Category Archives: EuroSun2008-4

A Numerical Investigation on a New Type Radiation Energy Supply End System

Jianbo. Ren1 , Yiping Wang1,2, Li. Zhu2*, Qunwu Huang

1 School of Chemical Engineering and Technology of Tianjin University, Tian Jin 300072, China
2 School of Architecture Tianjin University, Tian Jin 300072, China
Corresponding Author, zly13920679110@tiu. edu. cn

Abstract

A new radiation energy supply system for space heating and cooling was designed. Heat is conducted out from the metal tube loop buried underneath the corners through adjacent conductive glue layer to the wall and floor constructions, and then is dispersed inside mainly by radiation. The theoretical model of the system was established and the heat transfer process was simulated by the finite element analysis method. The analysis results show that the system has good cooling and heating performance. The space cooling and heating needs

in an energy-saving reconstructed building can be met with the 40°C heating medium in

winter and 10°C cooling medium in summer. This provides possibilities of more generous

requirements on the heating or cooling supply temperature. The heat flux from the floor and wall surfaces respectively contribute 60% and 40% to the total energy supplying in summer. The system performance is affected mainly by the tube diameter, fluid medium temperature. Keywords: Radiation energy supply; Heat transfer model; Thermal flux

1. Introduction

The building sector plays a significant role in the global energy consumption, especially in developed countries it accounts for 30%-50% of the whole energy consumption [1]. Use of radiant floor heat systems as an efficient way to achieve occupant thermal comfort in buildings with low — energy demands has increased.

There have been several studies dealing with floor heating systems in the literature. Weitzmann et al. [2] developed a two-dimensional dynamic model for simulating the floor heating systems. Lin et al.[3] conducted an experimental research on the thermal performance of under-floor electric heating system with latent heat thermal energy storage plate. They also prepared an analytical solution for this problem [4]. Laouadi [5] developed a model for analyzing radiant heating and cooling systems which can be used in building energy simulation software. Recently, Bozkir and Canbazoglu[6] studied unsteady thermal performance analysis of a room with serial and parallel duct radiant floor heating systems using hot airflow. Olesen [7] presented a comprehensive discussion on radiant floor heating systems and investigated on their underlying principles.

For the sake of reducing the investment, cooling supply is added into the floor radiation system to make it a dual-function energy supplier. Corina [8] said that radiant cooling system can be operated in any US climate with low risk of condensation, and can save on average 30% of the energy consumption and 27% of the peak demand compared to the traditional all-air system. Mumma [9][10] discussed the major concerns given about the radiant cooling system, such as condensation, cooling capacity and initial cost, and thus confirmed its relevant advantages.

In this paper, a new type radiation energy supply system for space heating and cooling was put forward. It was designed to meet the energy requirements by utilizing the heat conduction ability of both wall and floor by burying the metal tube in the corner. The paper constructed a two-

dimensional steady-state mathematical model of this system and studied this system by means of finite element method. Then influences of the diameter of embedded pipe, location of embedded pipe, water supply temperature on the temperature distribution and thermal flux on the floor and wall were investigated.

Description of the system

Подпись:
Figure 1 represents the DX-SAHP system studied in this work, which consists of the following components: thermodynamic solar collectors, a hermetic refrigeration compressor, a plate heat exchanger as condenser, an electronic expansion valve (EEV), a hot water storage tank with an auxiliary supplier of heat, heat exchangers inside the building and two water pumps.

A thermodynamic solar collector essentially consists of a flat aluminium plate without any glazing or back insulation and with a selective surface. It presents a centrally located tube running longitudinally where refrigerant gets evaporated thanks to heat absorbed not only from solar radiation, but also from the environment. The evaporated refrigerant is sucked in by the compressor, which drives it to the condenser in elevating its pressure and temperature. The energy rejected by the refrigerant in the condenser heats the water pumped to the storage tank. This tank

supplies hot water to heat exchangers in the building, which achieve and keep thermal comfort conditions in rooms.

Подпись: Fig. 1. Heat pump cycle on the pressure - specific enthalpy diagram

The process undergone by the refrigerant can be represented by the vapour compression cycle illustrated in Figure 2. Here, 1-2 represents evaporation and superheating of the refrigerant at evaporating pressure. The isentropic compression process is represented by 2-3, however we are interested in the polytropic compression process 2-3’. After reaching condensation pressure, the refrigerant condenses until becoming subcooled liquid (point 4). Then, the refrigerant is pumped through the EEV and the throttling process takes place. At point 1, we have a two-phase fluid at evaporating temperature, which is lower than ambient air temperature.

Operational and architectural aspects for building integrated Concentrating PV/Ts

Considering the building integrated Concentrating PV and PV/T systems, there are some operational and architectural aspects. A first notice is that in PV/T systems the cost of the thermal unit is the same either the PV module is crystalline-silicon (c-Si), poly-crystalline silicon (pc-Si) or amorphous-silicon (a-Si). Thus the ratio of the additional cost of the mounted thermal unit per PV

module area cost is different and is almost double in case of using a-Si compared to c-Si or pc-Si PV modules. The complete PV/T systems include the necessary additional components (Balance Of System, BOS, for the electricity and the BOS system for the heat) and therefore the final energy output is reduced by about 15% due to the electrical and thermal losses from one part to the other. Water hybrid PV/T systems can be used during the whole year for the pre-heating of water, since the temperature of water in the water supply network is no more than 20° C, even during summer months. Considering the installation of solar energy systems on building roof or facade, the combination of PV/T collectors with solar thermal systems have some aesthetic problems due to the different size and appearance. The problem can be overcome if there is a harmony in size and if solar thermal collectors have absorbers with same or similar color to the color of PV cells.

image193

Based on the investigated CPV and CPVT systems some new architectural designs have been performed, giving a better idea about the aesthetic integration of them on building structure. In Fig. 7 the design of an industrial building with booster reflectors on the roof shows an example of an effective integration of them on building roof. Another design with collectors on building roof is that of Fig. 8, where CPC type solar collectors with PV/T absorber are used. The Fresnel lenses with linear absorbers are shown in the three designs of Fig. 9. The first design is the case of the sunspace (Fig. 9, left) and the other two designs demonstrate the case of atrium with the absorbers out of focus (Fig. 9, center) and on focus (Fig. 9, right), showing the effect of shading.

image194
The design of Fig. 10 is referred to the integration of CPC reflectors on building structure. The building balconies can be used to put the reflectors, which can have the parabolic form and the reflected radiation to be focused on the back of the front building. In case of using stationary reflectors the focal line is moving up and down, depending on sun height, while in case of moving reflectors the focal line can be stable. In both cases, the sun faced building surface is lighted by the not reflected radiation and the reflected diffuse radiation illuminates the back of the front building.

3. Conclusions

Low concentration solar energy configurations have been investigated and studied regarding the effect of the concentrator type to the performance of CPV and CPVT systems. We have studied flat diffuse reflectors, which provide an almost uniform distribution of solar radiation on PV surface, linear Fresnel lenses, which achieve additionally solar control of interior spaces and CPC reflectors, which can effectively combine PV strips with flat solar thermal absorbers. The absorbing solar radiation increases cell temperature and reduces electrical efficiency, but several modes for efficient and cost effective heat extraction can be applied and the most appropriate, according to the application requirements, can be selected. These new concentrating collectors can be integrated on buildings being adapted with their architecture and contributing to the energy and the aesthetic requirements of it.

Acknowledgements

The contribution of the architect Maria Tripanagnostopoulou to the designs of the paper is warmly acknowledged

References

[1] Gordon J. M., J. F. Kreider and P. Reeves, Solar Energy, Vol. 47, 245-252, (1991)

[2] Klotz F. H., Noviello G. and Sarno 13th European PV Conf. 23 — 27 Oct., Nice, Paris, 372 — 375, (1995)

[3] Fraidenraich N. Progress in Photovoltaics: Research and Applications, Vol. 6,. 43 — 54 (1998)

[4] Poulek V., M. Libra, 16th European Photovoltaic Solar Energy Conference, 1-5 May, Glasgow, UK,

2453-2546, (2000)

[5] Goetzberger A., Proc. 20th IEEE Photovoltaic Specialists Conference, Las Vegas, 1333 — 1337 (1988)

[6] Zanesco I. and E. Lorenzo, Progress in Photovoltaics: Research and Applications, 10, 361 — 376, (2002)

[7] Mohedano R., P. Benitez and J. C. Minano, 2nd World Conference and Exhibition on Photovoltaic Solar

Energy Conversion 6-10 July, Vienna, Austria, 2241 — 2244, (1998)

[8] Garg H. P. and R. S. Adhikari, Int. J. Energy Res. 23, 1295-1304 (1999)

[9] Brogren M., M. Ronnelid, B. Karlsson, Proc. 16th Europ. PV Solar Energy Conf., 1-5 May, Glasgow,

U. K. Vol. III, 2121-2124 (2000)

[10] Nilsson J., R. Leutz and B. Karlsson, 19th European Photovoltaic Solar Energy Conference, 7-11 June,

Paris, France, 2094-2097, (2004)

[11] Edmonds I. R., I. R. Cowling and H. M. Chan, Solar Energy 39, 113 — 122 (1987)

[12] Yoshioka K., S. Goma, S. Hayakawa and T. Saitoh, Progress in Photovoltaics: Research and

Applications, Vol. 5, 139 — 145, (1997)

[13] Zacharopoulos A., P. C. Eames, D. McLarnon and B. Norton, Solar Energy, 68, 439 — 452, (2000)

[14] Nabelek B., Maly M. and Jirka VlRenewable Energy, 1, 403-408 (1991)

[15] Kaminar N., J. McEntee, P. Stark and D. Curchod. 22nd IEEE Photovoltaic Specialists Conference, Las

Vegas, 529 — 532, (1991)

[16] Jirka V., Kuceravy V., Maly M., Pokorny J. and Rehor E. WREC V Part III, 1595-1598. (1998)

[17] Wheldon A., R. Bentley, G. Whitfield, T. Tweddell and C. Weatherby, PV systems. 16th European

Photovoltaic Solar Energy Conference, 1-5 May, Glasgow, UK, 2622-2625, (2000)

[18] Whitfield G. R. and Bentley R. W. 2nd World Conf. On photovoltaic solar energy conversion, Vienna,

Austria, 2181-2184, (1998)

[19] Verlinden P. J. Terao A., Daroczi S., Crane R. A., Mulligan W. P., Cudzinovic M. J. and Swanson R. M.

Proc. 16th European PV Conference, Glasgow, U. K., 1-5 May, 2367-2370, (2000)

[20] Tripanagnostopoulos Y., Nousia Th., Souliotis M. and Yianoulis P. Solar Energy, 72, 217-234, (2002)

[21] Y. Tripanagnostopoulos, M. Souliotis, S. Tselepis, V. Dimitriou, Th. Makris. In CD Proc. 20th European

PV Solar Energy Conf. Barcelona 6-10 June (2005)

[22] Y. Tripanagnostopoulos, Ch. Siabekou and J. K. Tonui. Solar Energy, 81, 661-675 (2007)

[23] Y. Tripanagnostopoulos, M. Souliotis, R. Battisti and A. Corrado. Progress in Photovoltaics-Research

and Applications 13, 235-250 (2005)

[24] Y. Tripanagnostopoulos. Presented in 21st European PV Conf. Dresden, Germany 4-6 Sep (2006)

[25] Y. Tripanagnostopoulos and A. Iliopoulou. Int Conf. 22nd PVSEC, Milan, Italy, 3-7 Sep. (2007)

Zero net energy balance

The passive house PH+ concept is applied to a series of typical office building geometries and possibilities for on-site renewable energy production to reach zero net energy buildings are explored with the following assumptions:

• PV installation on the roof and facades (with total efficiency of 11%)

• Solar thermal system installation on roof and/or south facing facade (total efficiency of 56%)

• Roof cover 90%, facade cover 40%

image298

The results of the parametric study are summarized in Figure 4. Here, energy consumption figures are shown in MWh/a in order to be able to balance the energy with on-site generation. It can be seen that only four building geometries provide enough floor and facade area to produce enough energy to balance the energy demand of the design. It can further be seen that energy concept PH requires only a small amount of heating energy but a large amount of electrical energy. The energy concept with reduced internal gains (PH+) reduces overall energy consumption but increases the amount of heating energy demand. This is still beneficial for zero net energy buildings since PV and solar thermal systems can be applied to the building envelope. Depending on the building form there is enough roof and faqade area to generate renewable energy necessary fulfilling the ZNE definition. In Figure 4 building forms and energy concepts with ZNE (without red and white columns) are buildings with 2 floors (1a and 1b) and 3 floors (2a and 2b). All other building forms do not provide enough building envelope to fulfil ZNE definition. Here, other energy concepts might be applicable in order to reach ZNE.

3. Conclusion

Newly introduced building regulations aim to reduce energy consumption in buildings. In heating dominated climates like Norway this implies more stringent building envelope requirements to reduce heat losses. Insulation and air tightness of the building envelope ensures this but can as a result lead to overheating problems during the summer period. Consequently, especially in buildings with high internal loads like commercial buildings, cooling equipment may be needed that uses additional energy. This will have consequences for the design of energy efficient buildings in Norway in the near future. Bioclimatic design strategies which focus on avoiding active cooling might be applied more regularly. Especially architects and engineers have to be

aware of the consequences of design decisions in regard to building form and envelope. It is proposed that thermal comfort criteria should be developed and coupled to energy calculations. Especially thermal comfort in (a hotter) summer will be reduced and should be put in focus when designing buildings and energy concepts.

The results show that the air tightness of the building envelope and the efficiency of the heat recovery system are the most sensitive parameters (together with the outdoor climate). Therefore, the energy concepts focused on those two parameters. With appropriate energy concepts applied to different building geometries it could be shown that ZNE office buildings are possible. There are some difficulties in implementing the passive house standard to office buildings due to differences in internal heat loads and occupation hours, resulting in differences in heating and cooling demand. Consequently, internal heat loads were reduced (PH+) and this resulted in more building shapes that allow for ZNE.

Another challenge is that climate change predictions for Norway forecast an increase in mean temperature and precipitation [8]. This has the potential to increase the overheating problems in future summer periods and might even extend it to autumn and spring seasons. Especially in western parts of Norway this may also lead to hot and humid summer periods. Further work is needed in order to quantify the extent of energy related implications. Also cost implications have to be studied in order to be able to optimize the design options.

References

[1] I. Andresen, 0. Aschehoug, Bell, M. Thyholt, Energy-Efficient Intelligent Facades. A state-of-the-Art, in: SINTEF (Ed.), SINTEF Building and Infrastructure, Trondheim, 2005.

[2] K. A. Dokka, T. H. Dokka, User Guide SCIAQ version 2.0, SCIAQ Pro — Simulation of Climate and IndoorAir Quality, 2004.

[3] T. H. Dokka, I. Andresen, Passive Houses in cold Norwegian climate, 10th International passive house conference, 2006.

[4] T. H. Dokka, Hermstad, Energieffektive boliger for fremtiden — En handbok for planlegging av passivhus og lavenergiboliger, SINTEF Byggforsk, SINTEF Byggforsk, Trondheim, 2006.

[5] Enova, Bygningsnettverkets energistatistikk 2006, enovas bygningsnettverket, enova, Trondheim, 2007.

[6] M. Haase, I. Andresen, Energy-efficient office buildings in Norway — from low energy standard to passive house standard, Passivhus Norden, Trondheim, Norway, 2008.

[7] M. Haase, I. Andresen, Key issues in energy-efficient building envelopes of Norwegian office buildings, in: C. Rode (Ed.), 8th Nordic Symposium on Building Physics, Copenhagen, Denmark, 2008.

[8] K. R. Lise, A. G., S. Eriksen, K. H. Alfsen, Preparing for climate change impacts in Norway’s built environment, Building Research & Information 31 (3-4) (2003) 200-209.

[9] K. J. Lomas, H. Eppel, Sensitivity Analysis Techniques for Building Thermal Simulation Programs, Energy and Buildings 19 (1) (1992) 21 — 44.

[10] PHI, PHPP 2004: Passive House Planning Package 2004, in: T. P.H. Institute (Ed.), The Passive House Institute, Darmstadt, 2004.

[11] I. Sartori, A. G. Hestnes, Energy use in the life cycle of conventional and low-energy buildings: A review article, Energy and Buildings 39 (3) (2007) 249-257.

[12] TEK, Energi — Temaveiledning, in: StatensBygningstekniskeEtat (Ed.), Vol. TEK 2007, Norsk Byggtjenestes Forlag, 2007.

[13] P. Torcellini, S. Pless, M. Deru, D. Crawley, Zero Energy Buildings: A Critical Look at the Definition, National Renewable Energy Labratory, 2006.

[14] K. Voss, G. Loehnert, S. Herkel, A. Wagner, M. Wambsganss, Buerogebaeude mit Zukunft, Solarpraxis, Karlsruhe, 2006.

[15] T. Wigenstad, T. H. Dokka, T. Pettersen, l. Myhre, Energimerking av nsringsbygg, SINTEF Building and Infrastructure, Trondheim, 2005.

PV Systems

The Solar Building XXI has in the south fagade 96m2 of PV (Polycrystalline), which correspond to 12kWp, plus a 6kWp (Amorphous silicon) in the car parking in front of the building (see figure 12). These two systems are responsible for an overall PV annual production of 12 MWh (in the fagade) plus a 8MWh (in the car parking), which correspond to around 76% of the electricity consumption of the building.

image357

Fig. 12.The Solar XXI Building and the PV system in the car parking.

2. Acknowledgement

To INETI and Program PRIME by the financial support to this Project. To Arch. Pedro Cabrito and Isabel Diniz authors of the Architecture Project and all colleagues which participate in the Project Alves Pereira; Manuel Nogueira, Antonio Joyce and also to all the workers which make it possible.

References

[1] Helder Gongalves, Pedro Cabrito, Passive and Low Architecture Conference Proceedings, Vol. 1 (2006) 821­825.

Simulation model

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A simulation model was a detached house. The plan and elevation views are shown in Fig. 6, the building material specifications in Fig.7, and the schedules of various items in Fig.8. The areas for the analysis were Sapporo(43°4’N, 141°21’E), a cold weather region, Tokyo(35°39’N, 139°41’E) and Osaka(34°41’N, 135°30’E), mild weather regions, and Naha(26°12’N, 127°41’E), a hot weather region. The Standard year of expanded AMeDAS weather data 1981-2000 for each area were used.

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Natural room temperature and thermal load when the roof is assumed to be autonomous perspiration (autonomous perspiration mode) were compared with those of non-perspiration mode in summer (Sapporo: from July to August, Tokyo and Osaka: June — September, Naha: May — October). The specific sense temperature of the autonomous perspiration mode was set to five kinds (20, 25, 30, 35 and 40 °С). The air-conditioning case (living & dining room, master room and children room are air — conditioned) shown in Fig.8 and un-air-conditioning case (all rooms un-air-conditioned, i. e., natural room temperature calculation case) were both investigated, and in the former case, the room set temperature and humidity were 27°C, 60% during the cooling period. The areal supply water temperature, changed monthly, was assumed to be equal to the monthly average outdoor air temperature in each area.

Air channel mass flow rate and average temperatures

The air channel is modelled through a stationary one-dimensional volume, with a linearly temperature variation in the vertical direction. Three different working conditions are possible: q”ef = q”g1f; q”ef ^ q”g1f, and adiabatic condition q”ef; q”g1f = 0. Concerning the nature of the flow, three situations are considered: natural, forced (wind or mechanical fan) and mixed convection. In the natural convection situation, two flow regimes exist: the laminar and the turbulent. Besides, a distinction between thin and wide channels is considered. The governing equation, once integrated, can be expressed as:

mcp (Tf — Tf, i) = {q’”f + qgif ]Wy (6)

Where: Tf is the air average temperature at height y (°K); y is the height (m); Tf is the average temperature at the inlet (°K); m is the mass flow rate (Kg/s); Wis the width of the fa? ade (m) and q”ef and q”g1f are the convective heat sources. They are calculated as: qf = q”cond — q"”g and

q"g1f = q"g1 cond + q””g1. Assuming the Newton law of cooling for the heat convection sources, making

some arrangements and integrating along the fa? ade height we can obtain the average wall and outlet temperatures.

The flow-rate is obtained by equating the sum of the pressure differences which drive the flow (wind and buoyancy) with that of those opposing it (hydraulic and friction losses):

A • ( + ) + B h+J — 1 [(fappR”m )• H ++X Kh ] = 0 (i0)

Подпись:Подпись: Dh is the buoyancy term; B = —w  pv2H2 + H 2SG”

where: H =————- is the dimensionless height; A = —

2R”Dh b P”

is the wind-induced term; S is the stratitification coefficient; Kh are the inlet and outlet hydraulic losses; fapp is the apparent friction factor and Dh is the hydraulic diameter (m). The term fappR” is likewise depending on H+, which means that an iterative process must be set up. The expression of fappR” depends on the boundary conditions, on the flow nature, and on the flow situation: in laminar free convection and symmetric uniform heat fluxes, the correlations of Kaka? [9] were validated through CFD simulations. In the case of adiabatic wall boundary conditions, the correlations of Kaka? [9] are also accepted if G”D < 105. In the case of asymmetric conditions, the new correlation
(see equation 3) is used. In turbulent free convection, the term fappRe is no longer linear dependent on H. For this situation only correlations for forced turbulent fully developed flow exist [6]. In mixed flow convection, a composition of the previous correlations will be used. The pressure difference term (APw) is calculated using a dynamical pressure expression dependent of the pressure coefficients.

Once all the terms are determined, a Newton Raphson method is used to solve Equation 10.

Use of hydrophilic coatings to avoid optical distortion from external. condensation on high performance windows in a northern European climate

A. Roos1* , A. Werner2 and A. Rolandsson3

1 The Angstrom Laboratory, Uppsala University, Box 534, 751 21 Uppsala, Sweden
2 AF-consult, Fleminggatan 7, PO Box8133, SE-104 20 Stockholm, Sweden
3 Pilkington Floatglas AB, Box 530, SE-301 80 Halmstad, Sweden
* Corresponding Author, arne. roos@angstrom. uu. se

Abstract

External condensation on high performance windows is a recent problem occurring during clear nights when the window U-value is very low. The phenomenon begins to appear with U-values below around 1.5 W/m2K. In this paper we show experimental results indicating that using an external low-e coating on a window the formation of condensation can be delayed or even avoided. In another experiment it was shown that a hydrophilic coating on the external surface reduces the light distortion caused by the condensation and also that the formed condensation tends to evaporate faster than from a window without the hydrophilic coating.

Keywords: external condensation, high performance windows, hydrophilic, low emissivity

1. Introduction

In heating dominated climates a low U-value is required to reduce heat losses through windows, preferably in combination with a high solar heat gain. A fundamental drawback with very low U — values is that they can lead to the formation of water condensation on the external surface of the window in certain weather conditions. The condensation forms small drops of water on the glass surface, which cause refraction of transmitted light to the extent that the view through the window is sometimes completely obstructed. This effect has led to a surprisingly high resistance against energy efficient windows in several countries. Many customers have been given the advice to avoid the very best windows in order to avoid “the problems with external condensation”. Yet the problem is not really a very big one since the phenomenon only occurs during special weather conditions with high humidity, a clear sky and no wind. The condensation is predominantly formed during periods of falling outside temperature (evening, night, early morning) so that the dew point of the outside air is only a degree or so below ambient temperature. This means that the surface temperature of the glass pane falls below the dew point due to radiative cooling and the low heat losses from the inside are insufficient to heat the surface above this dew point. Condensation is thus formed in the same way as on the windows on a car parked in the street during the night. On building windows it mainly happens during autumn and spring and on average over the year only a few days per month, depending on the nature of the surroundings and on the actual U-value of the window [1-3].

Modern low-e coating technology has led to this situation with window U-values being well below 1 W/m2K for the best windows [4]. In this paper we show that modern glass coating technology can also be utilized to avoid the problem without sacrificing the high performance of the glazing.

Two concepts have been tested. Since the surface temperature of the glass pane falls below the dew point of the surrounding air due to radiative cooling, one obvious solution to the problem is to use a low-emissivity surface on the external surface. This keeps the surface temperature of the glass surface above the dew point and condensation is never formed [5].

Another concept that has been experimentally studied is to use a hydrophilic coating on the external glass surface. Such a coating has almost the same emissivity as uncoated glass (around 0.84) and does not influence the surface temperature of the glass pane. Thus condensation is formed at the same weather conditions as for uncoated glass. However, due to the hydrophilic nature of the surface the wetting angle of the water drops is very low and the condensation tends to form a sheet of water rather than individual drops on the surface. This means that the optical distortion caused by light refraction in the water drops is reduced, and the external view through the window is only marginally distorted [6].

2. External condensation

Подпись: 1 2 3 4 5 6 7 8 9 10 11 12 Month Fig. 1. Number of hours with condensation during a year in Stockholm for a window with U-value 1 W/m2K and an external surface emissivity of 0.85

In northern Europe external condensation predominately happens during the spring and autumn, and when the centre-of-glass U-value is below around 1.5 W/m2K. It becomes more frequent as the U-value decreases. Even with a very low U-value the total number of hours with external condensation is small, and it is only a problem during those hours when somebody is present. A number of attempts have been made to simulate the occurrence of external condensation. This can be done by calculating the temperature of the external surface hour by hour and count how many hours this temperature is below the dew point. Such calculations can lead to a frequency diagram as shown in Fig. 1 [7]. The calculations indicate the number of hours condensation can be formed, however, and there is an uncertainty since the time it takes for the condensation to evaporate is more difficult to calculate. Once condensation has been formed it can stay for a long time even when no more humidity is condensed on the surface. In Fig. 1 there are more hours of condensation in the summer than in spring and autumn. A more detailed analysis of exactly when condensation occurs also reveal that condensation is mostly formed during the very early hours in the morning before sun rise. Summer condensation is therefore usually not noted since it is gone by 7 am.

3. Experimental

3.1. Condensation detection test box

In the investigation a number of different surfaces were tested in a specially designed test box [8]. The tested surfaces were tilted 45 degrees towards the sky to optimize the growth of condensation, and the box was heated internally to simulate the heat flow through the window in a real situation. The condensation was detected by a light deflection system. A beam of light was going through the tested glass pane to a detector inside the box. When condensation was formed the light beam was scattered and the detected signal decreased. During the experiment the temperatures of the tested panes were measured using thermocouples attached on the internal test surfaces. The external and internal air temperatures were also measured.

The Solar House

The Centre Product Design for Sustainable Development (PDSD) develops research and advanced human education on specific issues focusing on solar energy conversion and environment, [11]. The Centre hosts solar-thermal and solar PV systems, [12], installed on the roofs that are used for research on increasing the efficiency of the solar energy conversion and adapting to the mountain clime, Fig. 1:

The Solar House, under construction, was designing considering the use of the solar-thermal system

(1) and heating pump (8) for heating.

The solar thermal system (S/T) will provide domestic hot water during summer and heating agent in the radiant flooring, fully meeting the heating demands during autumn and spring and partially, during winter, Fig. 2. It comprises of six flat plate collectors and three vacuum tube collectors, two heat storage tanks (1000L), a bi-valent boiler for domestic hot water (400L), and the auxiliary safety parts.

The heating pump (HP) provides supplementary heating during winter and cooling during summer,

Fig. 3. It has a 600m long pipe circuit, a collector/distributor, the soil-water pump (10kW) a buffer heat

image223

accumulator (400L), a bi-valent boiler (300L), and auxiliary parts. As back-up source a condensation heater, using methane gas is installed. The solar-thermal system and the heating pump are designed as independent systems, with a monitoring and data acquisition line installed for each.

The Solar House has, beside the energy provided by the above-mentioned systems, a specific design considering the passive use of the solar radiation. The egg-shape of the house, Fig. 4, favours controlled warm and cold air circuit supporting the heating/cooling loads during the extreme temperatures in summer and, respectively, in winter. The transparent walls (double glazing fenestration) have specific role in lighting and in thermal insulation and are designed with blinds automate operated during warm periods. The radiant flooring acts as a thermal mass, storing the heat resulted as the greenhouse effect during sunny winter days.

S/T

PV

Fig. 4. The Solar House

The 10 kWp PV array in the Centre is grid-connected but an energy balance based on the power used in the Solar House and the power fed in the grid can complete the global picture of the energy consumption in the House.

Meteorological data

A comparative analysis of summer meteorological data measured in the Geneva region over the 1990-2005 period yields following results [1], as illustrated in fig. 1:

• As well in urban as in rural areas, classified summer temperatures are very similar from one year to another, never exceeding 35°C. As a notable exception, 2003 however characterizes by around fifteen days with peak temperatures exceeding this threshold.

• Night temperature always drops lower in rural than in urban areas, whereas day temperatures do rise to similar peaks.

• To the contrary of the dry temperature, the wet bulb temperature of 2003 remains close to that of other years. This indicates a stable potential of evaporative cooling, to the contrary of direct night cooling and other techniques examined further down.

2. Building

Representative of the administrative building stock, the architectural typology is that of a low depth building, with 20 m2 / 50 m3 offices distributed on both sides of a broad central corridor. In terms of simulation, this typology results in a thermal model made up of three zones: two offices, on opposite facades and separated by the central corridor, with lateral boundary conditions given by identical interior climate (neighbor offices).

The parameters that govern the thermal behavior of the building are as follows:

• Thermal mass is mainly determined by 28 cm thick slabs, in heavy option (full concrete: 510 kJ/K. m2) or medium option (combined wood structure with concrete filling: 350 kJ/K. m2). In both cases, separation walls between offices add an additional 170 kJ/K. m2 (relative to ground surface).

• Thermal insulation is any of low 1980’s quality (6 cm, double glazing windows), or high quality as given by the Swiss Minergie standard (20 cm, triple glazing insulating windows).

• Solar access is determined by an E-W orientation on a low 5° horizon, along with a 50% window-to-wall ratio. Efficient external solar protection (overall g-value: 13% with 1980 windows, 7% with Minergie windows) are activated when direct radiation on the facade exceeds 10 W/m2.

• Internal gains are 10, 20 or 35 W/m2 (during occupation: 8-18 h).