no mechanical devices except fans. However, in mechanical ventilated buildings, air collectors can use the building s ventilation fan to draw fresh air through the solar preheating system. The collectors are virtually maintenance free due to the thermostatic control system. 2. LOW ENERGY BUILDING The investigated low energy building has a compact shape and specially divided zones to increase the effect of passive solar heating in the various rooms. The external walls and windows are well insulated, the heat transfer coefficient for the opaque external walls is U=.27 W/m 2 K and for the glazing U=1.3W/m 2 K. Other energy efficiency improving measures in the building are solar air collectors. Two types of the collector were investigated by numerical analysis [3]. The first, placed on the roof, are traditional flat panels, covered by glass. The volume of the collectors equals 1.1m 3 and the total aperture area is 17m 2, while the absorber surface 15m 2. The collector is situated on the southern sloped roof of the building. An angle of the collector relative to horizontal is 45 o. The roof collector captures heat, which is transfer to the bedroom situated on the northwest part of the building. The total volume of the bedroom is 53m 3 and the area of a floor equals 18m 2. As a second collector, an attached sunspace was taken into consideration. The sunspace is located on the southwest side adjacent to the south wall of the living-room. The sunspace has a much bigger volume than the collector. The volume is 33.1m 3. The floor area is 7.6m 2. All internal surfaces: the floor and the walls inside the sunspace are constructed so as to store as much solar energy as possible and transfer it to the living-room. The outer layer of those constructions is 25-3cm thick red granite, which absorbs solar energy during the day and utilises it during the night. Warm air from the sunspace is transferred to the livingroom on southwest part of the building. The volume of the living room is 114m 3 and the floor area is 39m 2. The collectors are connected with the building spaces by an airflow network defined according to the scheme presented in figure 1 and 5 respectively for sunspace and roofcollector. Figure 5a represents the situation when the temperature inside the sunspace does not exceed 2 C. The scheme in figure 1b shows the situation when the warm air inside the sunspace is additionally transferred to the living room by the fan. Casual gains are defined for a family of five people. The rooms are naturally ventilated by fresh air through the cracks around the windows. The air changes in the rooms during the selected two-week period are presented in figure 2 and figure 6. Real weather data for Warsaw is used and a two-week intermediate period from 18 March 1985 to 31 March 1985 was selected for the simulations. That transition period was chosen because during this period the air temperature in the collector reaches the control point temperature of 2 o C and the effect of warm air transfer from the sunspace can be estimated.

4 3. NUMERICAL SIMULATION The numerical analyses were carried out using the ESP-r (Environmental Systems Performance) software. ESP-r uses advanced numerical techniques to solve the heat and mass transfer problem in buildings based on the mathematical models described by Clarke [4]. Figure 1. The airflow networks representing a) the cold air is transferred from the sunspase only via the cracks, and b) the warm air is transferred from the sunspace by mean of a fan Air changes [1/h] 2 1,8 1,6 1,4 1,2 1,8,6,4, Air changes[1/h] 2 1,8 1,6 1,4 1,2 1,8,6,4, Figure 2. History of ventilation air flow from the greenhouse to the living room: a) only through the cracks; and b) through cracks and by an additional fan In the thermal analysis of the building behaviour the following cases have been consider both, for the sunspace and for the roof collector: gc when the air flows between the sunspace and the living room via the cracks only, gf when the air flows between the sunspace and the living room via the cracks and the fan, cc when warm air is transferred from the roof collector to the bedroom, cf without additional heat transfer from the roof air collector to the bedroom.

5 6 5 T Living Room T Greenhouse Heating load,8,7 6 5 T Living Room T Greenhouse Heating load,8,7,6, ,5,4,3 Heating load [kw 4 3 2,5,4,3 Heating load [kw,2,2 1,1 1, Figure 3. History of temperatures in the living room and the sunspace compared with heating loads: a) the cold air is transfer via the cracks only; and b) the warm air is transferred by the fan 4 35 Living Room gc Greenhouse gc Living Room gf Greenhouse gf Figure 4. History of temperatures in the living room and the sunspace for the different cases defined in the text Figures 3a and 3b present the resultant temperature inside the sunspace and the room. Additionally the heating load evolutions are compared to find a difference for both cases. When the air temperature in the sunspace exceeds 2 o C the fan starts to operate. The output of the fan is.29m 3 /h. The operating fan causes that the air changes between the greenhouse and the living room increases (figure 2). Warm air from the sunspace heats the air in the living-room by nearly 3K and reduces the heating load about 5% during that period.

6 Figure 5. The airflow networks for: a) no air flow between the collector and the bedroom; and b) the warm air is transferred from the collector by the fan Air changes [1/h] 2 1,8 1,6 1,4 1,2 1,8,6,4 Air changes[1/h] 2 1,8 1,6 1,4 1,2 1,8,6,4, , Figure 6. History of ventilation air flow in the bedroom a) no air flow between the collector and the bedroom b) the warm air is transfer from the collector by the fan 6 5 T BedRoom cc T Collector cc Heating load,6,5 6 5 T BedRoom cf T Collector cf Heating load,6, ,4,3,2 Heating load [kw 4 3 2,4,3,2 Heating load [kw 1,1 1, Figure 7. History of temperature in the bedroom and roof collector compared with heating loads: a) no air flow from the collector b) the warm air is transfer from the collector

7 6 5 BedRoom cc Collector cc BedRoom cf Collector cf Figure 8. History of temperature in the bedroom and collector for the different cases defined in the text A similar effect is noticed for the bedroom connected with the roof air collector. Additional heat coming from the collected solar energy increases the temperature in the analysed room and reduces the conventional heating loads and number of heating hours. In this case the output of the fan is.15m 3 /h and less than for the greenhouse. The output was decreased because of the thirty times smaller volume of the collector. However all of these parameters should be set individually for individual cases. Finally, the effect of additional heat transferred from the collectors on the thermal performance of the rooms was estimated. The efficiency factor, defined as a ratio between the heat gains from the sun and heating energy requirements, was calculated for both cases as follows. Qu η = Q + Q where: Q u is the heat gain from solar energy Q p is the heat supplied by the auxiliary heating device u For the selected two week period the energy demands for heating are 2722kWh for the living room and 1139kWh for the bedroom. The solar energy captured by the collectors and utilised for heating is 18kWh and 35kWh respectively. This gives the following of efficiency factors for the solar air collectors: p η 1 =.38 for the sunspace η 2 =.3 for the roof collector

8 4. IN CONCLUSION The energy potential of a sunspace and a roof collector connected with rooms in a building have been investigated in the present paper for Polish climatic conditions. It was observed that the properly designed solar air collectors could significantly contribute to the reduction of heating load during intermediate heating season periods. However, the solar heat contributions negligible during the whole heating season. But preheating of the ventilating air seems to be feasible. The obtained calculation results in that particular case shown in this paper allow to suggest the following conclusions: 1) The solar energy captured in the collectors can be efficiently utilized to cover the heating energy demand from the second half of March to the end of the heating season. 2) The operating fan increases the ventilation rate by about 3% but the number of air changes per hour was less than 2ac/h during the whole period. 3) Additional heat transferred to the rooms as a ventilation airflow rate increased the internal temperature and decreases the heating loads and the number of heating hours. 4) During the analysed two-week period the total solar energy contribution from the sunspace was three times higher than from the roof collector. 5) The efficiency factor of the large and small volume solar air collectors in this study are.4 and.3 respectively. 5. BIBLIOGRAPHY [1] Solar architecture in Europe, Design, Performance and Evaluation, Brussels - Luxembourg [2] Heating, Ventilating and Air-Conditioning Applications, ASHRAE Handbook, Atlanta, [3] BARTNICZAK J.: Budynek ekologiczny Bartek-8 dynamiczna analiza cieplnego zachowania się budynku zintegrowanego ze słonecznymi kolektorami powietrznymi, praca magisterska, Politechnika Łódzka, 22. [4] CLARKE J.A.: Energy simulation in building design, Adam Hilger Ltd., Bristol and Boston, 1985.