Category Archives: EuroSun2008-11

Design and Simulation of a High Concentrated Photovoltaic System. with Solar Cells Immersed in a Dielectric Liquid

Xinyue Han1, Yiping Wang1,2, Qunwu Huang1*and Li Zhu2

1 School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China 2School of Architecture, Tianjin University, Tianjin 300072, China Corresponding Author,


In order to further decrease costs of solar electricity, even higher concentrations are needed for photovoltaic concentrating systems. But under high luminous flux, effective cooling is indispensable for concentrator photovoltaic solar cells to keep them operate in high efficiency. In this paper, a high concentrated photovoltaic system with solar cells immersed in a dielectric liquid is designed. The dish is used to provide high concentration and silicon oil is considered as the dielectric liquid to effectively cool the solar cells. The advantages of solar cells immersed in a dielectric liquid and high solar concentration technologies are combined into a CPV system to increase the solar energy conversion efficiency. An analytical model to simulate the thermal behavior of the CPV receiver is proposed. Theoretical analysis shows that concentration ratio, fluid volume flow rate and fluid inlet temperature have great effect on solar cells temperature, and solar cells temperature has little change with environment variables including ambient temperature and wind speed.

Keywords: photovoltaic, high concentration, dish, dielectric liquid, model

1. Introduction

PV power generation systems are attracting considerable attention as sources of electrical power that can replace or complement the current power generation systems that utilize fossil and nuclear fuels. So far, the costs for the solar PV systems are not competitive with other power generation systems. But photovoltaic concentrating systems are a promising path to reduce the cost of solar electricity [1]. In order to further decrease costs, even higher concentrations are needed. However, when concentration of sunlight gets onto solar cells, only a fraction of concentrated sunlight striking the cell is converted into electrical energy (a typical efficiency value for concentrator cells is 27.3% [2]). The larger fraction of concentrated sunlight is converted into thermal energy in the cells which may lead solar cells to high temperature with no cooling aids. It is well known that the efficiency of solar cells decrease with increasing temperature [3]. So cooling of photovoltaic cells is one of the main concerns when designing concentrating photovoltaic systems. Design considerations for cooling systems include low and uniform cell temperatures, system reliability, sufficient capacity for dealing with ‘worst case scenarios’, and minimal power consumption by the system.

Many researchers have attempted to develop HCPV systems and their cooling systems [4,5]. But their costs and reliability for large scale production are yet to be confirmed.

In this work, parabolic dish concentrator was used to provide high-concentration. It is well known that for densely packed cells under high concentrations (> 150 suns), an active cooling system is

necessary. In our HCPV system, solar cells were immersed in a circulated liquid. Solar cells immersed in a dielectric liquid under certain conditions have an increased operating efficiency, and the liquid can provide an effective cooling to the solar cells. Instead, the advantages of solar cells immersed in a dielectric liquid and high solar concentration technologies are combined into a CPV system to increase the solar energy conversion efficiency. The aims of the present paper are: firstly, to preliminarily design a HCPV system and, secondly, to propose and analyze an analytical model that simulated the thermal behavior of receiver.

Measurement Results Pilot Plant in Limburg

During the project two pilot plants have been investigated by the ISFH. The pilot plant Limburg is measured since September 2006, the pilot plant Klein-Koeris (near Berlin) since November 2007. Due to the short measuring period of the HPS Klein-Koeris, only the results of the plant Limburg are presented in this paper.

The HPS Limburg supplies a 300 m2 single-family house with heat and consists of 14 BHE each with a depth of 17 m, a 16 kW heat pump and 44 m2 roof integrated, unglazed solar collectors pro­duced by the company Rheinzink (QUICK STEP SolarThermie).

The measured monthly mean values in Fig. 2 show a strong seasonal variation. Therefore it is nec­essary to analyse the total energy fluxes and the performance factor at least on a one year lasting time scale.




Подпись: CDПодпись: CO s_ CD Q_ E CD 10



The HP delivers 33.3 MWh/a of heat energy, which covers the demand of the floor heating system (70%) and the domestic hot water system (30%). The annual HPS performance factor (HPF) amounts to 4.0, if the power demands of collector and borehole pump are neglected. With these additional consumers the HPF is 3.5.

The measurements show a 20% higher electric power consumption of the HP compared to the manufacturer information. With the promised efficiency of the HP, the HPS would come up to an annual performance factor of 5.0. The measured collector yield of 545 kWh/m2a covers the annual heat demand on the evaporator side of the HP.

Further results can be derived from the measurements:

• The unglazed collector charges the borehole during summer. The borehole reaches a maximum monthly mean outlet temperature of 20°C during HP operation.

• A high specific collector yield of 545 kWh/m2a is reached. Due to collector operation during night with convective and condensing gains the collector yield in December is higher than the irradiation.

• The averaged measured outlet temperature of the BHS is 6°C. Typical design temperature of a HPS without solar thermal system amounts to approx. 0°C.

Подпись: 6:00 10:00 14:00 18:00 22:00 Fig. 3. Temperature at borehole in- and outlet in the course of day (8-Apr-2007, plant Limburg). The figure shows the temperatures while collector pump and HP are running.

On summer days with high irradiation a temperature of 20°C to 25°C is reached at the source side of the HP (Fig. 3). In the following night the source side temperature can drop down to 10°C. This high temperature difference in the course of a day emphasizes the need of dynamic simulations to describe the accurate behaviour of HPS.


The collector model regarding EN 12975-2 is much more complex for unglazed than for glazed SC. In addition to the irradiance and the ambient temperature the wind velocity and the infrared radiation exchange is needed too. For collector fluid temperatures nearby or even below ambient temperature condensation on surface of unglazed collectors has to be regarded. The literature men­tions condensation heat gains amounting to 5% up to 30% of the collector yield. A condensation model was developed [2] and validated by measured data and finally implemented in a TRNSYS collector type for unglazed SC.

Simulations of the Limburg plant show a total share of the condensation gains in the annual col­lector yield of 4%, maximum daily shares of up to 30% have been observed. During the winter period condensation heat gains reaches highest fractions of the collector yield (average value 14%). Considering condensation the difference in simulated and measured collector yield drops from 8.4% down to 5.3% (Fig. 4). It is assumed, that the residual difference is primarily caused by fun­damental problems in measuring the representative wind velocity and ambient temperature over the large area of the unglazed collector array.

2. Simulation Results and Conclusion

The single components and the whole HPS were modelled in TRNSYS and validated using the measured data. A long period of validation is very important, because the operational behaviour of the HPS shows a significant difference between summer and winter.

Subsequent to validation a reference system is built up in TRNSYS and the correlation between annual HPS performance factor (HPF) and the main influencing factors such as collector area, borehole length etc. is investigated. The boundary conditions of the reference system are shown in Tab. 1.

Tab. 1. Boundary conditions of the reference system simulated in TRNSYS



Space heat demand

60 kWh/m2a

Radiator heating


Domestic hot water, 4 persons

170 l/d at 45°C

Heat pump

7.5 kW thermal

Total heat demand

11 MWh/a

Collector tilt angle

45°, azimuth 0° (south)

Weather region

TRY 7 (Kassel)

Heat conductance of the soil

2 W/mK

Fig. 5. shows as simulation results the course of HPF as a function of the borehole length an the collector area. If the power consumption of the collector pump is considered, the HPF is reduced by AHPF = 0.2 (energy class A) and AHPF = 0.4 (energy class D)[1] respectively.

There is not only one target variable for system dimensioning according to Fig. 5. The choice of collector area and borehole length leads to two possible targets: energy saving and investment cost. The standard borehole dimensioning of the reference system based on VDI 4640 leads to a bore­hole length of 70 m and a simulated HPF of approx. 4.0. Starting from this design point the HPF increases from 4.0 up to 4.4 by adding 20 m2 collector area to the HPS. The same improvement may be achieved by using a borehole depth of 130 m instead of 70 m. The same HPF in the refer­ence system may be achieved, if a short borehole of 35 m will be regenerated by a 10 m2 collector array.

Beside saving of investment cost (BH length) and current cost (electric energy consumption) the usage of collector in HPS enhances the planning certainty of the whole system. The curves of the systems with collector are much more flat than the curves without collector (Fig. 5). Thus extreme supercooling of the BHE caused by improper dimensioning of the system can be prevented by in­tegrating a collector. E. g., a difference in thermal conductivity of the soil of 1 W/mK instead of

2 W/mK decreases the HPF of the reference system from 4.0 down to 3.7. The system with 20 m2 collector area is almost unaffected by this modified thermal conductivity. Similar positive effects of the collector may be stated, if the real heat demand of the building is higher than assumed for the design process.

Подпись: Fig. 5. Correlation of HPS performance factor (HPF), collector area and borehole length. Electric power consumption of the pumps is neglected.
Borehole length in m

Beside collector area and borehole length additional parameters influence the HPF. Based on a solar assisted HPS (15 m2 collector area, 70 m borehole length) the influence of other parameters on the HPF is investigated (Tab. 2). Changing the heat conductance of the soil or the weather region effects the system by up to AHPF = 0.2. On the side of the heat consumption (supply tem­perature of the space heating and domestic hot water system) the influence on the HPS is much stronger.

Tab. 2. HPF sensitivity of reference system with collector to changes in system parameters (design point:
collector area 15 m2, borehole length 70 m, HPF = 4.4)

Influencing parameter

Value in ref. system




COP 0/35 = 4.6

WP 1 COP 0/35 = 4.4 WP 2 COP 0/35 = 4.7



Heat conductance of the soil

2 W/mK

3.6 W/mK 0.4 W/mK



Space heating system

Radiator 40°C

Radiator 50°C Floor heating 35°C



Domestic hot water system

Tstorage = (45+3)°C

Tstorage = (60+3)°C without DHW



Space heat demand

60 kWh/m2a

30 kWh/m2a 100 kWh/m2a



Weather region

TRY 07

TRY 15, TRY 06


It may be summarized, that a HPS without SC is much more sensitive towards modifications on the source side. The solar collector in a HPS minimizes the risk which may be caused by unforeseen high heat extraction or a lower ground heat conductance.


The project „Unglazed Solar Collectors in Heat Pump Systems: System Concept and Dimensioning” is funded by ’’Deutsche Bundesstiftung Umwelt (DBU)” (AZ 21098). The authors gratefully acknowledge this support and carry the full responsibility for the content of this paper.


[1] R. Tepe, M. Ronnelid, B. Perers, (2003). Swedish Solar Systems in Combinations with heat pumps, ISES Solar World Congress, Goteborg.

[2] Eisenmann W., Muller O., Pujiula F., Zienterra G. (2006): Metal Roofs as Unglazed Solar Collectors, Coupled with Heat Pump and Ground Storage: Gains from Condensation, Basics for System Concepts. EuroSun 2006 (6th ISES-Europe Solar Congress, 27.-30.6.2006, Glasgow).

Determination of the optimum collector tilt angle

The subsequent simulation runs were carried out in order to calculate the best collector tilt angle for the thermosiphon system under the climatic conditions of Rome, a typical target region for thermosiphon systems. The goal behind these simulations was to achieve the best solar fraction even during winter times. The basis for the simulated collector tilt angles was the latitude of Rome, which is 41° 51’N. Therefore the first tilt angle was chosen to be 42°, all the other angles were simulated in steps of 10° plus or minus. The simulated draw off takes place every evening at 6.00 pm and lasts until the mixed water temperature turns below 45°C.


Table 2. Determination of the best collector tilt angle in terms of hot water production

month collector tilt angle [-]









nthly hot water production [l]









































































































Table 2 shows the simulation results in terms of monthly hot water production. It can be seen that the thermosiphon system is able to produce almost the same amount of hot water in the range from 22°.. .52°. Lower system angles, e. g. 22°, should be preferred if hot water is only needed between spring and autumn, e. g. for campsites. If hot water is needed throughout the whole year, a collector tilt angle comparable to the latitude should be preferred, in case of Rome 42°. With collector tilt angles of 62° and more it is not really possible to increase hot water production during winter times as is known from pumped solar hot water systems.

Simulation software

To test the energy management unit of chapter 4, the behaviour of the overall system has to be simulated dynamically. For this task, the software package Matlab/Simulink with the Carnot-Blockset Toolbox has been used and will be presented at the poster at the EuroSun2008.

3. Conclusions and Outlook

The presented project is supported financially by the Austrian “Klima — und Energiefonds in der Programmlinie ‘Energie der Zunkft’”, project partners are Elektrizitatswerke Wels AG, Wels Strom GmbH, ASiC — Austria Solar Innovation Center, University of Linz and BlueSky Wetteranalysen.


[1] G. Steinmaurer, H. Dehner, R. Mittasch: Solarthermische Fernwarmeunterstutzung Wels: Rahmenbedingungen, hydraulische Einbindung und Lastmanagement“, ‘Gleisdorf Solar 2008’, 3.-5.9 2008.

[2] : G. Steinmaurer, H. Dehner, R. Mittasch: "Entwicklung des Lastmanagements fur eine solarthermische Fernwarmeunterstutzung am Beispiel Wels", 18. Symposium Thermische Solarenergie, Bad Staffelstein, 23. — 25. April 2008.

[3] G. Steinmaurer, „Optimale Steuerung von Leistungsquellen mit Zwischenspeicher“, VDI-Verlag, ISBN-Nr.: 978-3-18-513008-3

Greenius — A simulation tool for renewable energy utilisation

J. Dersch1*, K. Hennecke1 and V. Quaschning2

1 DLR — German Aerospace Center, Linder Hohe, 51147 Koln, Germany
2 FHTW Berlin, Berlin University of Applied Sciences, Marktstrasse 9, 10317 Berlin, Germany
* Corresponding Author, juergen. dersch@dlr. de


Greenius, a software tool for fast calculation of the technical performance and economical figures of merit for different renewable energy systems has been extended to simulate also the usage of concentrating and non-concentrating solar collectors for process heat generation. A case study for a site in southern Spain was made, varying the temperature level of the generated heat. The results show that parabolic trough collectors may provide process heat with similar specific annual yield like CPC collectors at sufficient direct irradiance and at temperatures above 140°C. They offer the potential of much higher annual yields if the can be equipped with vacuum tube receivers, which CPC’s are already using.

The study shows also the flexibility and the broad range of application of the new Greenius version.

Keywords: Renewable energy, Software, Parabolic trough, CPC, Flat plate collector

1. Introduction

The software tool Greenius has been developed at DLR since several years [1] and it is customized to perform fast calculation of the technical performance and economical figures of merit for different renewable energy systems. The calculation is done on an hourly basis for a full year using a typical meteorological data set with this temporal resolution for the specific site of interest.

Technologies already implemented in Greenius are: parabolic trough solar power plants with and without thermal storage, non-concentrating and concentrating PV, dish/stirling systems, wind energy and fuel cells. An additional technology was added now — process heat utilization by non­concentrating and concentrating collectors.

The current version of Greenius offers a great flexibility to define the collector type, the collector orientation, solar field size, storage size etc. Furthermore different load profiles (with hourly resolution for a whole year) may be defined by the user. Heat losses of the field piping are considered as well as cooling down of the whole plant during non-sunshine hours.

Many industries have a large demand for process heat in the range from 60°C to 200°C. At sites with high solar irradiation this demand could be often met by the usage of solar collectors. Due to the high oil and gas prices the usage of solar energy becomes more and more economic and at the same time it reduces the emission of green house gases.


Precondition for an economical usage of solar energy is the knowledge of expected system output and system costs in an early stage of the project. The development of a numerical tool for fast assessment of design and technology options was carried out in the context of the IEA Task 33/IV.

Fig. 1. Sketch of a process heat generating system with parabolic trough collectors

2. Mathematical models

. Functionalities of macro-spatial management

The macro-spatial management is constituted of 5 distinct blocks:

Installed photovoltaic systems and types of application;

Description of type, operacional situation, fault and model of all the components of the installed photovoltaic systems;

Maintanance and training centers;

Phase of installation according to PRODEEM and Infrastructure of transmission lines, roads and airports.

Fig. 3 shows the photovoltaic pumping systems installed in the states of Paraiba (a) and Pernambuco (c). It also shows for each type of application the possibility of a more detailed filtering through a menu where the benefits and some specific aspects can be chosen: in (b), for example, the water pumping systems with higher discharge rate than 6 m3/h installed in Paraiba and in (d), the photovoltaic pumping systems in Pernambuco with manometric heights greater than 30 m.

Fig. 4 shows the batteries (a) and inversors (b) installed in Pernambuco. Additionally it shows the possibility of a more detailed filtering by a menu where specific model can be chosen: in (a) make PVX 12105 batteries and in (b) the Starpower inversors.

Fig. 5 discriminates the photovoltaic systems installed in Alagoas and Pernambuco showing the installation phase according to PRODEEM, in (a) the phase V in Paraiba and in (b) illimination in Pernambuco.

Calculation of thermo-mechanical loads on polymeric collectors

The geometry of polymeric collectors differs from standard flat plate collectors due to material properties, the mentioned production methods and the resulting degrees of freedom in design. Various approaches to design a polymeric collector are under discussion. Mechanical stresses

occur during operation and stagnation due to thermal expansion and external loads on the collectors. These stresses have to be determined for investigating the durability of the product.

The efficiency calculations mentioned above lead to operating conditions of the collector which the chosen materials have to resist. Stress distribution and deformation show potential risks for the stability and durability of the collectors. They are analyzed for various material combinations and geometries with the models used in FEM-Simulations.

Подпись: Fig. 6. Model of extrudable geometry with mesh used in the COMSOL Multiphysics to calculate the mechanical stresses The temperature distribution depends on the position of the absorber layer whereas the temperature level is mainly influenced by the amount of irradiance, inlet temperature and the thermal losses of the collector. These parameters are varied to simulate different operating conditions. The temperature distributions during normal operation and stagnation are calculated using COMSOL Multiphysics. They are compared and validated with the results from the above mentioned simulations and common flat plate collector performances. They are used as input parameters for the numerical simulation of mechanical stresses due to thermal expansion.

Half a fluid channel of two meter length with glazing and thermal insulation was modelled 3- dimensionally including the possibilities to vary certain geometry properties and materials. The mesh and the model geometry used to calculate the mechanical stresses for an extrudable geometry are shown in Fig. 6. Due to symmetry only half of the channel had to be modelled. At both sides of the model the boundary conditions were set to be symmetrical. The calculation of the temperature distribution was done with the general

heat transfer module in COMSOL. The convective top and bottom losses were described with constant heat transfer coefficients. The ambience does not have to be modelled because of these boundary conditions. The convection in the air gaps was neither calculated instead a heat transfer coefficient was applied, too. These parameters were validated with Fluent calculations and literature references. A 2D model of half a collector crosswise to the channels was used for the simulation of an extrudable geometry. Its temperature distribution was transferred from the 3D model.



The distribution of thermo-mechanical stresses shows high gradients in certain areas of the collector. Therefore the mesh was improved by choosing small finite elements in regions with big stress gradients. Due to higher temperatures of its material, the expansion of the absorber is higher than the expansion of the glazing and back frame area. Since the glazing and the back frame are connected to the absorber, the highest stresses appear at the connection between the absorber plates and the channel dividing bars. Fig. 7 shows a close view on the connecting bars. In this example the whole collector is made of the same polymer e. g. PMMA and therefore the deformation is negligible.

A co-extruded collector of two polymers with different thermal expansion coefficients shows an increasing deformation for increased inlet/fluid temperatures. The variation of the inlet temperature and resulting stress distribution are shown in Fig. 8. The images show the distributions at the outlet of the two meter long fluid channel, where the highest temperature gradients appear. In general, the mechanical stresses due to thermal expansion are acceptable for the considered geometries.

The steady state simulations show the maximum stresses and deformations which occur in the simulated operating conditions at one certain point of time and not the deterioration of the material caused by fatigue stresses. Fig. 8. Examples of simulated mechanical stress

distributions during operation and stagnation for an External loads such as snow load were examined co-extrudable collector geometry of polymers with

with a 2D model, as well. Fig. 9 shows the Van — different thermal expansion coefficients

Подпись: Fig. 9. Top load on an extrudable geometry simulated as a 2D model with variation of external loads and thickness of the plates and channel dividing bars

Misses-Stress distributions and deformations. The variation of the bar thickness indicates that the deformations due to top pressures increase strongly for thin bars. In EN 12975 the maximum permitted deformation at 1000 Pa top load is given as 0.5 %. This threshold can be realised by increasing the thickness of the bars to about 2.7 mm for this particular design. For a load of up to 3000 Pa, the maximum stress is in acceptable range regarding PMMA with a bar thickness of 3 mm. It becomes obvious that external operational demands have to be taken into account when designing polymer collector with maximized efficiency.

4. Conclusion

Computational fluid dynamics (CFD) and FEM-Simulations are a useful tool for the identification of characteristic problems of polymer collectors. The analysis of different collector and absorber layer geometries by simulation can be used for optimisation of the parameters regarding efficiency interrelationships as well as for the discussion of durability aspects in terms of mechanical loads. It shows that the heat transfer into the fluid can possibly be improved at higher fluid velocity and further variations of the absorber geometry. The effect of the collector design on thermo­mechanical stresses could be investigated by FEM-Simulations. Main aspects of a polymer collector are now better understood and the outcomes of this work show that both the efficiency and mechanical stability can be further optimised.

5. Outlook

We want to continue with the evaluation of candidate materials and start accelerated and real time tests on selected materials to identify the most promising polymers. Further, the aim is to build demonstration collectors and to perform further tests on durability and efficiency. We would appreciate to do this research in collaboration with industrial partners.

Using numerical simulations we want to calculate and compare the energy output of different designs and material combinations and optimize these designs to develop price efficient collectors of a good quality. Additionally, we want to identify the load levels at various service conditions and investigate different absorber coatings and their application ranges.

The daily temperature cycles and the stagnation situations in the summer, which the collector materials have to resist, cause material aging. Therefore time stepwise simulations using weather data and system operating parameters can lead to information on material aging, and to an estimate of the energy gain by taking into account the changes in the material parameters due to the thermal-mechanical stresses and other degradation appearing at operating conditions in future simulations.


The work was funded by the German Federal Ministry of Education and Research (BMBF FKz:

01RI05201).The authors also want to thank all the partners of IEA SHC Task 39 for their input,

especially the French colleagues from INES and CEA.


[1] Clough, Roger; Billingham, Norman; Gillen, Kenneth (editor): Polymer Durability: Degradation, Stabilization, and Lifetime Prediction. American Chemical Society, Washington 1996

[2] Duffie, John; Beckman, William: Solar Engineering of Thermal Processes. 3rd edition, Hoboken, New Jersey : John Wiley & Sons, 2006

[3] Kohl, Michael; Franke, Hannes; Stricker, Eva; Weifi, Karl-Anders: Polymeric materials for solar thermal collectors — a feasibility study. In: ESTIF: ESTEC 2007, 3rd European Thermal Energy Conference (proceedings). Freiburg : ESTIF, 2007, S. 223-229

[4] Jack, Steffen: Simulationsgestutzte Qualifizierung neuer Konzepte zur Gestaltung von thermischen Solarkollektoren auf Polymerbasis. Freiburg : Fraunhofer ISE; FHTW Berlin, 2008

Validation of Simulation Output

The vacuum tube collector implemented and simulated in OptiCAD was also investigated experimentally at the outdoor test facility of the “Test Centre for Solar Thermal Systems (PZTS)” at the Fraunhofer ISE according to the guidelines of EN 12975-2. The tests include the measurement of the optical efficiency p0 and the IAM-values in the longitudinal and transversal plane.

The following problem has to be considered: The measured optical efficiencies are evaluated with respect to the measured global radiation. However, the simulations in OptiCAD are conducted by applying direct radiation. In order to compare simulated IAM-values with measured ones the specific fraction of diffuse radiation recorded during the measurement has to be taken into account. Moreover, during the IAM measurements — which are outdoor measurements — different levels of diffuse radiation may have occurred. This is considered in the comparison to the simulated IAM- values. In the measurements, also the diffuse radiation is not isotropic but may vary over the hemisphere and the surroundings within the viewing angle of the collector. However, in order to simplify the process the adjustment of the measured values is carried out by assuming a constant isotropic distribution. The tilt angle of the collector is also not taken into account.

Figure 6 shows the simulated transversal IAM of the vacuum tube collector over the incidence angle in transversal direction. Furthermore, the measured transversal IAM-values at their corresponding incidence angle of 20°, 40° and 60° are charted. Those values were adjusted as described above.

It can be seen that the measured values and the adjusted ones are almost exactly the same. It should be mentioned that in the measurements the fraction of diffuse irradiation is rather constant which contributes to the fact that the adjustments are small. Furthermore, the figure shows that the simulation result fits very well the measured data whereby the simulation output can be regarded as validated.

2. Conclusion

Ray-tracing simulations can be used as a powerful tool for the development of collectors [5]. Furthermore, the acceptance of diffuse radiation of a collector expressed as the diffuse IAM can be determined by utilisation of simulated IAM-values for direct radiation.

Once a certain collector type (single glazed flat-plate collector, vacuum tube collector with a given distance of vacuum tubes, with or without reflector, etc.) is modelled in OptiCAD, it is rather quick to carry out parameter variations and investigate different development possibilities. This may concern geometrical and material options. However, the result shown in Figure 6 requires detailed information about the collector parameters. The quality and exactness of the input data directly influence the quality of the results of ray-tracing simulations. With respect to the comparison of simulated against measured IAM-values, the model for the distribution of the diffuse radiation on the hemisphere has to be taken into consideration.


[1] Hefi, Stefan: Raytracing-Untersuchungen fur die Entwicklung von Prozesswarme-Kollektoren. In: 18. OTTI Symposium Thermische Solarenergie. Proceedings. Bad Staffelstein, 23.-25. april 2008, p. 416 — 421.

[2] Rommel, Matthias (Fraunhofer ISE); Weiss, Werner (AEE INTEC): Process Heat Collectors. State of the Art within Task 33/IV. Gleisdorf, Austria: AEE INTEC 2008.

Available: http://www. iea-shc. org/publications/downloads/task33-Process Heat Collectors. pdf

[3] Welford, W. T., Winston, R.: The Optics of Nonimaging Concentrators: Light and Solar Energy. San Diego, California; London: Academic Press, Inc. 1978

[4] TRNSYS 16: a TRaNsient SYstem Simulation program. Volume 5 — mathematical reference. Type 71, p. 5-343

[5] Geisshusler, Simon: IAM (Winkelfaktoren) Raytracing Simulationen von Solarthermischen Kollektoren. In: 17. OTTI Symposium Thermische Solarenergie. Proceedings. Bad Staffelstein, 09.-11. may 2007, p. 248 — 250.

Modelling complex systems within TRNSYS SIMULATION STUDIO

S. Kuethe*, C. Wilhelms, K. Zass, R. Heinzen, K. Vajen and U. Jordan

Kassel University, Institute for Thermal Energy Technology, 34125 Kassel, Germany
* Corresponding Author, solar@uni-kassel. de


Solar thermal systems progress in complexity as the technology advances. Development and optimization of these systems is often carried out with the help of flexible numerical simulation tools like TRNSYS. With increasing complexity, maintaining flexibility and structure of the simulation model is an important issue. Therefore, Version 16 of TRNSYS comes along with the graphical user interface SIMULATION STUDIO. It assists the user in setting up the model. Furthermore, a graphical system representation is available automatically. However, for complex systems containing a lot of components and interconnections the system understanding and usability of the model gets difficult. Due to this fact this paper describes a new structure level introduced to SIMULATION STUDIO. Like TRNSYS itself, it also pursues a modular approach by organizing a model in several subsystems, where a subsystem is a collection of components. These subsystems are connected to each other with uniform interfaces only. This results in modularity on the component as well as on the subsystem level and allows the replacement of subsystems in SIMULATION STUDIO in a simple way. The solar thermal system developed within Task 32 of the Solar Heating and Cooling Programme of the International Energy Agency (IEA-SHC Task 32) has been implemented using the new approach.

Keywords: solar thermal, simulations, modelling complex systems, TRNSYS

1. Introduction

TRNSYS [1] is the most commonly used numerical simulation tool for development and investigation of solar thermal systems. Since Version 16, it comes along with the graphical user interface SIMULATION STUDIO. Besides the assistance of generating the TRNSYS ASCII input file, the graphical representation of the model is supposed to improve the maintenance of it. In addition, it should help to decrease the time for third party users to get familiar with new models. However, with increasing complexity of a model the graphical representation gets confusing due to the crossing of many interconnections between the several components (see Fig. 1). This fact leads to difficulties of the system understanding regarding the interaction between the components.

image320Portugal /

image295 image296 image297

No_Parsons (Type9а) Internal Bams (Type 9a)

Fig. 1: Graphical representation of a complex model with a lot of overlapping interconnections between the

components in SIMULATION STUDIO.

Furthermore, the replacement of system parts like a collector loop of a solar thermal system is not possible in a simple way. SIMULATION STUDIO already has got two options in terms of structuring components. On the one hand components can be arranged on several layers, which can be hidden and unhidden by demand, and on the other hand components can be collected in macros, where the input and output dialog of a macro contains all inputs and outputs of the components within the macro. In the graphical representation the collected components of the macro are replaced by one symbol only. Unfortunately, both features only help to improve the design quality but do not provide simple to handle removals or transfers of complete system parts to other models. Therefore, a new structure level is introduced to SIMULATION STUDIO. It follows the modular concept of TRNSYS by defining subsystems, in which components are grouped. The subsystems are connected via uniform implemented interfaces containing all required inputs and outputs of the subsystems. As interfaces simple EQUATION blocks are used in SIMULATION STUDIO and connections of subsystems are realized via the defined interfaces only. The interfaces are implemented in a way that a replacement of subsystems is possible without deleting or reconfiguring graphical links between the subsystems.

Which way should it face?

Influences of climate, load profile and energy storage on the optimum inclination and orientation for thermal and photovoltaic solar energy


Brian Norton

Dublin Energy Lab Dublin Institute of Technology Dublin, Ireland president@dit. ie Abstract

Climatic and operational factors influencing the optimal orientation of solar energy, primarily photovoltaic, collectors are discussed in the context of relevant previous research. An example is provided of how being in a climate with diffuse components of insolation and winter loads that are generally high gives rise to an optimal inclination with a far greater slope angle than the latitude.

Keywords: Photovoltaics, system design, climate, inclination, orientation.

1. Introduction

When building integrated photovoltaics (BIPV) walls, roofs, and awnings provide fully — integrated electricity generation while also serving as part of the weather protective building envelope [1 — 4]. BIPV can also serve as window shading devices, semi-transparent glass facades, exterior cladding panels, skylights, parapets or roofing systems [5 — 19]. BIPV system output depends on [20 — 22] (i) the availability of and access to solar radiation as determined by climate, inclination and orientation of available building surfaces [23, 24], (ii) PV efficiency and its degradation with time [25], (iii) efficiency of balance of system components. [26], coupling to the electrical network, electrical wiring resistance and voltage drop in diodes [27,28] and (iv) shading, over-shading [29] and accumulation of dirt, dust or snow.

2. Design Issues

Grid-connected PV system economic viability depends on electrical loads and utility prices [30, 31]. PV output varies according to their specific spectral selectivity as direct and diffuse insolation spectra alter with air mass and relative humidity [32; 33, 34], so better spectral matching raises PV efficiency [35]. Larger air mass at low sun angles (the incident spectrum being towards red) decreases PV efficiency. [36]. PV surface reflection loss depends orientation, inclination and location [34, 37, 38]. Measured yearly reflection losses have ranged 6.7% to 10. 8% [39, 40]. Reflection losses are lessened by the inclusion of anti­reflection coatings. Accrual of dirt on a PV surface reduces insolation transmission annually by 2% to 8% [37, 41, 42] but in dry summers could be over 20% [43; 44] depending on the PV surface, local dust sources, cleaning frequency (by rain or manually) [45] and PV surface

inclination [46]. The annual system performance ratio [1] of a roof-mounted BIPV at latitude of 54°N was 18.1% lower than the maximum [47]. At 35.7°N latitude maximum annual energy was obtained for the surface with tilt angle 29°. [48]. For both these locations, these more-inclined optimal inclinations show the contribution diffuse insolation can make to the total solar energy incident annually on a BIPV array. As illustrated in figure 1, there is a broad trend in Europe for diffuse components to be larger (i. e., the clearness index is lower) at higher latitudes [24].


Figure 1. Annual variation of clearness indices with latitude for representative European

locations [24]

For seasonally-tracking arrays, annual PV output can be 94% to 96% of the maximum annual PV output when optimum tilt angle is selected only once a year and 99% of the maximum annual PV output if the optimum angle is adjusted twice a year [49]. Different methods are available to obtain optimum tilt of a PV system based on the latitude, local climates, insolation conditions and energy demand [47, 50 — 56] and location-pecific measurements have been reported of the seasonal dependence of PV system performance [57 — 64]. An autonomous solar energy system is defined as not being grid-connected: such are common from free-standing urban street furniture such as solar powered parking charge maters and solar lit and powered ticketing in bus shelters to remote systems ranging from buildings in isolated locations to solar water pumps in deserts [65 — 66]. Such systems satisfy a particular temporal load pattern in the specific weather variations associated with the prevailing climate. From a solar energy system design perspective, climate and load primarily determine the optimal inclination of collection. This interaction is, however complex as the load a function of the climate either directly or indirectly, in time or magnitude. Using a validated simulation model, the maximum annual system performance ratio of a roof mounted BIPV system at a latitude of 54°N in the UK was found to be for a south-facing surface inclined at 20°.[47]. Figure 2 shows the breakdown of influences that cause the optimal inclination in this particular example to 30° different from the latitude


The example in figure 3 does not have energy storage. The function of energy storage is to enable loads to be met by solar energy systems nocturnally and, more generally, when insolation would be insufficient. The optimal inclination with appropriately sized storage would be the optimal plane position for the collection of both the prevailing direct and diffuse insolation components. Similarly, with storage, there is seldom benefit from non-equatorial — facing (i. e., south-facing and north-facing in the northern and southern hemispheres respectively) orientations. However in applications that make transient use of solar energy such as cooking, incorrect inclination and orientation can lead to poor performance.


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Simulation based energy consumption calculation of an office building using solar-assisted air conditioning

S. Thomas* and P. Andre

Department of sciences and environmental management, University of Liege 185 Avenue de Longwy, 6700 ARLON, Belgium Corresponding Author, sebastien. thomas@ulg. ac. be


To minimize environmental impact and CO2 production associated with air-conditioning system operation, it is reasonable to evaluate the prospects of a clean energy source. The targets of the study are to evaluate cooling energy consumption to maintain thermal comfort in an office building and to point out solar energy to satisfy these cooling needs. Simulations were carried out with three different cooling systems in the same operating conditions to determine as accurately as possible the potential use of solar energy. For comparison purpose, the base case is a classical air-conditioning system (heat pump for cooling, gas boiler for heating). Two other configurations were simulated: a classical vapour compression system fed by photovoltaic panels and electricity grid as back-up and, absorption chiller fed by solar thermal panel field and by gas boiler. In the three chosen locations (Paris, Lisbon and Stockholm), results shown that installing photovoltaic panels on the roof is really interesting from the primary energy consumption point of view.

Keywords: Solar cooling, absorption, photovoltaic, TRNSYS

1. Introduction

The present energy context is characterized by the imminent end of the era of fossil fuels and the environmental impact of their operation. Energy demand growth, local pollution, global climate change, … are problems that must be taken into account. It seems urgent to reconsider our way of life and design equipments as to minimise their energy consumption. Most air-conditioning equipments are currently electrically driven vapour compression systems. According to the International Institute of Refrigeration (IRR) [1], 15% of the world’s electricity is used for refrigeration and cooling. Moreover, the growth of cooling needs and consequently electricity consumption for cooling is an indisputable fact.

Using renewable energy such as solar energy is really feasible for air conditioning in building. Different technologies exist and can be compared based upon different characteristics: energy consumption, cost of the whole equipment, … The study is dedicated to office buildings which require especially high comfort levels and have often high heat gains due to glazed facades and electrical equipment. Small scale (round 5-10 kW) applications became recently market available. For large scale absorption and adsorption chillers the technology is more mature because of existing system driven by waste heat (coming generally from a cogeneration unit). In this work the emphasis is put on available technologies, key equipments are market available and data is taken directly from manufacturer.

The comparison exercise is realized on a theoretical office building. To point out use of solar energy in this building, three test cases have been defined. The first one is used as reference as it is a usual system implementation in real office building. For each case, three locations have been simulated using Meteonorm data files: Paris (Montsouris station), Stockholm (Arlanda station) and Lisbon.







Energy source






Vapour compression system




Sun, Electricity as back-up

Vapour compression system, PV panels


Gas, Sun

Boiler, solar thermal panels

Sun, Gas as back-up

Absorption chiller, solar thermal panels

Table 1. Three cases heating and cooling system

Especially for the last two cases, several aspects are to be addressed in order to provide a suitable solar air conditioning solution:

• Solar collectors

• Refrigeration equipment (chiller)

• Building

• Climate

Simulation of all these elements as well as links between them requires a very flexible simulation software. The dynamic simulation environment TRNSYS [2] is applied in this study. It makes possible the whole system simulation as well as the implementation of new models.