Andreas Fieber, Division of Energy and Building Design,

Department of Construction and Architecture, Lund University P. O. Box 118, 221 00 Lund, Sweden andreas. fieber@ebd. lth. se Phone +46-46-2227347 fax +46-46-2224719 Helena Gajbert, Division of Energy and Building Design, Lund University Hakan Hakansson, Division of Energy and Building Design, Lund University Johan Nilsson, Division of Energy and Building Design, Lund University Tobias Rosencrantz, Division of Energy and Building Design, Lund University Bjorn Karlsson, Division of Energy and Building Design, Lund University

The building industry and the solar energy industry calls for innovative and attractive building integrated active solar thermal and PV systems, in order to widen the acceptance and use of solar energy. As an answer to a widened understanding of building integration, a multifunctional wall element has been developed.

A PV/T component on the inside of an antireflective insulation window with concentrating mobile reflector screens makes the system fully integrated into the building, even its interior. The Solar Window provides PV electricity and warm water, besides passive space heating and day lighting. Simultaneously, the reflector screens act as sunshades and added internal insulation for the window. The reflectors have an optical concentration factor of 2.45, which decreases the required, cost-intensive PV cell and heat absorber area. The hybrid technology has synergetic effects such as cooling the PV cells for increased performance, and making use of heat generated in the cell. The climate protected system is a visible element in the exterior and particularly in the interior, and its performance is directly connected to the user behaviour, due to the operation of the reflectors, which can be switched between a closed, concentrating mode or an open, transparent mode.

Performance of a 1 m2 prototype of the system, regarding its sun shading and U-value properties and its photovoltaic and active thermal output, has been measured. For a two-pane anti-reflective window, the U value is reduced from 2.8 to 1.2 W/ m2K with the reflectors closed. The annual transmittance through the window is estimated to 609 kWh/ m2, of which approximately 10% is expected to be delivered by the PV modules. About 20 % will be delivered as active solar heat and 30% as net passive space heating. The distribution is highly dependent on the daily operation of the reflectors, which to some extent could be automated.

Maurizio Catalano, Dipartimento di Architettura e Urbanistica — Politecnico di Bari — Italy Tel +390805963347, Fax +390805963348, e-mail: m. catalano@poliba. it Guido R. Dell’Osso, Dipartimento di Architettura e Urbanistica — Politecnico di Bari Francesco lannone, Dipartimento di Architettura e Urbanistica — Politecnico di Bari

Introduction

Design and realisation of natural ventilation systems constitute an important subject in the research field on the ability of buildings to respond to climatic conditions. This study aims to evaluate how the stairwell can be an essential element of natural ventilation systems in low-rise buildings.

This research evaluates how parts of buildings act as an indoor microclimate control system. Computational Fluid Dynamics codes (CFD) are used in order to design/verify the behaviour of the building components as a natural ventilation system.

This study focuses on building types that are very common, like blocks of in-line houses (three — to five-storey with a single stairwell and with two apartments on each floor).

The natural ventilation system studied is characterized by easy implementation in energy retrofitting of buildings and by inexpensive installation and management. Furthermore, the related operation is quite easy.

In this study, the main innovation is the different architectural and functional conception of traditional building components, such as the stairwell. The stairwell is not only used as a chimney in order to increase the air-change rate in the cold season, but it is also used as a wind-catcher in summer.

The main results of CFD simulations concern the design of the stairwell openings (location, size, aerodynamic characteristics) and the design of an aerodynamics control system.

The behaviour of the examined natural ventilation system is governed by a very large number of parameters. The results shown concern only a certain number of the typical boundary conditions. However, these results can be useful for the designing of other similar natural ventilation systems.

Future research projects will concern the evaluation of different boundary conditions. The further aim is the definition of the most relevant parameters for the designing of similar systems.

The derivation requires two generalisations: a pressure potential versus flow relationship, and a system pressure drop versus flow relationship. A very simple but useful assumption for the relationship between pressure potential and volume flow is:

pp = KPVm (1)

where Kp = ppref/Vrefm is determined at a reference point (Vref, pref) near the optimum, and m is a negative exponent. Fig. 1 shows pressure potential lines for a few values of m. Note that if m = 0, then pp = Kp, denoting a constant pressure potential.

A useful assumption for the system pressure drop in incompressible flow is: Pl = K|_Vn

where n will typically be 2 when system pressure drop is dominated by minor losses, and closer to 1.75 when the pressure drop is dominated by Reynolds number dependent wall friction losses (White, 2003). The solid line in Fig. 2 represents the system loss curve.

Note that the effect of the variation of density with temperature rise through the system is disregarded, but may be included in the choice of K and n in the vicinity of each operating point. The turbine pressure drop is then: pt = pp — pL = pp — KL V (3)

Since the change in density across a solar chimney turbine is typically small (Apt < 2 %) we can regard the air flowing through the turbine as incompressible, i. e. the fluid power is equal to the product of the volume flow and total pressure drop across the turbine:

P = pt V = (pp — Kl Vn) V (4)

The shaded area in Fig. 2 represents the fluid power. The power generation rate of the turbine depends not only on the characteristics of the flow system it is part of, but also on those of the turbine itself. In the present paper, however, we assume that the turbine efficiency does not vary appreciably with changes in flow rate, or, if it does, the variation in turbine losses may be accounted for in the system pressure losses.

Teclemariam G. Nemariam, Royal Institute of Technology, Dept. of Energy Technology, Div. of Applied Thermodynamics and refrigeration, Brinellvagen 60, SE-100 44 Stockholm Prof. Per Lundqvist, Royal Institute of Technology, Dept. of Energy Technology, Div. of Applied Thermodynamics and refrigeration, Brinellvagen 60, SE-100 44 Stockholm

Abstract

In this paper an analytical study is performed on solar energy utilization in space cooling of a building using a solar driven single-effect absorption refrigeration system. It is modeled with a transient modeling tool TRNSYS, Transient Simulation Program. The main components of the system are, solar collector, hot water storage tank, auxiliary heater, absorption chiller, and different components of building. Two types of solar collectors: double-glazed and evacuated tube collectors are used in this study. Two different locations: Assab, Eritrea and Nicosia, Cyprus are chosen to see how system performance and efficient vary.

The effect of collector area and hot water storage tank volume on the solar energy extraction is studied and discussed. The effect of hot water and cooling water temperatures on the performance of the absorption machine is studied.

The effect of sizes of insulation thickness, shading devices, overhang and wing wall on cooling load of the building is calculated and discussed.

The highest system efficiency is obtained when evacuated tube solar collector is used in both locations, but is higher for Assab for a given collector area. The generator inlet water temperatures for Assab are 86°C and 85°C when evacuated and double-glazed collectors are used respectively, while for Nicosia they are 86°C and 83°C. The yearly cooling load for Assab is 265 MJ and for Nicosia it is 78.6 MJ. The highest cooling load for both locations is obtained during July and for the non-insulated building it is 30.5 MJ for Assab and 16.02 MJ for Nicosia. The lowest cooling load is obtained when 0.2 m insulation thickness and 1.5 m overhang and wing wall is added. The cooling load reduces 34% for Assab and 25% for Nicosia in the first addition of 0.05 m insulation thickness.

Introduction

Electricity and some natural gas are the common energy sources for air conditioning systems. Alternative energy sources are needed for near future since the demand of air conditioning and the cost of energy is increasing. Solar energy is one of the possible alternative energy sources for cooling systems and one of the advantages of using solar energy, as energy source is that the maximum energy is obtained when the cooling load is at its peak.

Absorption refrigeration system is one of the available technologies, which use solar energy as heat source. Most solar-powered absorption cooling projects to date have used single-effect lithium bromide absorption systems. Single-effect absorption system gives best results in the temperature range of 80 to 100oC and limited in COP to about 0.6 to 0.8 [1]. If one considers an absorption refrigeration application system to include the solar collectors, storage tank auxiliary heater, building pumps, piping systems etc, it is not only the absorption chiller, solar collectors and other components which must minimize energy usage, but the cooling load of the building must also reduce as much as possible.

Constructing a hardware model of a building cooling system and performing a test in order to obtain all parameters that are needed for a complete design of a system consumes more time and money compared to that of computer modeling and
simulation. Many researchers have studied and modeled different solar assisted air conditioning systems. Comparison has been made between conventional and solar cooling systems [2], between solar assisted single-effect, double-effect and triple­effect cooling systems [3], between flat plate solar collector, evacuated tube collector and compound parabolic collector [4], and between cooling systems of various combinations of solar collectors and absorption cycles [1]. Most of them were interested in energy supply and cooling systems and the application of each model is limited to a particular condition. The cooling load minimization has not been taken into consideration.

The purpose of this work is to model and simulate a solar assisted air conditioning system for two locations; Assab, Eritrea and Nicosia, Cyprus and comparison has made in terms of optimum collector slope, solar fraction, system efficiency, hot water inlet temperature and cooling water temperature. In addition, the cooling load of the building is calculated with and without insulation, overhang and wing walls. The system is modeled for one year using a TRNSYS program together with meteorological weather data of both locations. System performance of two different collector models: evacuated solar collector and double-glazed selective surface flat plate collector has been done. The optimum system efficiency and solar fraction of each system is calculated and compared based on the appropriate area and slope of collector, size of storage tank, insulation thickness, overhang and wing walls.

In the framework of IEA Solar Heating and Cooling Task 25 activities, a new software is currently developed /6/. The aim was to produce a user-friendly tool, which would allow the user to design a system without extensive software training.

The simulation program calculates the hourly energy demand of the solar-assisted air­conditioning sub-systems. The calculations are performed on an hourly basis and are summed up to calculate the annual energy demand.

The outputs of the software include the following: electrical energy demand for fans, pumps and compressors; energy demand of the (thermal) back-up system; water consumption. The annual total costs of the system are calculated based on the annual energy demand of the components and their investment, maintenance and capital costs.

The building loads can be calculated or imported if evaluated through building simulation programs like e. g. TRNSYS, EnergyPlus, ESP-r.

Several solar assisted air-conditioning technological solutions can be simulated using this tool. Numerous types of solar collectors are available in a selection diagram that already includes all the necessary coefficient/performance settings and a minimum storage capacity calculation routine. The user can define the collector area and orientation. In general, the routines dealing with the solar energy supply allows a detailed calculation of the performance capability of the plant.

The refrigeration module enables calculation of the following types of cold water production: Vapour compression machine, absorption system, adsorption system, free cooling by means of cooling tower, use of well water. Calculations can be made for air-based systems as well as for combined water-air systems. Depending on the selected systems (with/without air handling unit, with/without chiller), the conditioned space can be supplied with mechanical ventilation or infiltration only, and chilled ceiling or fan-coil systems are possible for sensible cooling.

The design tool will be available for free for the first year after publication in summer 2004.

Truus de Bruin-Hordijk, Siuhang Chan and Marinus van der Voorden Building Physics Group, Faculty of Architecture, Delft University of Technology P. O. Box 5043, 2600 GA Delft, The Netherlands E-mail: G. J.de Bruin-Hordijk@bk. tudelft. nl

The world population is still growing and people will always need more space for living. Problems arise especially in the urban environment. One of the possibilities to solve the problems is to make constructions and buildings below the earth’ s surface. However, underground spaces give many people negative associations with cold, dampness and most of all darkness. It feels gloomy and unsafe. People need light, natural light where possible, and they like to be connected with their surroundings.

Daylight is dynamic and gives information about the weather and the time. The possibilities for the use of daylight in underground spaces has been investigated.

1. THE SCOPE OF THIS PAPER

In order to investigate the possibilities for the use of daylight in underground spaces we have limited our study to one underground space with only one light entrance, a tube in the ceiling, to measure the level of illuminance for different dimensions of the space and the tube. One light entrance for one space was a precondition made beforehand, because it is the most simple model and one light entrance above earth surface can be easily integrated into the surroundings or can be hidden by landscaping.

First, preliminary experiments in the daylight chamber of our faculty were done.

After that, a simulation model of an underground exposition space is made with the computer program (desktop) Radiance [1]. Computer simulations with a diffuse sky and a clear sky are done. After the first conclusions, different variants for the light entrance are simulated in Radiance in order to avoid the negative effects of the direct sunlight. At the end of this paper a design concept for the light entrance is shown.

The most convenient, reliable and precise solution to describe and follow the illuminance distribution and efficiency of the skylight is to perform scaled model measurements in the artificial sky. The main advantage of these physical model measurement it is possible to follow and analyse all the necessary and investigated material and geometry properties and characteristics — even without knowing the exact mathematical and physical background of the events. In case of the artificial sky the Input can be precisely determined — since any standard CIE sky condition can be set-up. With the help of the scaled, physical model of the daylighting system — the Output can also be measured enabling to analyse, compare and evaluate all the investigated illumination characteristics of actual skylight. The accuracy of this method meets all practical requirements. The sky itself is a 6 m diameter hemisphere, illuminated around its parameter under its horizontal plane. All required standard sky conditions — i. e. CIE Overcast Sky — can be achieved and maintained throughout the measurements. The daylighting model can be placed in the middle of the sky, and inside the model all the illumination values can be measured and the efficiency of the system can be precisely analyzed.

Existing Daylighting Calculation Methods

The table above (Table 1.) indicates some of the existing investigating methods, and it also lists the properties, taken into consideration to determine the illuminance distribution in the space bellow. None of the listed existing methods or computer software consider all the properties of the skylights, which are playing an important and basic role in the light distribution and efficiency.

These methods are not able to differentiate between shiny or matte surfaces and they are not able to handle all the geometrical properties of a certain light modulating structure. At the calculation of diffused surfaces the transmission values are taken into consideration on a very base level, because these formulae are not including inclination angles of surfaces, which can play very important and determining role in some case.

These facts are proving that the existing calculation methods are all neglecting some important features, this is leading to the necessity to handle the problem using a new and different system of investigation. The model measurements in the artificial sky are able to provide a more exact and global answer to this question.

The thermochemical accumulator (TCA) is an absorption process that uses a working pair, not only in the liquid, vapour and solution phases but also with solid sorbent (Olsson et al., 2000). This makes it a three-phase system, with significantly different properties from the traditional absorption processes, where there are only two phases: either solution + vapour or solid + vapour. Figure 2 shows the schematic of a single TCA unit, which is similar in
principle to that of Figure 1. In a practical unit the vessels are evacuated and the solution is pumped over a heat exchanger to increase the wetted area and improve heat transfer.

During desorption in the reactor, the solution is saturated and further desorption at the heat exchanger results in the formation of solid crystals that fall under gravity into the vessel. Here they are prevented from following the solution into the pump by a sieve, thus forming a form of slurry in the bottom of the vessel.

This gives the TCA the following characteristics:

•

High energy density storage in the solid crystals.

• Good heat and mass transfer, as this occurs with solution.

• Constant operating conditions, with constant ATequ for a given solution temperature.

For discharging, where the process is reversed, saturated solution is pumped over the heat exchanger in the reactor where it absorbs the vapour evaporated in the evaporator using the cooling load. The solution becomes unsaturated on the heat exchanger, but when it falls into the vessel it has to pass through the slurry of crystals, where some of the crystals are dissolved to make the solution fully saturated again. In this way the solution is always saturated and the net result is a dissolving of the crystals into saturated solution.

LiCl Porperties

The first TCA units have been built using water/LiCl as the active pair. The physical properties of this pair have been summarised in the literature (Conde, 2004) and empirical equations have been created for them based on data from a large number of studies over the last 100 years. The solubility line for LiCl can be seen in Figure 3 a), where it is readily apparent that there are several different hydrates for LiCl. However, for the operating range of the TCA, the solution is generally operating at temperatures of 20-50°C for discharge and 65-95°C for charge, all of which are within the monohydrate range for saturated solution. The figure shows that the mass fraction for saturated solution is a function of the solution temperature, and thus Equ. 1 is simplified to Equ. 2 for the TCA. This in practice means that ATequ is constant for a given set of boundary conditions resulting in constant operating conditions during charging/discharging.

ATequ ~ fsat (Tsol) Equ. 2

The equations for vapour pressure derived by Conde were used to create a Duhring chart for LiCl, Figure 3 b). This shows that the maximum value for ATequ is for the saturated solution, and that for the operating conditions of the TCA with an ambient temperature of 35°C, ATequ is 37°C for discharging (comfort cooling), and 53°C for charging.

SHAPE * MERGEFORMAT

Figure 4 shows the relationship of ATequ to the temperature of the saturated solution. ClimateWell have made their own measurements at different times with the mixture of LiCl that they use in the TCA. The lower line shows the correlation ClimateWell use in their control system, whereas the filled squares represent data for the latest measurements. The data from 2004 agree well with Conde’s equations at higher solution temperatures, but deviate somewhat at lower solution temperatures, as can also be seen in Figure 3 b).

A solar-driven ejector refrigeration system consists of two main sub-systems: a solar collector sub-system and a refrigeration sub-system. The major components in the system include solar collectors, a storage tank, an auxiliary heater, an ejector, a condenser, a regenerator, an evaporator, an expansion device and pumps.

The solar collector subsystem

This subsystem consists of solar collectors, a storage tank and an auxiliary heater. Solar radiation is converted to heat by the solar collectors; the heat is transferred to the heat supply medium (water) in the solar collector, then it is stored in a storage tank before being supplied to the refrigeration subsystem in the generator. The auxiliary heater is placed between the storage tank and the generator. The lowest generating temperature of the refrigeration subsystem is set at 80°C. The temperature difference between the heat source fluid (water from the storage tank) and the refrigerant in the generator is assumed
at 10 K. If the temperature of the heating medium is lower than 90°C the auxiliary heater will start and the working medium is heated until it reaches the set point.

In the model of the flat plate solar collector in TRNSYS, the solar collector efficiency is calculated from the heat balance in the flat plate solar collector by the Hottel-Whillier-Bliss equation. It is basically defined in the form of the average Bliss coefficient (FR(xa)e) and the heat loss coefficient (FRUL).

An ejector refrigeration subsystem

The main energy supply to this subsystem is heat, but a small amount of electricity (supplied to the pump) is required to circulate working fluid inside the system. In the refrigeration subsystem, the high velocity vapor stream (from the generator) goes through a converging-diverging nozzle in the ejector resulting in the vapor being sucked from the low temperature evaporator. Suction occurs, as the pressure is low at the narrowest section of the ejector. The stream from the evaporator reaches subsonic velocity. In a mixing zone at the end of the converging section, the two streams are mixed. After mixing, the combined stream becomes a transient supersonic stream and the velocity of the combined fluid must be high enough to increase the pressure after deceleration in the diffuser to a suitable condensing pressure. The vapor from the ejector goes to the condenser, condenses and heat is rejected to the environment. After the condenser, part of the liquid refrigerant is pumped to the generator and the rest goes to the evaporator, reaching an evaporating pressure by the expansion device. The process inside the cycle can be shown in figure 2.

The necessary heat input to the generator (Qg) is

Q = m (h — h . )

g g g, out g, rn

The cooling capacity at the evaporator (Qe) is,

Q = m (h — h. )

e e e, out e, in

At the ejector, the energy balance at the mixing point is written as,

(mg + me) ■ hm = me ■ he + mg ■ hg, exp

The efficiency of the ejector system can be expressed by both an entrainment ratio (•, a ratio between the evaporation mass flow rate and the generation mass flow rate) and a coefficient of performance (COPejc). Neglecting the electricity supply to the pump, the COP of the ejector refrigeration system is defined as the ratio between cooling capacity and necessary heat input.

The ejector is the key component of the refrigeration subsystem, it is used to maintain the pressure difference between the condenser and the evaporator; the better the ejector, the higher the system performance. From the mass conservation, the impulse law and an energy balance around the ejector, the entrainment ratio (*) can also be written as:

# = me 1 mg = (cg 1 cc)“1 = [(h3 — h4 )/(h6 — h5 F -1 (6)

The ejector efficiency selected for this paper is typical for ejector performance as reported in the literature of Lundqvist (1987).

System Performances

Energy inputs to the system are heat to the generator, electricity to the pumps (both in the refrigeration subsystem and in the solar collector subsystem) and electricity to the auxiliary heater. The performance of the system can be defined as the system thermal ratio (STR).

STR =

Qsu

The electricity input to the pump is very small comparing to generator, thus it is generally neglected when calculating the performance can be shown as the system thermal ratio, which product of the collector efficiency and the COPejc.

STRideai = n„ • COPjJc

As detailed in Andersen (2002), three types of graphical representations were developed to provide various visualization possibilities of the transmitted or reflected light distribution features, in addition to a recombined view of the six calibrated images, gathering the latter into a unique orthogonal projection:

• the projection of the BT(R)DF values on a virtual hemisphere, allowing a precise anal­ysis of the angular distribution;

• a photometric solid, representing the BT(R)DF data in spherical coordinates with grow­ing radii and lighter colors for higher values, illustrated in Figure 6;

• several section views of this solid, providing an accurate display of the numerical values distribution.

BRDF visualization*: photometric solid (hemispherical light reflectance* = 0.01)

285

270 I 10.05 255

‘ I.

(a) Opalescentplexiglas, (вг, фг) = (40P, CP) (b) Holographic film (HOE), (вг, фг) = (OP, OP)

An in-depth validation of both BTDF and BRDF was conducted, based on different ap­proaches (Andersen, 2004): [13]

• bidirectional measurements of systems presenting a known symmetry and verification against standard luminance-meter data or analytical calculations;

• empirical validation based on bidirectional measurements comparisons between dif­ferent devices; in case of disagreement, however, no conclusion can be established;

• assessment of hemispherical optical properties by integrating BT(R)DF data over the whole hemisphere and comparison to Ulbricht sphere measurements (Commission Internationale de l’Eclairage, 1998);

• comparison of monitored data with ray-tracing simulations to achieve a higher level of details in the BT(R)DF behaviour assessment.

These studies led to a relative error on BT(R)DF data of only 10%, allowing to confirm the high accuracy and reliability of this novel device.

Conclusions

This paper presents the conception and construction of an innovative, time-efficient bidi­rectional goniophotometer based on digital imaging techniques and combining BTDF and BRDF assessments. To allow reflection measurements, a controlled passage of the incident beam into the measurement space was created, minimizing parasitic reflections around the sample. Openings in the detection screen for the situations where it obstructs the incom­ing light flux were also required, made as small as possible to restrict the produced blind zones; to remove these elliptic covers, a motorized extraction and repositioning system was developed and tested successfully

This design proved efficient and reliable, for both the light beam penetration into the mea­surement space and the passage through the obstructing screen. The high accuracy achieved for BTDF assessments was checked to be kept for BRDF measurements as well, placing re­liance on the assumptions made in the construction of the instrument.

Acknowledgements

This work was supported by the Swiss Federal Institute of Technology (EPFL) and the Com­mission for Technology and Innovation (CTI). The authors wish to thank Pierre Loesch and Serge Bringolf for their contribution in the photogoniometer’s mechanical development.