Category Archives: EuroSun2008-3

Model based optimisation of distributed generation with respect to electric grid restrictions

Bernhard Wille-Haussmann*, Thomas Erge, Jochen Link, Christof Wittwer

Fraunhofer-Institut fur Solare Energiesysteme ISE, Heidenhofstrafle 2, 79110 Freiburg, Germany
Corresponding Author, bernhard. wille-haussmann@ise. fraunhofer. de


To integrate high volumes of renewable and decentralised energy resources (RES & DER) into the electricity grids it will be necessary to manage generation and loads much more than today. Within this paper we discuss operation strategies for cogeneration plants, which can contribute to active grid management with high volumes of RES. Typically cogeneration plants (CHP) are thermally driven, which is enforced through guarantied fixed feed-in tariffs (e. g. the KWK-G feed in tariff in Germany). With the growing share of CHP up to 25% of electricity production in 2020 (18% EU [1]), the rules of control devices increase and the feed-in tariffs are expected to change to a flexible structure. To manage cogeneration in the distribution grid we investigated additive variable local tariff components for the electricity production. The influences of different CHP operation strategies to the power quality are evaluated in a load flow simulation of an exemplary low voltage grid. The results show that with the local tariff based operation a bigger part of the electric load profile is covered by local generation and also the variation of voltage in the grid decreases.

Keywords: cogeneration, optimised operation, load flow analysis

1. Introduction

Driven by the liberalisation of the electricity markets and the support for distributed, climate friendly generation technologies, the production system of electricity develops more and more from a centralised to a decentralised structure. Especially lower voltage levels are faced with supply by distributed and/or fluctuating power plants (RES), such as photovoltaic, wind and cogeneration. This leads to new operation requirements in the grid. Management strategies for those grids with a high penetration of DER must be developed [2]. To integrate a great amount of these technologies, the grids will not be allowed to stay passive. They must become active and communication between the different participants is necessary. Those “smart grids” should be designed and managed by model based methods.

Cogeneration plants produce heat and electricity simultaneously. Due to the fact, that electric and thermal demand profiles do not match, a thermally driven CHP can not supply electricity peak demands when needed. With thermal storage systems it is possible to decouple the electric and thermal delivery, which offers a higher degree of freedom for the operation management. In contrast to most renewable generators, whose production fluctuates with the availability of sun or wind, cogeneration plants can be used quite flexibly, as long as the thermal restrictions are respected. This fact offers the possibility to use them in the grid to stabilise the power quality, e. g. the voltage range. With a high

penetration of RES the number of factors to be considered by grid management increases enormously. To manage those complex grids new optimisation and model reduction techniques are necessary to get rid with the complexity. Within the project NetMod [3], which is supported by the German “Bundesministerium ftir Bildung und Forschung”, together with several partners Fraunhofer ISE develops methods for such a management.

Подпись: Fig. 1. Grid and generation structure of a low voltage grid.

In this paper we discuss integration and management strategies for RES in a showcase of a low voltage grid. The structure of this grid is shown in Fig. 1. The grid consists of a long branch which feeds several houses in a rural area. Typically these structures are dimensioned weak, so that the high potential of renewable generation in this area is limited by the transport capacity of the local grid. For this showcase we assumed 4 houses with a nominal power of 10 kW each. At each house also one PV plant with a peak power of 25 kWp is installed. At the last house also a CHP plant with a nominal electric power of 25 kW (pel = 30 %, pth = 65 %) is installed, which supplies a local heat grid. To increase the degrees of freedom for each CHP a thermal storage with a capacity of 3000 l is installed.

In the following chapter we will discuss several possibilities to operate cogeneration plants. Based on this we introduce an optimisation algorithm for CHP operation, which is based on variable tariffs. For the showcase two price functions (global and local) are introduced. The operation of cogeneration only makes sense if the produced thermal energy can be used simultaneously or can be stored for a later use. There fore knowledge about the thermal load profile is necessary to generate optimal schedules. The prognosis of the thermal load will be subject of the following paragraph. Decentralised generation will influence the power quality in the grid. The following chapter evaluates the influence of different operation scenarios to the power quality by a load flow analysis and closes with suggestions for a grid optimised operation of decentralised power plants.

Using concentrators in PV/T approach

Silicon cells are the most expensive element used in photovoltaic systems. This reason and shortage of monocrystalline silicon lead us to developing concentrator approach.

There are a number of companies already acclaims about producing concentrator PV/T systems. Among them are Swedish cleantech company Arontis AB (Solar8, Fig.3, left), Canadian company Menova Energy Inc. (Power Spar, Fig.3, right), Spain company TRIGEN Solar S. L (GSH-20X, Fig.4, left), UK company HelioDynamics (HD16c with possible applications, Fig.3, right) [4].

Concentrator PV/T system built in the framework of research VEI project consists of 2D rotational frame, concentrator made with flat mirror facets and PV/T receiver. The project is devoted to


designing, building and testing of PV/T concentrator system based on solar cells with vertical p-n — junctions (Fig. 5).

Fig. 3 Solar8, left and Power Spar, right.

Подпись: image175

Fig.4. GSH-20X, left and HD16c with possible applications, right.

Подпись: Fig. 5. 3D image of VEI PV/T system.

PV/T receiver includes row (array) of solar cells with vertical p-n-junctions in a case with quartz windows and special cooling liquid inside. PV/T receiver is designed for two side illumination because of ideal symmetrical design of solar cells with vertical p-n-junctions.

1.2. Solar cells with vertical p-n-junctions

Receiving element and solar cells for it are most important parts of concentrator PV/T systems.

The solar cells with vertical p-n-junctions are known as a good alternative of planar concentrator solar cells. They have a number of advantages: high temperature tolerance, low series resistance, bifacial sensitive accepting surfaces suitable for bifacial illumination, low equilibrium temperature,

quite simple technology of manufacturing etc. Low series resistance gives them to work under high concentration ration.

Подпись: Fig. 6. The technology of fabricating solar cells with vertical p-n-junctions (assembling of wafers with p-n- junctions and aluminum and silumin inter-layers foils in stack - on the left, soldered stack with showed cut planes - in the center, vertical p-n-junctions solar cells with external contacts - on the right).
A method of manufacturing solar cells with vertical p-n-junctions using thermal compression bonding with silumin was developed in VEI and defended by Russian patents [5, 6]. The main goal of this technology is to provide a structure for a photovoltaic cell which gives as a result improved characteristics: improving quality of interconnecting soldering, eliminating of compensative influence of aluminum to n-layers, high temperature tolerance and mechanical firmness. The main stages of technology of producing solar cells with vertical p-n-junctions (without antireflection coating) are showed at Fig.6.

According to our approach, the silumin (alloy of Al and Si) layers prevent a penetration of compensative impurities (Al) into n — region. The produced solar cell structures give as a result the high quality mechanical and electrical contacts, high fill factor for I-V-characteristic (more than 0.8), and increased tolerance to high temperature under high intensity and higher tolerance to penetrating particle radiation (for space applications).

For optimization of technology processes the modern high resolution scanning microscopy methods of investigating vertical p-n-structures are used.

Photovoltaic system output

A model was constructed for computation of the maximum power point output of a photovoltaic system from radiation components on the horizontal plane, based on general transposition formulas for beam radiation, the Hay and Davies model for diffuse radiation and assumed seasonal variations in ground reflectance [12]. All radiation components are modified by an incidence angle modifier, expressed by [13], that depends on the effective angles of incidence. The conversion efficiency of the photovoltaic array, dependent on ambient temperature and standard NOCT measurements for solar cells, was modeled from [12]. Standard silicon solar cell parameters were
used and temperature and radiation data were generated for Stockholm with the climate database program Meteonorm 6.0. [14].

Evaluation of a Parabolic Concentrating PVT System

Luis Ricardo Bernardo*, Bengt Perers, Hakan Hakansson and Bjorn Karlsson

Energy and Building Design Division, Lund Technical University, Box118 SE-221 00 Lund, Sweden
* Corresponding Author, Ricardo. Bernardo@ebd. lth. se


The purpose of this study was testing and performance simulation of an innovative tracking hybrid solar system being developed by the Swedish Company Arontis. The Solar8 collector produces both electrical and thermal energy in one system. Its performance was compared with conventional photovoltaic panels and solar thermal collectors working side-by-side which are already on the market. The solar8 sample tested in Lund is a prototype designed for small demonstration projects and further development is ongoing.

The evaluation shows that the thermal collector has an overall heat loss coefficient of 3.1 W/(m[5] [6].°C), an optical efficiency of 65% and an electrical efficiency at 25°C of 8% per active glazed area. If we account the total glazed area instead, the thermal collector has an overall heat loss coefficient of 2.5 W/(m2.°C), an optical efficiency of 52% and an electric efficiency at 25°C of 6%. The electric efficiency of the bare cells is 16%. Annual performance simulations were carried out for the Swedish (Stockholm), Portuguese (Lisbon) and Zambian (Lusaka) climate. From the simulations one can conclude that: Solar8 can be replaced by a traditional PV-thermal collector side-by-side system using less space and producing the same electric and thermal outputs; tracking around one axis placed in North-South direction is considerably better then tracking around an axis set on East-West direction; the global irradiation on a static surface is always higher when compared with the beam irradiation towards a tracking concentrating surface; the ratio between electric and thermal output decreases when Solar8 is moved to the equator.

Keywords: Solar8, Solar Hybrids, Photovoltaic Thermal Concentrators, PVT

Подпись: Fig. 1. Solar8 trough in Energy and Building Design Laboratory, Lund Technical University (LTH).

output is limited by the output coming from the poorest cell since all the cells are series connected. This is one of the challenges to overcome in this new technology. Local diodes installed in each cell can be able to bypass the current over the poorest cells and help reducing the problem with uneven radiation.

It is important to notice that the production of both heat and electricity is favoured by lowering the operating temperature. However a minimum water temperature is generally required by the given application involving higher working temperature on the cells. Due to lower insulation, the hybrid system thermal losses are higher when compared to a normal solar collector. Hence, it is expected that a flat plate hybrid system will deliver approximately 10% less electricity and 10% less heat compared to a thermal collector beside a PV module with the same cells amount [1]. It is also important to have in mind that when an electric load is connected to the PV cells, the thermal efficiency is further decreased since part of the radiation is converted into electricity. [7]

Canadian tests

The testing at NSTF was done indoors with a solar simulator and included panels from three manufacturers, BP Solar, Evergreen Solar and UniSolar. These tests were conducted on the 10

Подпись: Figure 1. PV modules mounted onto transpired collector

square metre (108 ft2) test panel previously used for certification tests of the SolarWall solar air heating system.

Evergreen supplied six of their new 170 Watt panels. Two rows of three panels covered 90% of the 10 m2 test panel.

BP Solar supplied six of their 160 Watt panels which when placed onto the test panel covered approximately 76% of the test panel surface leaving 24% of the transpired panel exposed to the sun.

UniSolar supplied eight of their peel and stick 68 Watt modules which fit onto the test panel in a horizontal configuration and covered 90% of the test panel.

It was decided to perform two sets of tests at two flow rates for each panel, one set at NOCT conditions and the other at the solar thermal conditions. The air flow heat removal rates selected were 36 m3/h. m2 (2 cfm/ft2) and 108 m3/h. m2 (6 cfm/ft2) of gross collector surface which represent low and medium air flows typical for heating ventilation air.

The Evergreen panels and UniSolar panels have similar coverage area of 90% but are significantly different in shape. The Evergreen and BP panels are similar type and shape with the main difference being the coverage area, BP panels only cover 76% of the surface of the transpired panel compared with 90% for the Evergreen ones.


Components sizing

An excel sheet has been created including a methodology to size each machine, heat exchanger, tank and pump of the selected layout. The outputs of this procedure are used for first simulations to assess the performance of the overall system. Depending on the outcome of the simulations, the procedure is repeated and followed by further simulations with the aim to find out the optimal sizes.

First, the sizing procedure requires the selection of the DS, as this choice fixes the hot and cold streams temperatures to be provided by the facility. As already mentioned, radiant panels (40/30 °C) and fan coils (8/13 °C) have been respectively selected for heating and cooling. Secondly, the heat and cold power demand of a user building is needed. As previously said, the monitored heating and cooling power demand of EURAC has been given as input to the sizing procedure. Then, the following main steps are implemented:

• The summer heat demand is calculated on the basis of the cooling load rate that is wanted to be supplied by an absorption chiller and of a typical COP of such machine;

• On the basis of the duration curve (Fig. 3.), the co-generator size is selected in order to ensure 3000 running hours per year at least;

• The biomass boiler size is obtained by the maximum requested heat power minus the heating capacity of the cogenerator;

• The heat exchanger between the facility and the DS is sized depending on the needed heating and cooling power and the desired logarithmic temperature differences, thus the maximum flow rate in the overall facility is determined;

• The mass flows for all the machines are calculated on the basis of their capacities and the temperature gaps they have to provide.

For determining the values to be assigned to some variables, such as the collectors surface and the storage tanks capacity, no defined calculation procedure is applied as this selection can depend on practical aspects, e. g. the maximum building surface which is exposed to the sun for the collectors or the available volume space for the tanks.

Подпись: Solar Collectors 200 m2 1 Cogenerator 200 kWth 1 Biomass Boiler 480 KWth 1Absorption chiller 330 kWc 0 Compression chiller 0 kWc 1 Solar tanks 8,000 l 1 Cogenerator tank 4,000 l image074

The outcomes of the first application of the sizing procedure are used for the first simulations (Fig. 5.): they only represent a starting point for the optimization process, especially for the solar collectors surface, the storage tanks and the absorption chiller size.

Fig. 5. First selection of sizes and simulation of the heating flow demand and of the flows deriving from each

heat production system in a typical winter day.

Electro-Mechanical Structure

The operational subset of the tracking system, named Electro-Mechanical System, is presented in figs 2 and 3.

This structure has two DOF, motorized by stepper motors with incorporated encoders, in order to track exactly the prescribed path.

The mechanical system was designed using standard industrial Aluminium profiles in order to obtain a simple and economic structure.

The mechanical structure is mainly composed by Bosch-Profiles and Aluminium plates.

The two motorized axis are composed by Step-motors assembled to Aluminium shafts.

Figure 2 illustrates the several main components of the mechatronic system:

Part n. 6 = Step-Motor to control axis 1;

Part n. 7 = Step-Motor to control axis 2;

Part n. 8 = Photovoltaic cell (150mmx150mm).

The PLC-tracker has the complete operational management of the tracking system. The main tasks performed by the system are:

— Control of the two step motors;

— Processing the data from both encoders;

— Processing the voltage signal coming from the PV-Cell;

— Processing the data from the external proximity sensors that informs the system about the hard-home position reference.

This PLC controls directly the tracking system and commands all other PV-Panels, from the solar Park, according to the developed algorithm. Figure 5 shows an example of a solar park with several PV-Panels.

Building and PVT System Simulation

1.2. Model setup

Подпись: Fig. 2. IDA ICE model of Sydney house

A model of the house was constructed in both TRNSYS and IDA ICE, allowing comparison of the heating demand simulated by each program. Figure 2 shows the 3D visualisation of the house created in IDA ICE.

The heating energy requirement of the house is determined by implementing a heater with unlimited heating capacity in each zone, setting the mean indoor temperature to 18°C and modelling the house separately from the PVT system. The simulation results for the heating energy delivered to the zones then provide values for the energy required by the zones to maintain an average temperature of 18°C. Heat gains of 3W/m2 due to lighting, equipment and occupancy were included in the modelling. An air change rate of 1 ACH was used for all simulations.

In order to determine the output energy from the PVT system that could be supplied to the house, the TRNSYS Type 567 PVT system was also simulated separately using TMY2 weather data for Sydney and the parameters detailed in Table 1.

Local Management of CHP

Most feed-in laws for decentralised and fluctuating generation guaranty a fixed payment for each kWh fed in. Typically an operator will try to maximise the operation hours of his plant to increase produced electricity and with it his return. This thermally driven operation does not regard any restrictions of the electricity grid. To motivate the local operator for an electrical and thermal coordinated operation of the CHP we suggest a variable and local component to the price curve for the produced electricity, which represents the transport capacity of the distribution grid.


Nanodiagnostics of solar cells with vertical p-n-junctions

3.1. Contact area and scanning equipment

Contact area between two adjacent p-n-n+ junctions is most important for investigation in order to achieve better characteristics and parameters of solar cells with vertical p-n — junctions such as high efficiency and fill factor, low series resistance, high mechanical and thermal tolerance. Cross-section of the contact area is shown at Fig.7.

Nanocenter of the MPEI (TU) is participating in our research activity. Using INTEGRA Prima scanning probe microscope (SPM) we are analyzing quality of soldering processes, relief and physical properties of boundaries of vertical semiconductor structures. SPM gives an opportunity to carry out studies of spatial, physical and chemical properties of objects with the typical dimensions of less than a few nanometers (Fig.8). Owing to its multifunctionality, availability and simplicity, atomic-force microscope has become useful tool.

3.2. Experimental results

Below the several examples of taken images are shown. With help of video-microscope it is very convenient to observe quality of soldering structures at the different magnifications (Fig.9).


Fig.9. Images of SCs with vertical p-n-junctions at the consecutive magnifications (from left to right)

Three-dimensional image taken with help of atomic force microscope is depicted at Fig. 10.


Fig. 10. Three-dimensional image of contact area (10 p scale divisions are shown)

Topology of Al layer and adjacent area is shown at the Fig.11. At the next stage of research we are planning to use scanning capacitance microscopy, which allows mapping variations in electron carrier concentration across the sample surface with the unprecedented sensitivity, but we need to solve potential problems with samples that are too rough.


Fig. 11. Topology of Al layer and adjacent area (scan field 170 p, resolution 670 nm); topology of the surface, marked out by green square at first scan (scan field 70 p, resolution 270 nm); topology of the surface, marked out second (scan field 11p, resolution 40 nm).

2. Acknowledgements

Author thanks Federal Agency on Science and Innovations of RF for support of this research project.

3. Conclusion

Concentrator PV/T technologies are prospective alternative of traditional solar thermal collectors and PV modules.

First results of diagnostic of solar cells with vertical p-n-junctions show high informative and useful data obtained with the help of modern methods of investigation of semiconductors. As for thickness of inter-layers connecting p-n — junction structures in developing SCs with vertical p-n junctions we have a definite reserve i. e. decreasing thickness of inter-layers it is possible to increase sensitive surface of SCs.

It is necessary to improve quality of the surface on the cutting stage of bonded (soldered) structures for improving technology and quality of solar cells and potential using scanning capacitance microscopy for detailed study of carrier’s distribution.


[1] ] I. Tyukhov, S. Clouston, R. Rogers, D. Strebkov, F. Vignola New solar combined concentrator technology in Oregon. Proceedings of 33rd ASES Conference "A Solar Harvest: Growing Opportunities", Ed. R. Campbell-Howe, 2004, July 11-14, Portland, Oregon, USA, p. 223-227.

[2] G. Jongerden et al. Colored solar cell unit United States Patent 7227078, June 5, 2007.

[3] I. Tyukhov, D. Strebkov, Y.. Kuzhnurov, E. Tveryanovich, S. Kivalov Integrating PV/thermal concentrator systems into buildings Proceedings of the 2005 Solar World Congress, Edited by D. Y. Goswami, S. Vijayaraghaven, R. Campbell — Howe, American Solar Energy Society, International Solar Energy Society, CD, ASES 2005.

[4] PVT ROADMAP, A European guide for the development and market introduction of PV-Thermal technology, The 6th Framework Programme, 2006.

[5] V. Simakin, D. Strebkov, I. Tyukhov Silicon multi-junctional solar cells with vertical p-n-junctions: evolution, technology, applications, and new opportunities. 14th Intern. Solar Conference “EuroSun 2004”, Proceedings, PSE GmbN, Freiburg, Germany, v. 3, p. 357-366.

[6] V. Simakin, D. Strebkov, I. Tyukhov Solar cells with vertical p-n-junctions: technology and new and new opportunities. The 2nd International Conference on Physics of Electronic Materials PHYEM’05 Kaluga, Russia May 24-27, 2005, p.149-152.

2. Experiment set up

This experiment was conducted in November 2006 in the solar energy laboratory, Kanagawa Institute of Technology, Atsugi — Japan. This PV/T experiment was a part of a water heating system with a dual-source heat pump system.

As shown in Fig. la, the dimension of PV/T to collect the solar energy is 780x770mm which gave an effective collector area of 0.606 m2. The dimension of PV/T included of frame is 820x790mm. The PV/T is layered by a glass cover, laminated to the PV cell with adhesive layer and the absorber. Fig. lb is the configuration of the water heating system. During this experiment, the heat pump was not operated in order to measure only the PV/T performance. A constant temperature layer is located closed to the PV/T to supply the constant flow rate and constant inlet water temperature. A Data logger is functioned to collect the useful information such as: collector surface temperature Tcoll, inlet water

temperature to the collector T in, outlet water temperature from the collector Tout, ambient air temperature Ta, wind speed Vwind, and solar radiation I. All data were collected from 12.000 to 14.00

for every 30 seconds. This experiment was lead under the condition of constant flow rate and constant inlet water temperature to the collector of 41/min and a 120C, respectively.