Category Archives: EuroSun2008-4

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).

Estimation of avoided CO2 emissions

The amount of CO2 emissions avoided is proportional to the quantity of PV electricity produced. For the estimation of CO2 emission avoided (Figure 3) we considered that the electricity produced in the schools by the PV systems will be equal in value to the reduction of electricity produced by the Portuguese energy mix. In the Portuguese energy mix 1.2kg of CO2 is emitted per koe of primary energy consumed [7]. The conversion factor between primary energy and electricity is

0. 290 koe of primary energy per kWh of electricity [2]. Therefore, the CO2 emitted is given by:

CO2 emitted [kg/kWh]=1.2[kg/koe] x 0.290 [koe/kWh] =0.35 kg of CO2/kWh (1)

In this estimation, the CO2 emissions due the PV system assembling, manufacturing, and maintenance were neglected.

The CO2 emission trading market is not very stable yet. Still, in the future it will also be relevant to approach CO2 as a financial application. For this estimate we used a reference CO2 value of 24€

per ton of CO2 emitted. In the present case the total emission certificate value is 170-260 K€/year, see Figure 6.

Figure 6 — CO2 emission avoided (in blue) and their financial value (in grey)

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Using data from the International Energy Agency [8] is possible to see that the total PV production at schools will represent 0,06% of the electricity consumption in Portugal, and the CO2 avoided will be about 0,02% of the total Portuguese CO2 emissions.

Radiation transmission parameters

The optical transmittance and transmission coefficients for these films given in table 1 show that these parameters increased with increasing wavelength from 300 nm up to about 800 nm. From this point, these parameters were found to decrease with wavelength. Tables 1, 2 and figs. 1-3 also show that the parameters decreased with increasing thickness of films. For example, maximum visible transmission coefficient for Snl2 film with a thickness of 5.2×10-9m is 9.6x106m-1, that for a thickness of 11.7×10-9m is 8.8×106 m-1 while for a thickness of 16.7×10-9m; it is 9.0x 106 m-1. The values of transmission coefficients for Snl2 films of thickness 11.7×10-9is from 2.7 to 4.7 x o6 m-1 in the UV and from 6.5 to 4.7 x 106m-1 in the NIR. The values of the transmission coefficients for MnBr2 and FeCl2 follow this pattern. High values of radiation transmission coefficients in any region indicate high transmittance (transparency), whereas low values indicate opacity of the films in that region. Thus, the high values of these parameter in the VIS region indicates that the films are highly transparent to visible radiation while the low values of the parameters in the UV and NIR show that the films are opaque in these regions.

Facade solar collectors

The integration of solar thermal collector into building facade brings several essential advantages in comparison with solar collectors mounted separately from building envelope (in front of envelope or on supporting structures above the roof). Facade collectors have been investigated in number of research projects [1-3] to reveal advantageous synergy effects of facade integration. Additionally to the basic function of solar collectors, facade collector serves also as protecting shield against atmospheric effects (weather protection) and improves the thermal properties of the building with respect to passive solar gains in winter season. Furthermore, integration of collectors into building facades leads to aesthetically and visually more attractive solution compared with collector fields placed on flat roofs (most of residential buildings in urban housing estates), which create industrial appearance of the buildings. Facade solar collector can be either thermally coupled with envelope (direct integration) or thermally separated by means of ventilated air gap (indirect integration). Advantage of facade direct integration comes in higher thermal efficiency of solar collector due lower front heat loss through glazing (vertical air gap results in lower Nusselt number than inclined) and due to lower back and edge loss (building insulation layers) at normal incidence of solar radiation.

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Fig. 1. Annual profile of solar irradiation for different slopes (TRY Prague).

 

Behaviour of solar systems with facade collectors has been analysed through usability of solar gains to cover energy demands for hot water and space heating systems and interaction of facade solar collectors with building indoor environment (heat gains through envelope in winter, interior overheating in summer) in detail [4-6]. For climatic conditions in central Europe, the maximum annual irradiation is received with 45° sloped surfaces oriented to the south. In the case of facade collectors with a vertical slope, the reduction in the annual irradiation sum is around 30 %. Fig. 1 shows the annual profile of average daily solar irradiation for roof (45°) and facade (90°) collector based on the test reference year for Prague. The comparison shows a large difference between the summer peak and winter season values for the case of roof solar collector and a relatively uniform profile for the facade collector which corresponds closely to the heat demand profile (approx. constant with a decrease in the summer season, in the case of DHW system). Thus, solar DHW systems with facade collectors require approximately 25 % higher collector area to achieve solar fraction 50 — 60 % compared to optimally sloped collectors (45°). Required faqade collector area becomes equal to roof installations in the case of oversized DHW systems with higher solar

fraction (above 65 %) but with much shorter stagnation periods due lower portion of nonutilisable energy gains in summer [4]. For solar combisystems (DHW and space heating) the situation is different. Due to relatively uniform irradiation profiles, facade collectors provide the same yearly energy gains in comparison with standard roof installations but with radically reduced frequency and level of stagnation conditions [5, 6] for usual design parameters.

Matching the heat demand to the availability of solar heat

An alternative to using a storage vessel to maximise the use of solar heat is to match the heat demand to the availability of solar heat. This can be achieved by allowing appliances like the dish washer or the washing machine to start operating only when sufficient solar heat is available. Instead of setting the start of the washing program, the latest allowable time of stop of the program can be set.

Simulations are carried out for the base case, which includes a 3 m2 vacuum collector and a 150 l vessel. Different DHW and hot fill patterns are compared, including the one described in chapter 3.2.3, a pattern with heating demand only during the day time, one with heating demand only during the night time and one where the daily heating demand for DHW and hot fill is ‘smeared out’ over 24 hrs.

The results show a difference in primary energy savings between the different patterns in the order of 1%. This small figure is due to the fact that even a rather small 150 l storage vessel is an efficient way to match the heating demand to the availability of solar heat. The results suggest that rather than implementing complicated control devices, that may result in delays and therefore annoyance on the part of the occupant, the use of a storage vessel is a better way to maximise the use of solar heat.

General satisfaction

In the beginning of the questionnaire, LESO-SEB occupants were asked to state their agreement with the statement “In general, the lighting in my office is comfortable”. Average agreement with this statement was found to be 87%, a comparably high percentage: Average agreement with the same statement in the US had previously been found to be only 70% [9]. 78% of the 23 study participants think that the lighting within their ADS-equipped office is better than the lighting in other offices where they have previously worked. 22% judge the lighting comparable to their former offices. None of them feel that the lighting at the LESO-SEB is worse compared to the lighting in previous offices.

Theoretical study on a diurnal solar chimney with double air flow

J. Arce1,2,3, J. Xaman2, G. Alvarez2, M. J. Jimenez3, J. D. Guzman3 and M. R. Heras3

1 CIE-UNAM, Centro de Investigacion en Energia, Priv. Xochicalco S/N Col. Centro, Temixco, Morelos, CP 62580, Mexico

2 CENIDET-DGEST-SEP, Department of Mechanical Engineering, Centro Nacional de Investigacion y Desarrollo Tecnologico, Prol. Av. Palmira S/N. Col. Palmira.

Cuernavaca, Morelos, CP. 62490, Mexico

3 CIEMAT, Department of Energy, Energy Efficiency in Buildings Unit, CIEMAT,

Madrid, E-28040, Spain.

Corresponding Author, j earl@cie. unam. mx

Abstract

A theoretical study on a solar chimney for diurnal use with double air flow is performed.

The chimney’s dimensions are; 2.0 m height, 1.0 m width, and 0.15 m depth for both flow channels. The principal element is a metallic plate between double glass cover, one side toward the East and the other one toward the West. Glazing is used in order to decrease the radiation and convection losses to the environment. Two air inputs at the bottom and two air outlets at the top are used. For some established environmental conditions, the energy conservation equations are resolved for each element of the chimney in order to calculate the temperature distribution, the efficiency of the system and the mass flow rate. The model was verified with the results reported in the literature for a reduced problem of a single flow chimney, obtaining very good agreement. For an irradiance of 500 W/m2, a maximum mass flow rate of 0.06 kg/s was calculated for the double air flow chimney.

Keywords: Solar chimney, natural convection, ventilation

1. Introduction

Nowadays, natural ventilation in the design of buildings is one of the important requirements, mainly because contamination or energy savings. Ventilation is the movement of air from the exterior to the interior. It is very important to replace the inner air in any building in order to enhance the air quality and the thermal performance. In the last three decades, the study of ventilation has been increasing among the scientific community. Earlier, there existed very few studies of natural ventilation systems in the literature. This fact may be due to the use of conventional systems, such as air conditioning systems and air heating systems to create artificial environments. However, it is well known that oil is rapidly decreased; besides air pollution is another issue that needs to pay attention. Passive systems such solar chimney may contribute to decrease the use of conventional fuels and also improve the environment of buildings. A solar chimney is defined as a kind of long ventilated heated cavity that should be installed in buildings in sunny places. Small and real scales models have been studied [1, 2-6]. However, the performance of solar chimneys is not completely understood.

2. Objective

The objective of this paper is to study numerically the thermal performance of a full scale two air flow solar chimney and modify parameters that predict the thermal performance of the system under particular environmental conditions.

 

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Glazing 1 ■

Blackened metallic absorbing plate ж

 

Glazing 2

Solar

radiation

 

Solar

chimney

 

Bottom

opening

 

Bottom

opening

 

Room

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(a) (b)

Fig. 1. (a) Schematic diagram of a solar chimney. (b) Physical model.

Towards the full implementation of the Program

2.1 Activities

The implementation program strategy in a jail includes the following activities:

• 600 hours training course on solar thermal addressed to prisoners for the achievement of a

professional qualification accredited by local public authorities,

• a preliminary energy audit of the site,

• a study on the feasibility of the solar thermal plant,

• the adaptation of the standard call for tenders to local peculiarities and needs,

• the evaluation of the projects submitted by the solar companies,

• the involvement of the prisoners in the installation of the plant,

• the monitoring of the energy performances of the system, as required by the Guaranteed Solar

Results (GSR) contract,

• the maintenance of the plant carried out by the prisoners, in cooperation with the technical staff of the

jails.

For a better management of the plants it is expected to install a System Integrated Hardware in order to record the parameters for a real time diagnosis and for proper maintenance.

The system should allow to store the solar systems data and offer the possibility for the institutions involved to carry out the constant monitoring of the plants operating all over Italy.