D. Chwieduk

Inst. of Fundamental Technological Research Polish Academy of Sciences, Swietokrzyska 21, Warsaw, Poland
Institute of Heating Engineering, Warsaw University of Technology, Nowowiejska 21/25, Warsaw, Poland

dchwied@ippt. gov. pl

Abstract

Nowadays, attic apartments with inclined external walls and windows (roof) surfaces have become quite “fashionable” in buildings erected in highly densely populated cities. The influence of solar energy and in consequence energy transfer through windows on the energy balance of such rooms is evident. Calculations of solar radiation incident on building surfaces, including attic envelope have been performed. To describe and solve problems of the dynamics of processes in the building envelope and surrounding, a mathematical model of energy transfer phenomena in opaque and transparent elements has been developed. The developed model enables to calculate energy transferred through windows into/out of the room at any time, and other energy balance elements, including heating/cooling energy needs. Selected results of simulation studies of attic rooms are presented. It turns out that overheating in summer due to high solar irradiation and energy transmitted and absorbed in glazing can be a real problem for rooms in attics.

Keywords: energy transfer through windows, energy balance of a room, rooms at attics

1. Introduction

Analysis of solar energy availability and its influence on the energy balance of attic rooms is the subject of this paper. Nowadays, when a building envelope is designed and constructed according to energy savings measures, with high quality of thermal insulation and other building materials, the heat transfer through the opaque external walls is a minor part in the total energy transport between the outdoor and indoor environment. Important elements of the energy balance of a building are the energy transfer through windows and energy needed for ventilation (supplied with or parallel to fresh air), even if recuperation of waste heat (from ventilation) is accomplished. Energy transfer through windows is especially important for rooms in attics.

Nowadays, attic apartments with inclined external walls and windows (roof) surfaces have become quite “fashionable” in buildings erected in highly densely populated cities. In many cities because of the high costs of land, especially in the city centres (downtowns), developers and construction companies construct buildings using every available square meter of land. If it is not possible to build high buildings because of existing spatial management plan of some regions of the city, then very often the last floor, at attics, is used as living space. However, it is against traditional way of the use of buildings. For many years in the past attics were used as stores and buffer zones, but not as apartment spaces. The living comfort at attics is not as good as at rooms located on floors below attics. The living standard can not be comfortable if no extra protection especially against too much solar radiation in summer is implemented. The problem of overheating of rooms at attics with special location (orientation), because of too much energy transfer through windows, is the main topic of this paper and problem to be presented.

A similar analysis was performed for both building sectors in a typical summer day. In the simulations, the openings added in the Health sector were considered closed, and the air chamber added in the Development sector was ventilated to avoid overheating. The thermal behaviour of the Health sector in summer (Figure 8) shows that, during the sunshine hours, the indoor air temperatures are around 30 °С with an average outdoor temperature of 22.9 °С. Only during the night the building enters inside the comfort zone. In the Development sector the mean air temperature is around 26 °С (Figure 9) with a mean outdoor temperature of 22.9°C..

(a)

Fig. 8. Summer hourly temperature in the Health Sector: (a) North thermal zones, and (b) South thermal

zones.

In this sector, the indoor temperature, even without internal gains, is always greater than the outdoor temperature. In the afternoon, the west windows cause glare and dazzling, because the sunlight incides on the corridor floor from 14PM to 16PM and on the internal wall connecting the corridor with classrooms and services since 14PM to sundown

In both sectors of the building, natural ventilation and ceiling fans are suggested. Aeolian extractors in the ceilings are suggested to remove the warm air. In the Health sector, the crossed ventilation cannot be practiced because of the location of the offices in the center of the building. In summer, when the sun exposition of walls should be minimized, the East and West envelopes of the Development sector are highly exposed. In addition, solar radiation from West will cause overheating and glare in the afternoon, so shading devices are imperative for glazed areas.

4. Conclusions

The improvements to the original design of the CIC building showed to be effective to reduce the heating and cooling loads and to obtain important energy-saving. The strategies to be applied were selected after a careful analysis of the thermal behaviour of the original design, that evidenced an uncomfortable indoor conditions in the Development Sector both in winter and summer: in winter, when high levels of solar collection are needed, this Sector only collects direct solar radiation from East and partially from West, due to the shading of neighbouring buildings; in summer, when direct solar radiation must be avoided, the Sector is unshaded and collecting both from East and West. Furthermore, in the afternoon, the west windows cause glare and increase overheating. The winter thermal behaviour of the Health Sector is better due to the high solar collection in North surfaces.

Some strategies were tested to improve the thermal behaviour of the building. Thus, to heat the offices of the Health sector in winter, the use of openings (1% of the wall area) connecting the North corridor and the offices was added, in order to introduce the hot air into the offices by natural convection. The openings must be closed during the night to prevent inverse air flow. In the Development sector, the addition of a tight air chamber in the roof (with ventilation in summer) was recommended.

In both sectors of the building, natural ventilation and ceiling fans are suggested for the summer period. The installation of aeolian extractors in the ceilings is proposed in order to remove the

warm air. The extractors can remove the warm air during the night. In the Health sector, the crossed ventilation cannot be practiced because of the location of the offices in the center of the building. In summer, when the sun exposition of walls should be minimized, the East and West envelopes of the Development sector are highly exposed. In addition, solar radiation from West will cause overheating and glare in the afternoon, so shading devices are imperative for windows and glazed areas.

It is expected that this project will serve as a demonstrative project, in order to to spread the application of passive strategies and solar equipment in public buildings of social use, and to minimize the impacts on the climate and the environment caused by the consumption of conventional energy. Also the training of professionals and design technicians of the Social Development Ministry, and the training of the people belonging to the cooperative work societies of Villa Zagala CICs in subjects like management, operation and control of CICs buildings is expected.

Aknowledgements

This project was partially supported by PAE (ex PAV) 22559, CIUNSa 1699, PICTO UNSa 36646, and the special project “Mejoramiento de la Envolvente y Equipamiento Solar con Fines Demostrativos y de Capacitacion para un Centro Integrador Comunitario (CIC) del Ministerio de Desarrollo Social en Villa Zagala — Partido de Gral. San Martin, Provincia de Buenos Aires”. SECYT.

References

[1] Righini R., Grossi Gallegos H., Raichijk C., (2004). Trazado de nuevas cartas de irradiacion solar global para Argentina a partir de horas de brillo solar (heliofania). Energias Renovables y Medio Ambiente, 14, 23-32.

[2] Hernandez A., (2003), Geosol: Una Herramienta Computacional Para el Calculo de Coordenadas Solares y la Estimacion de Irradiacion Solar Horaria, Avances en Energias Renovables y Medio Ambiente 7, 19 — 24.

[3] Flores Larsen S., Lesino G. A new code for the hour-by-hour thermal behaviour simulation of buildings. In: Proceedings of VII International Building Simulation Congress 2001, Rio de Janeiro, Brazil, 75-82.

[4] Flores Larsen S., Lesino G. Programa de diseno y simulacion de edificios. In: Proceedings of XI

Congresso Iberico e VIIbero-Americano de Energia Solar, Vilamoura, Portugal, 2002.

[5] LuzSol 1.1. http://www. labeee. ufsc. br/software/luzDoSol. html. Last accessed: July 29, 2008.

The assignment of this project was to design an autonomous PV system for beach cabins. The main criterion for the design was ease of use and ease of transport, since beach cabins are installed and de-installed once a year. The final design consists of different modules. Each module has a different function, ranging from battery, to charge regulator, to radio. A consumer can thus compose his/her own PV solar energy system. The connected Solar Blox form one console with a straightforward user interface in the cabin, instead of a collection of loose components. The use of modules facilitates transport of the system and installation of the system. And since the Solar Blox are specifically designed to be connected to each other, the consumer can be assured that the components are well matched together, thereby improving the reliability of the system.

Solar Blox are not limited for use in beach cabins. During the projects also in other markets (export) were considered for such a product. An order for a Solar Blox based PV system was already placed.

General statements regarding the economy of solar plants are very difficult to make as the reference cost of electricity varies by a factor of 5 worldwide and the efficiency of electrical cooling machines also varies. Solar radiation varies by a factor of approx. 2 between the various Locations we have measured, and additional use of the energy from the solar plant for heating and hot water purposes has to be individually assessed for each project. Grant assistance available for such projects also varies considerably.

However, it can be assumed that most of the projects will achieve a payback period of 5-15 years based on current energy prices. It is important to note that there is a minimum size for solar cooling

plants to be economic. This depends on location and certain conditions, and varies between 1000m2 — 2000m2 of area to be cooled for now.

Diagram 8: Energy Cabin for cooling and heating with solar energy.

The calculation methodology adopted in SolTerm, considers all solar thermal systems as pre-heat systems.

Simple schematic representation of the solar thermal system considered in SolTerm is given in Fig.2:

The main components of the system are:

• solar thermal collector characterized by the parameters: collector aperture area, zero-loss efficiency, heat loss coefficients determined according to EN 12975-2; section 6.1 (steady — state efficiency tests) [7]. The collector area is implemented by multiples of one collector and a data base of certified solar thermal collectors is integrated in the programme although the user can edit its own collector characteristics.

• collector loop including possibility to calculate pipe heat looses and efficiency of heat exchanger

• storage tank, characterized by its volume and heat losses coefficient

The energy delivered by the solar thermal system is a function of climate (climatic data base is included in the software tool) and by the load profile. A hourly load profile can be defined, as well as, week and week-end days and monthly load profiles.

The algorithm used, performs calculations in a simulation basis of the system behaviour in time steps on 10 min. Stratification of the storage tank is considered using an approach similar to the

one described in [6; chapter 8]. Transient behaviour of the system is not considered. The result of one simulation allows for the determination of:

a) monthly values of heat demand, i. e., Qsol, us, m, (calculated according to the load profile defined) where definition of load temperature and cold water temperature is included.

b) monthly values of energy delivered by the solar thermal system, i. e., Qsol, out, m The SolTerm version 5.0 was used in the comparison.

For comparison between the two methodologies, fixed values and procedures where adopted in SolTerm calculations in order to have input values as comparable as possible with the methodologie of EN 15316-part 4-3.

Since SolTerm allows for variations in Load profiles the option was to consider the same load profile seven days of the week with only one extraction between 17-18h.

1.1 Passivhaus standard

The standard Passivhaus was first defined by the Passivhaus Institut in the early nineties. It describes a highly insulated house with extremely low energy consumption for heating and controlled mechanical ventilation with a heat recovery to make the building air tight, to guarantee

The available illuminance level and the daylight autonomy for the three defined strips have been determined. Only the results of the central zones is presented in Figure 6 and 7 because this zone is the most unfavourable one.

One notice that the available illuminance in a classroom with a 10 % porosity of glazing, is higher to the 300 lux value required for d/h ratio lower than 0.65. These values of overhang are incompatible with the requirements of PERENE in term of solar protection. The associated daylight autonomy is slightly above 88%.

For values of d/h above 0.75, the illuminance level is near 250 lux with a daylight autonomy of 87% for d/h = 1.5. One can see that it is difficult to have efficient solar shadings combined with an acceptable daylighting level.

It would be necessary to reconsider the minimum value of illuminance level in the middle of the room. Maybe it is possible to propose a minimum threshold of 200 lux and to fix a mimimum percentage of autonomy in natural lighting around 87%.

4. Conclusion

This study is a first step in the combined taking into account of solar shadings and daylighting in buildings under tropical climates. The influence of solar shading on the daylight autonomy and the illuminance level of a room has been underlined. It is true that it is difficult to combine efficient solar shading with a good daylighting level.

The daylight autonomy in natural lie values of debordfor o ofbelow 0.,6ich are ncompatiwith the requirements of the PERENE standard in terms of solar protection. It would be necessary to reconsider the minimum value of illuminance level in the middle of the roomIt would be nblo of 250 luxd to fix a a percentage of daylight autonomy in natural libe an acceptable compromise. An acceptable

Furthermore, thanks to this study, we are capable of asserting that the PERENE requirement for solar shadings are too demanding to get an acceptable daylighting level. A proposal for reducing the solar factor values for windows is in progress. The results will allow the establishment of design rules for a future thermal standard for the French tropical island of La ReunionFuture studies will focus on other types of solar shadings and other typical rooms such as offices with the consideration of the glare risk as well.

References

[1] Garde F. et al (2005). Elaboration of Thermal Standards for French Tropical Islands : Presentation of The PERENE Project, Clima 2005, october, 2005, Lausanne.

[2] Adelard L. Garde F. (2008). Adaptation des cibles 04, 08 09 et 10 du referential HQE du CSTB au contexte des DOM. Actualisation Fevrier 2008. Convention ADEME/UR.

[3] Fontoymont. M, Tsangrassoulis. A, Synnefa. A (2004). Synthlight Handbook : chapter 2 : daylighting.

[4] INRS. (2005), Soudry C., aide memoire juridique TJ 13 : Eclairage des locaux de travail.

[5] Marchio, Abdessalam, Casari. (1998) Guide pratique pour le tertiaire : maitriser les apports par l’architecture (1998). Editeur : EDF

[6] OFEN., Morel N., Scartezzini J. L. (2003). Daylighting Buildings in the 21st century, IEA Solar task 31: rapport annuel 2003

[7] Sketchup, (2008). http://sketchup. google. com[8] Energy Plus, (2007). http://www. eere. energy. gov/buildings/energyplus/getting. html

[9] DaySim, (2007). http://irc. nrc-cnrc. gc. ca/ie/lighting/daylight/daysim_e. html

2.1. The Danish Building Regulation

New energy regulations, within the Danish Building Regulations from 2006, have been tightened with 25-30 % for new buildings compared with the prior Building Regulations from 1995. With the new energy regulations the energy frame will always have to be calculated for new buildings and includes energy for heating, hot water, cooling, ventilation and lighting. When the energy frame is calculated, electricity consumption is multiplied by 2.5 to compensate for the inefficiency of the power production. Energy production from on-site energy sources are subtracted from the calculated energy frame with the same factors: 2.5 for electricity and 1 for the other energy sources. This makes energy production from on-site PV-cells more profitable per kWh in regards to the energy frame compared to e. g. solar heating.

2.2. Barriers

One of the barriers in implementing PV-systems in buildings is the lack of experience among tradesmen and the cost of the PV-panels. The development of the PV-window is based on overcoming these barriers to make it easier to integrate the PV-window in buildings and to reduce the installation costs and hence make it more attractive to use PV-technology in buildings.

The results derived from building simulations, of window shading profiles in winter, indicate that if shutters remain closed on the North West orientated fenestration, the reduction of solar gains and consequently the drop of indoor temperature is insignificant (0.1 to 0.2 deg. Celsius Table 1, 1.0,

1.1, 1.2). This is mainly attributed to:

i) Area — The window area on North and West sides is limited.

ii) Orientation-There is no direct solar incidence on these facades.

However if South facing glazing is left shaded, in winter, the solar losses are considerable; an area of 10% of shaded South orientated windows causes a reduction of indoor temperature by 2.0 degrees Celsius and reaches 10.0 degrees reduction as the shaded window area increases. The reduction is attributed to the large amounts of solar gains received by fenestration on this orientation, due to the low solar path and the small angles of incidence. This results to rapid

deviation of indoor temperature from comfort levels (Tables 1, 1.3 and 1.4). When all windows are left closed the indoor temperature drops below outdoor temperature (Table 1, 1.4).

The above results emphasise the sensitivity of the manually operated shading devices and the associated uncertainties which could hinder the successful performance of the design of the “Zero Energy House”.

From 1st February 2006 until 31 July 2008, the daily average electrical energy delivered to the grid by the PV systems was 53.7 kWh, corresponding to 23.6 kWh produced in the park and to 30.0 kWh produced in the fagade, Table 8.

The average daily consumption of the Solar XXI building was of 74.6 kWh and the contribution of both PV systems to satisfy this consumption was about 72 %.

Ago 08

Table 8. Daily averages values of the energy produced by PV systems and consumed in the building.

2. Conclusion

The monitoring data of the two PV systems installed at Solar XXI building, shows very good performances. The daily average electrical energy delivered to the grid by the PV systems was 53.7 kWh, corresponding to 23.6 kWh produced by the amorphous silicon modules installed in the park and to 30.0 kWh produced by the multicrystalline modules in the fagade.

The measured average daily consumption of the Solar XXI building was of 74.6 kWh and the contribution of both PV systems to satisfy this consumption was about 72 %.

References

[ 1] A. Joyce, C. Rodrigues, R. Manso, 2001, “Modelling a PV System”, Renewable Energy 22, 275-280, Pergamon, Australia

[ 2] H. Gongalves, A. Silva, A. Ramalho, C. Rodrigues, 2008, “Thermal performance of a passive solar office building in Portugal”, EuroSun2008, Lisbon

[ 3] C. Rodrigues, 1997, “Dimensionamento de Sistemas Fotovoltaicos Autonomos”, master thesis Mechanical Eng., Institute Superior Tecnico, Lisbon