Category Archives: EuroSun2008-3

Summary and conclusions

PV-Wind-Hybrid systems are for all locations more cost effective compared to PV-alone systems. Adding a wind turbine halves the net present costs (NPC) for the coastal locations in the south of Sweden and cuts the NPC by one third for a location as Borlange with low wind speeds. The load that has to be supplied has of course a large impact on the system size and costs. The results from the simulations show that the NPC for a hybrid system designed for an annual load of 6000 kWh will vary between $48,000 and $ 87,000. Sizing the system for a load of 1800 kWh/year will give a NPC of $17,000 for the best and $33,000 for the worst location.

However, theses values are calculated for a capacity shortage allowance of 10%. The question is of course if such a shortage is acceptable in a single family house and if not what means could be applied to supply the remaining 10% and what would this cost. These questions have not been studied but as Figure 4 shows for most location it would increase the cost significantly if the last 10% should be supplied with the PV-Wind system. The cost per kWh electricity produced by a PV-Wind-Hybrid system varies between 1.4$ for the worst location and 0.9$ for the best location.

References

[1] Borowy, B. S., and Salameh, Z. M. (1994). "Optimum photovoltaic array size for a hybrid wind/PV system." Energy Conversion, IEEE Transaction on, 9(3), 482-488.

[2] Celik, A. N. (2002). "Optimisation and techno-economic analysis of autonomous photovoltaic-wind hybrid energy systems in comparison to single photovoltaic and wind systems." Energy Conversion and Management, 43(18), 2453-2468.

[3] Koutroulis, E., Kolokotsa, D., Potirakis, A., and Kalaitzakis, K. (2006). "Methodology for optimal sizing of stand-alone photovoltaic/wind-generator systems using genetic algorithms." Solar Energy, 80(9), 1072-1088.

[4] McGowan, J. G., Manwell, J. F., Avelar, C., and Warner, C. L. (1996). "Hybrid wind/PV/diesel hybrid power systems modeling and South American applications." Renewable Energy, 9(1-4), 836­847.

[5] Protogeropoulos, C., Brinkworth, B. J., and Marshall, R. H. (1997). "Sizing and techno-economical optimization for hybrid solar photovoltaic/wind power systems with battery storage." International Journal of Energy Research, 21(6), 465-479.

[6] Berruezo, I., and Maison, V. (2006). "Electricity Supply with PV-Wind Systems for Houses Without Grid Connection," Master thesis, Hogskolan Dalarna, Borlange.

[7] Pazmino, V. (2007). "PV-Wind Energy Hybrid Systems Techno-Economic Feasibility Analysis for Different Swedish Locations," Master thesis, Hogskolan Dalarna, Borlange.

Numerical analysis

2.1. General specifications

The configuration of the PVT module consists of a row of photovoltaic cells with a rectangular surface area of 1cm x 1m, placed at the top of the aluminium heat sink. The encapsulation can be divided into various elements.

1. — In the surface at which concentrated radiation is received, an EVA film is applied to the cells, and high absorption glass with low iron content is used as an outer skin. This reduces deterioration of the cells and minimises the thermal losses through the top of the module.

2. — Between the cell and the heat sink a strip of electrical insulation is inserted using a double sided adhesion (Chomerics Thermattach T404). This method considerably-simplifies the adhesion process, as it simultaneously serves to insulate the cell and to fix it in right position.

3. — Finally, the lateral and underneath faces of the heat sink are thermally insulated-with a plate of temperature resistant polypropylene.

image083 Подпись: Fig.2. Simulation Scheme.

The figure 2 shows a scheme of the proposed cooling system. Concerning to the boundary conditions they must be fixed in the numerical study. In the outer upper face of the cooling device a, Neumann boundary condition of 1000W/ m2 is applied. This represents the heat flow per unit surface area that the cells transmit to their back contact. The remaining lateral boundaries are considered to be adiabatic surfaces, assuming that the insulation is sufficiently wide to achieve this requirement. Finally, at the surfaces that represent the entry and exit of the fluid flow, the boundary conditions are fixed flow at the entrance and free flow at the exit.

As a line of symmetry exists in the geometry (Fig.1) only half of the system will be modelled.

The tube is made of aluminium (thermal conductivity k =202W/mK). The analysed cross sections are those of the usual commercial tubes with rectangular cross section. The only requirement is a fixed section at the upper face where to accommodate the row of PV cells (1cm width). In the table 1, a summary of the analysed sections is shown. The difference among these three sections relates to the height of the section, it varies from 7 to 27 and hence the aspect ratio increases.

Table 1. Dimensions of the commercial aluminium cross sections analysed in this research

a

(Hc/Wc)

H

(mm)

Hc

(mm)

W

(mm)

Wc

(mm)

L

(m)

1

15

7

1.5

7

1

2.43

15

17

1.5

7

1

3.86

15

27

1.5

7

1

The SDS process for silicon ribbon growth

Joao M. Serra*, C. Pinto, Miguel C. Brito, Jorge Maia Alves, Killian Lobato, Antonio Vallera

DEGGE/SESUL University of Lisbon
Campo Grande ED-C8, 1749-016 Lisboa, Portugal
Corresponding Author, imserra@fc. ul. pt

Abstract

A lot of research has been done to try to reduce the costs of solar cells by developing ribbon growth techniques that bypass the ingot/wafering step. The SDS-Silicon on Dust Substrate process, here described, is a technique to produce ribbons directly from the gas phase. Test solar cells were fabricated on SDS ribbons as a demonstration of concept of this new technique. Keywords: Photovoltaics; Solar Cells; Ribbons

1. Introduction

A lot of effort has gone into reducing the costs of solar cells by bypassing the ingot/wafering process, which is currently the dominant industrial process. Early on it was realised that such a wasteful process could be avoided by the direct preparation of silicon sheet by ribbon growth techniques [1][2]. The SDS-Silicon on Dust Substrate process, described here, is a new method for the growth of silicon ribbons for photovoltaic applications.

Description of the PV/T concentrator

The system developed at the University of Lleida (Figure 1) is a two-axis sun tracking system using water as a working fluid. The absorber has 72 mono-crystalline Si solar cells which are adhered on top of two circular tubes. Water runs in these tubes to cool down the cells thus enhancing their efficiency and the outlet water is guided to a storage tank where it can be further utilized for domestic hot water production or running a solar-driven chiller. The 12 m2 reflecting system (receiver) is composed of 30 white mirrors that reflect light onto the PV focus band of 0.10 m width and 3.90 m long. The mirrors are of equal length but have unequal widths and are installed at various tilt angles. The solar cells are illuminated by approximately 22 times the solar beam irradiance. The generator’s rated power is 1 KWectrcial and 4 KWthermal.

image058

Fig. 1. 22-sun PV/T concentrator

Operation of decentralised generation in the distribution grid

The different operation strategies for the cogeneration plants influence the power quality in the grid. To evaluate the power quality we used a load flow analysis [9]. For that analysis it is assumed that all load and generation are 3 phase symmetric. So the result will be the same for all phases. The grid is represented by the admittance matrix, which contains all elements of the used equivalent circuits. The algorithm for solving the load flow equations is implemented in the open source language R (www. r — project. org).

To evaluate the power quality a load flow analysis for the low voltage grid described above was made. For the analysis it was assumed that the electricity demand is distributed equally to the 4 houses. Also the 4 PV plants have the same generation schedule. The feed-in of the CHP was defined by the optimisation results described above. As slack node the node GRID (Fig. 1) was set. At this node the voltage is fixed the reference voltage of 395 V. This is less than the nominal voltage of 400 V to increase the input of decentralised generation in the grid. The slack node also must absorb/provide the needed/surplus power. The load flow analysis was repeated for each time step.

image121

Fig. 6 shows the simulated voltage profile at the regarded day in May at the connection point of the CHP system for five scenarios. In the “BASE” scenario no decentralised generation is available. In this traditional scenario the lowest voltages can be seen. Beginning from the slack node the voltage decreases depending on the height of load until the end of the line. In the scenario “PV only” the four PV plants feed in and change the load in the grid. This results in an increasing voltage during PV input to about 424 V, which almost reaches the upper limit of the allowed voltage range of (+6 %/-10 % of the nominal voltage). In these times additional CHP generation will violate the voltage criterion, which can be seen in the three CHP scenarios. In the scenario “KWK-G” the cogeneration plant is thermal driven and feed in according to the schedule shown in Fig. 3. It can be seen clearly that some of the operation blocks during high PV input times increase the voltage up to 435 V which violates the voltage criterion. A similar behaviour can be seen in the scenario “VDE” The highest payment for

produced energy is at 10 am where the transport capacity is restricted due to high PV input. In “LOCAL” scenario the high price times are in the early morning and evening. So the CHP operation blocks are shifted to these times. Because of this shifting all the produced energy of the CHP can be used in the local grid and no further violations are caused. The maximum voltage does not exceed the maximal voltage in the “PV only” scenario.

As it can be seen in the results of Fig. 6 the locally optimised CHP operation with a local price curve can help to control decentralised generation with respect to restrictions from the grid. In the scheduling process of generation it must be avoided, that all generators feed in simultaneously if there is little load and the grid is near its transport capacity. Because almost all plants do have local conditions to be fulfilled (e. g. heat restriction for CHP plants) it seems not to be possible to control all of them by a central device. These restrictions only can be known by the local operator. With the local price curve he also will automatically act according to the grid operator’s interests as much as he can.

2. Conclusion

Within this paper we demonstrate the potential of local management of decentralised generation to increase grid capacity towards RES and DER, taking cogeneration as example. For times of high grid utilization due to local excess energy production, lower pay-back prices for controllable electricity feed-in are defined by the local grid operator, while during times of low grid load prices are increased accordingly. This local component to the price curve for produced electricity stimulates local operators to shift their generation to high-price times. With this control method the grid operator can influence operation without direct access to cogeneration devices and the amount of RES can be increased without upsizing the grid facilities. This method can easily be extended to loads, which will be influenced by flexible tariffs. With the propagation of “Smart Metering” systems, the option of tariff driven loads and generation will be available in the near future.

References

[1] A European Strategic Energy Technology Plan: Technology Map, Commission of the European Communities, SEC(2007)1510, Brussels 2007

[2] Leprich, U.; Bauknecht, D.: Dezentrale Energiesysteme und aktive Netzbetreiber, BMU-Fachtagung Perspektiven Dezentraler Energiesysteme, Berlin 2006

[3] www. netmod. org

[4] Wille-Haussmann, B., Erge, T., Wittwer, C.: Decentralised Optimisation of Cogeneration in Virtual Power Plants, CISBAT 2007 — Renewables in a changing climate Innovation in the built environment, Lausanne, Switzerland, 4.-5.9.2007

[5] Bronstein, I.: Taschenbuch der Mathematik, Harri Deutsch 1999

[6] VDE-Studie: Smart Distribution 2020: Virtuelle Kraftwerke in Verteilungsnetzen, 2008

[7] Backhaus, K., Erichson, B., Plinke, W., Weiber, R.: Multivariate Analysemethoden, Springer 2005

[8] Faraway, J.: Practical Regression and Anova using R, 2002

[9] Spring, E.: Elektrische Energieverteilnetze, VDE-Verlag 2003

Coupling solar collectors and co-generation units in solar assisted heating and cooling systems

A. Napolitano 12*, G. Franchini1, G. Nurzia2, W. Sparber2

1 University of Bergamo, Department of Industrial Engineering, Viale Marconi n. 5, 24044 Dalmine (BG), Italy
2 EURAC, Institute for Renewable Energy, Viale Druso, n. 1, 39100 Bolzano, Italy
* Assunta Napolitano, assunta. napolitano@unibg. it

Abstract

The present work reports the main issues of coupling solar collectors and cogeneration units for heating and cooling purposes which have been derived from the detailed analysis of a study case. The work suggests a procedure for planning such systems so that solar collectors and a cogenerator do not interfere in their respective operation. The procedure includes the selection of a layout, the definition of a control strategy and the sizing of each component of such a plant. The outputs of the procedure are used in TRNSYS dynamic simulations to assess the performance of the planned plant.

Keywords: solar heating and cooling, co-generation, planning, simulation

1. Introduction

In Solar Heating and Cooling (SHC) plants, solar collectors are typically assisted by auxiliary technologies to match the entire user’s heat demand. Gas boilers are commonly employed as heat back up systems, but further machines fit to the same use. Among the possible supplementary technologies, Combined Heat and Power (CHP) generators, also known as Cogeneration systems, offer some advantages.

Many technologies are available for cogeneration but in this work only gas engine based CHP units are considered in combination with solar collectors, both for heating and cooling purposes. The latter is supposed to be achieved by means of an absorption chiller. When heat recovered from a CHP system is used to power an absorption machine, this not only meets the cooling load, but also reduces the peak electric demand caused by the cooling request [1]. Moreover, a balanced heat demand over the year, due to the presence of an absorption chiller, improves cogeneration application project economics by increasing its operating hours per year [2]. When CHP units are selected to assist solar collectors for heating and cooling purposes, further advantages are added to the above mentioned ones. In fact, CHP systems support solar collectors by providing a heat source which derives from thermal recovery. Hence, such a back up results in a more efficient energy supply and fuel saving compared to conventional systems (e. g. boilers), especially in cooling season as heat driven chillers require heat amounts larger then the electric demand of conventional chillers.

Despite of these advantages, coupling solar collectors and a CHP unit (SHC-CHP systems) for heating and cooling purposes present certain critical issues, as shown by the study case below reported. The main issue is to fit to each other a typically unsteady heat source (the solar radiation) and a system which needs steady working conditions (N. B. the primary function of an engine heat recovery
equipment is to cool the engine [1]), with the aim of meeting the heat demand, both in winter and summer. As planning such systems can be rather complex, a research work is being carried out to identify the main criteria for optimal designing and sizing. This work is based on dynamic simulations on TRNSYS platform to test on one hand first selected layout and control strategy, on the other hand the influence of various choices of sizes.

Electric/Thermal power ratio in Solar8

Knowing that the beam fraction of the global irradiation increases when we move closer to the equator, conclusions can be taken on Solar8 electrical/thermal output ratio depending on its location (Table 5).

Table 5. Solar8 electric and thermal annual outputs per square meter of total glazed area, on a N-S tracking axis and 50°C average working temperature. The total glazed are on Solar8 is 4.6m2.

Solar8 annual outputs per glazed area

(A Solar8= 4-6 m)

Stockholm

(lat=59.2°N)

Lisbon

(lat=38.7°N)

Lusaka

(lat=15.4°S)

Solar8 electric annual output per glazed area (kWh/m2,yr)

47.7

86.8

105.7

Solar8 thermal annual output per glazed area (kWh/m2,yr)

159.7

434.9

605.3

Ratio Electric/Thermal

0.30

0.20

0.17

The ratio between electric and thermal outputs decreases when Solar8 is moved closer to the equator where the beam irradiation values are higher. The electric output is proportional to the irradiation thus, a PV module as constant efficiency for the same working temperature. A solar collector as higher efficiencies for higher irradiances since the thermal output increases more than proportional when the irradiation increases.

4.3. Solar8 vs. traditional side-by-side system based on glazed area

There are many ways and factors to take in account when comparing the performance of a concentrating hybrid with a traditional side-by-side system composed by a PV module and a solar collector working separately. The following tables feature Solar8 comparison with the traditional side — by-side system based on their power outputs and total glazed area (Table 6 to Table 8).

Table 6. Solar8 electric and thermal outputs with a N-S tracking axis at 50°C average working temperature.

Solar8 annual outputs

(A Solar8= 4-6 m2)

Stockholm

(lat=59.2°N)

Lisbon

(lat=38.7°N)

Lusaka

(lat=15.4°S)

Solar8 total electric annual output (kWh, yr)

219.2

399.0

486.1

Solar8 total thermal annual output (kWh, yr)

733.9

1998.9

2782.1

Table 7. Traditional side-by-side-system electric and thermal outputs per square meter of glazed area. The PV module n0b=16% at 25°C. The flat plate collector q0b=80%, ai=3.5 W/m2oC and operates at 50oC average

working temperature.

Traditional side-by-side system

Stockholm (lat=59.2°N) Fixed tilt=40°

Lisbon (lat=38.7°N) Fixed tilt=30°

Lusaka (lat= 15.4°S) Fixed tilt=20°

PV module output per glazed area (kWh/m2,yr)

173.2

278.7

324.5

Flat plate collector output per glazed area (kWh/m2,yr)

478.7

999.7

1266.0

PV area needed to equal Solar8 electric annual output (m2)

1.3

1.4

1.5

Collector area needed to equal Solar8 thermal annual output (m2)

1.5

2.0

2.2

Table 8. Traditional side-by-side-system and Solar8 comparison based on total glazed area.

Side-by-side system vs. Solar8

(A Solar8=4-6m2)

Stockholm (lat=59.2°N) Fixed tilt=40°

Lisbon (lat=38.7°N) Fixed tilt=30°

Lusaka (lat= 15.4°S) Fixed tilt=20°

PV module area / Solar8 total glazed area (%)

27.5

31.1

32.6

Thermal collector area / Solar8 total glazed area (%)

33.4

43.5

47.8

Side-by-side system area / Solar8 total glazed area (%)

60.9

74.6

80.4

The traditional side-by-side system uses less area than Solar8 for the same electric and thermal outputs. This difference decreases when the systems are moved closer to the equator since Solar8 is exposed to higher beam irradiation values. In Lisbon, for instance, Solar8 can be replaced by 1.4m2 of PV module and 2m2 of thermal collector for the same outputs. Hence, it would use 74% of Solar8 total glazed area (4.6m2). Practically, two components require more space than one component.

Multi Solar (PVT) Co-Generation Power StationAmi Elazari

Millennium Electric T. O.U. Ltd.

P. O. Box 2646, Raanana 43650 Israel

Phone : 972-9-7439490 E-mail : Info@millenniumsolar. com Abstract

A new innovation technology includes construction of a Multi Solar Power Station using the Multi Solar (PV/T) Collectors System and a thermal (steam turbine) generator, using the excessive solar thermal energy produced by the Multi Solar System (MSS) which doubles the amount of electricity produced by the PVT Power Station while reducing the costs of the solar electricity produced to as low as under $3 USD per watt in certain countries.

The scientific basic principal of the MSS Co-Generation System is A built-in MSS that has the highest efficiency existing today — 85% (15% electricity, 35% hot water, 35% hot air or total 70% thermal energy). Each square meter of the MSS produces 150W DC electricity from PV panels (with 30% higher efficiency than the usual PV due to the cooling system of the PV) and a total of 700W thermal energy. This mass of thermal energy could be transferred into electrical energy with 25% efficiency by using a thermal turbine based on a low pressure steam generator.

1. Introduction

The MSS PV/T/A technology is the basic element of the Solar PV/T Power Station. The MSS is an innovative, patented (NO 5522944) Solar PV/Thermal/Air System that makes it possible to convert solar energy into thermal energy and electric energy at the same time using a single integrated collector. The Thermal Steam Generator (Turbine) is the complementary unit to the MSS collector. it makes use of the thermal energy produced by the MSS collector in order to provide an additional and equal amount of energy as is produced by the photovoltaic system.

2. Concept

Millennium proposes to establish a solar power station in an alternative structure, operated by 150°C steam generation and thermal turbine. The existing commercial steam turbines can reach 25% optimal efficiency by using solar thermal energy made by the MSS collectors. This decrease of the feeding temperature for the steam turbine leads to dramatic improvement of the economic feasibility, as a result of the smaller solar array required to provide the same output. This innovative technology should improve the ability of countries to increase solar energy production.

image049

MSS (PVT) Collector Drawing

The progress of solar technologies, the comeback of renewable energies and the development of the MSS collector which produces electricity from PV cells with 30% higher efficiency (by cooling of the PV cells using internal water pipes on the back side of the MSS collector and preventing the efficiency degradation of regular PV caused by excessive heat). The MSS is the appropriate technology for this innovative idea since it’s the only mature Solar PV/T technology which has been in operation for many years. The MSS PV/T/A technology which has been developed in Israel has been integrated in a variety of projects for over 16 years. The MSS has proven technology for commercial applications.

Since the proposed solar thermal technology is limited to an operation heat of 150°C, we are limiting the solar steam temperature to a maximum of 135°C, which is the feeding temperature of the thermal turbine. Our innovative idea is to increase the heat of the steam produced by the solar station, while reaching the optimal temperature for the thermal turbine. The system consumes the solar thermal energy produced by the MSS collectors at 55°C at first level. This temperature is being increased by the special solar thermal collectors, connected in 2 rows and transferred to the thermal turbine in temperatures of up to 150°C (steam). The thermal turbine produces electricity based on the thermal energy of 20-25% efficiency. In order to increase efficiency percents, the option to use tracking devices for the MSS collectors may be considered.

image050

Multi solar (PVT) co-generation power station drawing

 

Results and discussions

1.1. Pressure losses

From an engineering point of view an important parameter is the pressure drop within the channel as this parameter determines the requirements for the pumping power. To determine the pressure drop, it is convenient to work with the friction factor of Darcy [6], defined as:

image085 image086 image087

If the pressure variation is isolated from the equation 1 and integral transformation is carried, out the pressure drop is obtained as:

Where p is the fluid density, Vm is the mean fluid velocity, D is the hydraulic diameter and (x2-x1) is the pipe length.

image088 Подпись: (Eq. 3)

If we analyze the equation 2, it is noticed that the pressure drop is highly affected by the hydraulic diameter, which is directly linked to the shape factor (the bigger the Dh is, the smaller the AP). Once the pressure drop is know, the pumping power can be determined as:

Ignoring the effect of temperature on the cell performance, the efficiency of the electrical conversion of the cells is 20%. Therefore, for the cell surface area under investigation (0.01m2) and with an irradiation of 15000 W/m2, the electrical power produced by the cells is predicted to be 30 W.

Taking into account the necessary pumping power and the electrical power produced by the cells, the net electrical power is defined as:

pnet electrical Pelec, PV Pelec, pumping (Eq. 4)

In the figure 4, the variation of the net P with the Re and the defined aspect ratios is shown,

image090

In the figure 4, it can be noticed that the maximum net electrical efficiency has a quadratic form, and increases with growing aspect ratio. On the other hand, the slope of the parabola in the region which describes the power in the laminar regime is much greater in tubes with smaller aspect ratio.

Process description and experimental setup

In the SDS process (see flow diagram in Fig. 1) a bed of silicon dust, obtained from high purity gaseous feedstock, is prepared, acting both as a cheap substrate and as a “sacrificial detachment layer”. A thick film is then deposited on this bedding layer by fast CVD, at low temperature and atmospheric pressure. Finally, the detached free standing ribbon is recrystallised by a floating molten zone (ZMR — Zone Melting Recrystallization) technique.

The advantages of the SDS process are: (i) no substrate and therefore no associated cost and no contamination; (ii) low energy and thermal budget by use of atmospheric pressures and low temperature CVD; (iii) high quality, free standing, crystalline silicon sheet by float zone crystallisation, with no contact with foreign materials.

The SDS process is well suited for operation in a continuous mode. For example, at a 20 pm/min deposition rate (achievable with silane at ~900°C), 10 minutes are required to achieve a 200 pm thick pre-ribbon. This deposition rate is such that, during the recrystallization step, with a constant advance speed of 10mm/min, only a 100 mm long high temperature (900°C) zone is required.

 

Si powder
layer

 

deposition
by CVD

 

as-grown multicrystalline

nanocrystalline Si ribbon

Si ribbon

 

► ./’va

 

Separation

 

ZMR

 

Fig. 1. SDS process flowchart.