Category Archives: EuroSun2008-2

Scope of the work

The Solar Turbine Group (STG) was founded for the purpose of developing and implementing a small scale solar thermal technology utilizing medium temperature collectors and an ORC to achieve economics analogous to large-scale solar thermal installations. This configuration aims at replacing or supplementing Diesel generators in off grid areas of developing countries, by generating clean power at a lower levelized cost (~$0.12/kWh compared to ~$0.30/kWh for Diesel [9, 11]). At the core of this

Подпись: Figure 1 Solar ORC platform for off grid micro utility power generation Подпись: Figure 2 Solar ORC in field trials in Lesotho, southern Africa 2007 (Solar Turbine Group)

technology is a solar thermal power plant consisting in a field of parabolic solar concentrating collectors and a vapor expansion power block for generating electricity (figure 1). An electronic control unit is added for autonomous operation as sub-megawatt scale plants cannot justify the staffing of operating personnel. The design tradeoffs for maintaining low costs at small scales, include operating at a lower cycle temperatures (<200 DC) and using an ORC: lower temperatures enable cost savings in the materials and manufacture of the absorber units, heat exchangers, fluid manifolds and parabolic troughs.

Because no thermal power blocks are currently manufactured in the kilowatt range a small-scale ORC has been designed for this application (Figure 2). The design is based on off-the-shelf components with little modification, such as HVAC scroll compressors (for the expander), and car power steering pumps.

A novel control strategy and electronic control system is required for the components discussed above to work in concert and in a maximally efficient manner. Among the functions to be managed is the control of individual components, such as Solar ORC fluid machinery, as well as directing the energy flows between these components, battery storage and AC loads. Optimization of the control strategy, a major objective of ongoing research, will be based on theoretical and experimental characterization of all system components, and will, in practice, rely on developing a control system with feedback from diverse parameters ranging from ambient temperature to the particular load profile to be matched.

The work presented in this paper focuses on the characterization of the ORC system by developing and validating a model, which will be used to select the best components, working fluid and control strategies for the solar Rankine engine.

2. Modeling

The ORC model is built by connecting the models of its different main components. A volumetric pump and a scroll expander models are considered since they are the technologies selected for the ORC prototype presented in this paper. All models are developed under EES [5] using semi-empirical parameters that are identified with experimental data.

REDUCTION OF CO-EMISSIONS BY COMBINING PELLET. AND SOLAR HEATING SYSTEMS

F. Fiedler1*, T. Persson1 and C. Bales1

1 Solar Energy Research Center SERC, Hogskolan Dalama, S-78188 Borlange, Sweden
* Corresponding Author, ffi@du. se

Abstract

Emissions are an important aspect of a pellet heating system. High carbon monoxide emissions are often caused by unnecessary cycling of the burner when the burner is operated below the lowest combustion power. Combining pellet heating systems with a solar heating system can significantly reduce cycling of the pellet heater and avoid the inefficient summer operation of the pellet heater. The aim of this paper was to study CO-emissions of the different types of systems and to compare the yearly CO-emissions obtained from simulations with the yearly CO-emissions calculated based on the values that are obtained by the standard test methods. The results showed that the yearly CO-emissions obtained from the simulations are significant higher than the yearly CO-emissions calculated based on the standard test methods. It is also shown that for the studied systems the average emissions under these realistic annual conditions were greater than the limit values of two Eco-labels. Furthermore it could be seen that is possible to almost halve the CO-emission if the pellet heater is combined with a solar heating system.

Keywords: Carbon monoxide emissions, Pellet and solar heating systems

1. Introduction

The dramatically increased prices for oil and electricity over the last few years encourage many house owners with electric heating or and oil heating systems to convert their heating systems. Today in Sweden mainly heat pumps are installed but also pellet heating systems become more and more popular. Studies have shown that the combination of conventional boiler heating systems with solar heating is beneficial in terms of fuel savings and lower emissions since the boiler usually in the summer can be turned off when it’s efficiency is low [11; 14].

Emissions of harmful gases are important parameters in addition to the efficiency and the thermal performance of pellet heating units. The national building codes and emission regulations include limits of allowable emissions of noxious gases for wood pellet boilers. More stringent limit values are applied by the Swedish Testing Institute (SP) and eco-labels such as the Svanmark [7; 13]. The limit values can be expected to further sharpened when comparing the limit values from other European regulations and eco-labels [1; 12]. More stringent limit values have also been proposed by the Nordic eco-label Svanmark [6]. In Table 1 the official limit values for emissions and efficiencies for pellet boilers and pellet stoves are compared with the current limit

Regulation

Limit value for emission

NOx

CO

OGC

Particles

mg/m3 dry flue gas with 10 vol-% O2, 0°C, 1013 mbar

EN 303-5 (class 3)

3000

100

150

SP-Swedish testing institute,

2000

75

P-mark

Svan-mark

1000

70

70

Svan-mark, proposed 2006

340

400

25

40

Blauer Engd Pellet stoves [To be measured with 13vol-%

150

200 — 400

10-15

35

O2] Pellet boiler

150

100-300

5

30

Table 1. Limit values for emissions from automatic fed pellet heating units with a nominal combustion power smaller than 50 kW, CO-carbon monoxide, OGC-organic gaseous carbon.

In this study the emphasis has been on CO-emissions released from different pellet heating systems with different operating strategies combined or not combined with a solar heating system. CO­emissions from pellet stoves/boilers are highest during the start and stop phase. By operating the burner with modulating combustion power the number of starts and stops and consequently the start/stop CO-emissions can be reduced. On the other hand, the longer operation time leads to higher total CO-emissions during normal combustion. Both these effects are simulated in this study, and results are given for complete annual simulations with sub-hourly time step.

2. Method

This work compares and analyses the simulation results of six combined solar and pellet heating systems that have been chosen from a variety of design variants. Four of them were chosen to represent the range of commercially available solutions found in Sweden. The systems contain: a water mantled stove; an air cooled pellet stove; a store integrated pellet burner; and a standalone pellet boiler. The fifth system is similar to the system with the standalone boiler but uses a boiler with an adequate size of 12 kW. The sixth system is based on a completely new system concept using a very efficient Austrian pellet boiler. The pellet heating units in these systems had been previously tested at the Solar Energy Research Center, Borlange [10]. A detailed description of the systems can be found in [3] and [11].

The systems were modelled in the simulation environment IISiBat/TRNSYS [5]. The systems have been simulated for one year for the same boundary conditions. Particularly design parameters such as the boiler combustion control have been varied to study the effect on the CO-emissions of the systems. For comparison one system has also been simulated with only the boiler as main heat source and without solar heating system.

MEDESOL project

Owing to its features, membrane distillation process is believed to have a great potential to be employed for the production of high-purity water from seawater, brackish water and industrial wastewater [7]. Membrane distillation systems have the ability to work with low operating temperatures and high concentration brines, which reduces both the specific energy required per cubic meter and the impacts of large amounts of brine disposal. Membrane distillation, unlike other membrane technologies can be easily coupled with solar energy due to its compatibility with the transient nature of the energy. Despite the advantages of solar membrane distillation, few experimental systems have been developed compared with the mature technologies: solar PV-driven reverse osmosis and solar distillation. The comparison of PV systems and thermal driven ones, with respect to long­time performance and reliability needs further research and therefore, more data is needed in order to compare both technologies [1].

The main objective of the MEDESOL project is to develop an environmentally friendly cost-effective of minimum maintenance and easy to handle desalination technology based in SMD, for a capacity range between 0.5 to 50 m3/day, to supply rural areas with water difficulties. The design involves the innovative concept of multistage MD in order to minimize the energy requirements. The system is developed to be supplied by compound parabolic solar concentrators (CPC), specifically designed to be energy-efficient at working temperatures. The project is divided into the following phases:

1. Design and construction of three MD prototypes for multistage concept, and an optimised solar static collector.

2. First tests at PSA facilities with saline solutions connecting the modules to a existing solar collector field and evaluation of the collector prototype.

3. Assessment of two different scenarios in applying MD, in EU and developing countries respectively.

4. Techno-economical and environmental assessment of pre-commercial systems to both scenarios.

To date, stage 1 has been completed and stage 2 is being carried out at PSA facilities. This communication will focus on phase 2 results.

Pilot Plant for Solar Process Steam Supply

K. Hennecke,1* T. Hirsch2, D. Krtiger1, A. Lokurlu3 and M. Walder4

1 DLR — German Aerospace Center, Linder Hohe, 51147 Koln, Germany
2 DLR — German Aerospace Center, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany

3 Solitem, Susterfeldstr. 83, 52072 Aachen, Germany

4 Alanod, EgerstraBe 12, 58256 Ennepetal, Germany * Corresponding Author, klaus. hennecke@dlr. de

Abstract

An aluminium upgrading process will be supplied by steam directly generated in parabolic trough collectors. In this first of it’s kind installation in an industrial environment, steam at 4 bar will be fed into the existing distribution lines of the production to heat anodizing baths and storage tanks. The integration of the solar steam through separate heat exchangers in parallel to the existing system was also considered. In principle, due to the low temperatures of the baths, solar hot water systems could be integrated in the same way. However, the additional heat exchangers and pipelines between the solar system and the consuming process generate significant investment cost on top of the solar system. The selected integration of the solar steam generator in parallel to the existing boiler reduces the complexity of the system, saves cost by avoiding duplication of piping and heat exchangers, and provides flexibility in the operation to ensure security of supply and maximum use of available solar energy.

Keywords: Renewable energy, solar process heat, process steam, direct steam generation

1. Introduction

Industrial process heat accounts for about 28% of the total primary energy consumption for final uses in EU25. More than half of that demand is required at temperatures below 400°, making it a promising and suitable application for solar thermal energy. Nevertheless, only an almost negligible fraction of the total solar thermal capacity presently installed is dedicated to this application [1]. The collaborative Task 33/IV of the Solar Heating and Cooling program and the SolarPACES program of the International Energy Agency (IEA) aimed to support a more wide spread application of solar heat for industrial processes by

• Assessing the potential and identifying most promising applications for solar technologies in industry

• Developing methods for the design and integration of solar systems for industrial applications

• Developing and testing of collectors for medium temperatures up to 250°C, and

• Initiating and monitoring of pilot plants.

In support of these efforts, the P3 project (Pilot plant for solar Process heat generation in Parabolic trough collectors) was started in February 2007 with the goal to demonstrate the direct steam generation in small parabolic trough collectors for industrial applications. The pilot plant will be installed at the production facilities of ALANOD in Ennepetal, Germany. One of the products of this aluminium anodizing plant is MiroSun™, an aluminium based mirror also used as reflector

material in the SOLITEM PTC 1800 parabolic trough collector. Scientific support for the development of the solar system design, integration, operation and control concepts is provided by DLR. The future operation will be closely monitored and evaluated by Solar Institute Julich and ZfS Rationelle Energietechnik GmbH. Although the site provides less than ideal solar radiation conditions, precedence was given to the relatively close neighbourhood of the project partners supporting the successful realization of this innovative application. To limit technical and financial risks, the solar field is restricted to a moderate size of 108 m2 aperture area. An economic operation is not expected under these conditions. However, ALANOD regards the opportunity to demonstrate on site the utilization of their product in innovative applications as an additional benefit.

2. Background

Trough placement for sun tracking

The advantage of one-axis tracking concentrators is to require tracking the sun only in one direction, because in the other direction these collectors do not perform sunlight concentration. The axis of solar trough can either be placed parallel to the North-South direction of Earth rotation axis, or parallel to the East-West direction of Earth rotation plane.

If the solar trough axis is parallel to the North-South direction, the tracking system must follow the sun in its daily excursion and the sun’s altitude over the horizon depends on Latitude and day of the year. With this layout the solar rays impinge on the collector with an inclination dictated by Latitude and hour of the day.

If the solar trough axis is parallel to the East-West direction, the sun should not be tracked in its daily excursion. However the tracking system must follow the displacements in sun’s altitude occurring every day of the year. In this layout only at noontime the sun’s rays are perpendicular to the entrance aperture of solar trough.

The results of the studies presented in this paper are referred to sun’s altitude over the horizon, so they can be used in both placements of solar trough axis. Considering the North-South positioning, the sun’s altitude represents the daily excursion of the sun, which corresponds to the tracking path that the trough collector must follow. Whilst for the East-West placement, the sun’s altitude indicates the various positions of the sun corresponding to each day of the year, for the chosen Latitude.

Test of a Mini-Mirror Array for Solar Concentrating Systems

J. Gottsche1, B. Hoffschmidt1, S. Schmitz1, M. Sauerborn1*, Ch. Rebholz2, D. Ifland2, K.

Badstubner2, R. Buck3 and E. Teufel3

1 Solar-Institute Julich (SIJ), Aachen University of Applied Sciences (AcUAS), D-52428 Julich, Germany

2 Fraunhofer Institute for Reliability and Microintegration (Fh-IZM), D-82234 Oberpfaffenhofen, Germany 3 German Aerospace Centre (DLR), Institute of Technical Thermodynamics, D-70569 Stuttgart, Germany

Corresponding Author, sauerborn@sii. fh-aachen. de

Abstract

To reduce the construction costs of solar tower power plants an innovative new mirror system with mini mirrors is on the way as an uncommon alternative to the currently used heliostats with huge surface (actual up to 120m2). An about (0.6 x 0.6) m miniature test unit has now been con­structed. This first demonstration unit at first consists of a boxed array of 25 small parallel ar­ranged mirrors with a size of (10 x 10) cm each. All mirrors are linked by an agile, crossed stick stage with an integrated drive system to move them simultaneously into all necessary positions. This specimen, developed in cooperation with all partners, will be tested in the specially pre­pared artificial sun of the SIJ in different ways under changing conditions.

In a simulation an optimum field layout was calculated. With this data a theoretical comparison was made between a typical existing huge heliostat and the developed mini-mirror array unit with respect to the efficiency and cost-performance relations for a solar tower power plant. Keywords: mini mirror array, solar thermal tower power plant, heliostat field layout, raytracing

1. Introduction

The heliostat field of a solar tower power plant is with up to 50% a significant part of the investment cost. During the last 30 years of developing heliostats, it has been found that the key to essential de­crease of expenses lies mostly in innovative design modifications which strongly reduce the material input. The main cost driver of all low cost versions is still the steel structure, securing the mirror and the movement system that has to guarantee high accuracies even under high wind loads and extreme thermal stress situations.

The typical cost figure of heliostats depends on the stiffness requirements of the steel and is currently in the area of 130-150€/m2 (Solucar 120 m2). Because of the foreseeable increasing of the raw materi­als price the cost problem will in future gain even more relevance. Figure 1 shows a rough estimate of the maximal cost reduction potential. To simplify the calculation, the cost of the heliostat construction is reduced to the weight of the steel used (steel price actual > 2€/kg) and ignores the production ex­pense for further treatment (pillar, framework), higher techniques (actuator, controller, sensors) and the mirrors. When a wind load must be taken into account, the decrease of the expense is limited to max. 46% of the current mirror price.

In this way, only a heliostat design with a slight or even without sensitivity to wind load will offer the chance to scale down the mechanical layout to reduce the material input substantially. The easiest way to guarantee very small wind loads on the mirror system is to install the system in a box with a plane surface design. To solve this restriction and to keep the heliostat at a suitable size, small mirrors (such as (10 x 10) cm) could be used. The principle design of a Mini-Mirror Array proposed in this work is shown in figure 2.

Подпись: minimal expense

image091 image092 Подпись: 70€/m2

example: Solucar (Spain)

image094

An additional effect of the reduction of weight and the decrease of the associated friction losses will be the option to use a less robust actuator system for the mirror movement. So in the end the material in­put and the energy requirement of the system can be reduced effectively.

Current Study

The current study is an extension to the previous experimental study, however, the objective of the current study is to determine how the heat pump unit responds to a wide range of input temperatures. As well, a goal of the current study is to determine if the steady-state model previously developed in the first study can accurately predict the dynamic operation of the system. A varying power input to the heaters was applied ranging from 750 — 1500 W in a sinusoidal fashion, similar to that of a daily solar heat input. The fluid temperatures delivered to the evaporator varied between 15 and 37oC corresponding to the power output of the heaters. The results of the current study are compared to the simulated results of Freeman’s model, and will be used to eventually refine the model to better predict the actual operation of an ISAHP system.

Energy Performance and Optimisation

Tmopt program was used to couple the Tmsys simulations software with GenOpt, a generic optimisation program. Trnopt acts as an interface programme between the two software programs and streamlines the optimization process.

The optimisation process has as objective function the maximisation of the production of desalted water. The result is the optimal operating temperature of the fluid at the outlet of the solar field system coupled to the Rankine Cycle. This temperature has been optimised considering two different time periods: hourly or daily. The optimal daily and hourly temperature conditions are determined for both cycles considering two typical days corresponding to a typical situation in winter and in summer. The results for both cases are shown in Table 2 and Table 3.

As an example of the hourly results, Fig. 4 shows the results of the water system and working at a set point using the hourly and daily optimal temperatures in the selected summer day.

Table 2. Performance of the system operating at the optimum daily temperature

Cycle

Day

Solar

irradiation

[kWh/day]

Turbine

capacity

[kWh/day]

Optimal daily Temperature

[°С]

Desalted Water Production [m3]

Steam Rankine cycle — Reverse Osmosis plant

24 January

2585

150

235

44

26 June

6960

986

325

294

Organic Rankine cycle — Reverse Osmosis plant

24 January

2585

129

262

38

26 June

6960

768

354

228

Table 3. Performance of the system operating at the optimum hourly temperature

Cycle

Day

Solar

irradiation

[kWh/day]

Turbine

capacity

[kWh/day]

Optimal daily Temperature [°C]

Desalted Water production [m3]

Steam Rankine cycle — Reverse Osmosis plant

24 January

2589

151

variable

37

26 June

6960

988

variable

295

Organic Rankine cycle — Reverse Osmosis plant

24 January

2589

129

variable

39

26 June

6960

768

variable

229

Подпись: (a)

image051

(b)

Fig. 4. Results for the optimal operating temperature of the fluid of the solar system. (a) daily (b)

hourly

2. Conclusion

A model for the optimal integration of Rankine cycles and solar thermal plants to drive Reverse Osmosis desalination plants has been presented. This model combines rigorous models for the simulation of the Rankine and solar field subsystems. The objective is to calculate the optimal operation temperature to produce the highest amount of desalted water. An example using trough solar collectors and water and pentane as working fluids was presented.

Using the trough solar collector and the two selected fluids the results in terms of energy efficiency and production of desalted water are very similar operating the system at an optimal hourly temperature at the collector’s outlet or at an optimal daily temperature. However, significant differences where found between the optimal temperatures in winter and summer days. So, this would mean that the set point for the solar field outlet temperature should be changed throughout the year to obtain the best global system performance but this change will have little effect throughout the same day. The performance using water is similar to that of n-pentane due to the high efficiency of the selected trough collector at high temperatures and the high solar radiation available in the selected geographical location. In the future the developed model will be extended to study also other types of solar collectors and working fluids.

Acknowledgements

This work is financially supported by the Ministerio de Educacion y Ciencia of Spain, OSMOSOL project, ref. ENE2005-08381-C03-03.

References

[1] IDA Desalination, Yearbook 2007-2008, Water Desalination Report, Global Water Intelligence, UK.

[2] L. Garcia-Rodriguez, Seawater desalination driven by renewable energies: a review, Desalination, 143

(2002) , 103-113.

[3] L. Garcia-Rodriguez, Renewable energy applications in desalination: state of the art, Solar Energy, 75

(2003) , 381-393.

[4] S. A. Kalogirou, Seawater desalination using renewable energy sources, Progress in Energy and Combustion Science, 31 (2005), 242-281.

[5] E. Mathioulakis, V. Belessiotis, E. Delyannis, Desalination by using alternative energy: Review and state — of-the-art, Desalination, 203 (2007), 346-365.

[6] J. McHarg, R. Truby West Coast researchers seek to demonstrate SWRO affordability. Desalination & Water Reuse, 14 (2004) 10-18.

[7] Osmosol — Desalacion por osmosis inversa mediante energia solar termica, Memoria de proyecto, Proyectos de investigacion, Ministerio de Educacion y Ciencia, 2006. https://www. psa. es/webeng/projects/joomla/osmosol/

[8] S. Canada, G. Cohen, R. Cable, D. Brosseau, H. Price, Parabolic trough organic Rankine cycle solar power plant, NREL/CP-550-37077, Presented at the 2004 DOE Solar Energy Technologies, Denver (USA), 2004.

[9] G. Burgess, K. Lovegrove, Solar thermal powered desalination: membrane versus distillation technologies. Proceedings of the 43rd Conference of the Australia and New Zealand Solar Energy Society, Dunedin, November 2005.

[10] D. Manolakos, G. Papadakis, E. Sh. Mohamed, S. Kyritsis, K. Bouzianas, Design of an autonomous low — temperature solar Rankine cycle system for reverse osmosis desalination, Desalination 183 (2005), 73-80.

[11] D. Manolakos, G. Papadakis, S. Kyritsis, K. Bouzianas, Experimental evaluation of an autonomous low — temperature solar Rankine cycle system for reverse osmosis desalination, Desalination 203 (2007), 366-374.

[12] Delgado-Torres, A. M., Diseno Preliminar de un Sistema de Desalacion por Osmosis Inversa mediante Energia Solar Termica, PhD thesis, Universidad de La Laguna (Tenerife, Spain), 2006.

[13] J. C. Bruno, J. Lopez-Villada, E. Letelier, S. Romera, A. Coronas, Modelling and Optimisation of Solar Organic Rankine Cycle Engines for Reverse Osmosis Desalination, Applied Thermal Engineering (2008), doi:10.1016/j. applthermaleng.2007.12.022.

[14] TRNSYS 16 — A transient system simulation program, version 16, 2004.

[15] S. A. Klein, Engineering Equation Solver (EES), F-Chart Software, http://www. fchart. com

[16] ROSA — Reverse osmosis system analysis software, ver. 6.1.3, Dow Water Solutions, 2006.

[17] A. C. McMahan, (2006). Design and Optimization of Organic Rankine Cycle Solar-thermal Power Plants, Master of Science, Solar Energy Laboratory, University of Wisconsin-Madison (USA).

[18] A. M. Patnode, (2006). Simulation and Performance Evaluation of Parabolic Trough Solar Power Plants, Master of Science, Solar Energy Laboratory, University of Wisconsin-Madison (USA).

[19] J. Lopez-Villada, J. C. Bruno, E. Letelier, S. Romera, A. Coronas, Simulacion con Trnsys de Sistemas Solares Termicos para Desalinizacion mediante Osmosis Inversa, XIV Congreso Iberico y IX Congreso Iberoamericano de Energia Solar, Libro de actas 647-652, Vigo, 2008.

Scroll expander model

The scroll expander model has been previously proposed by Lemort et al. [5] and partly validated by tests with steam. In this model, the evolution of the fluid through the expander is decomposed into the following steps (as shown in Fig. 3):

• Supply pressure drop (su^ su,1,1)

• Cooling-down in the supply port of the expander (su1,1 ^ su,1);

• Isentropic expansion from the supply pressure down to the adapted pressure imposed by the internal expansion volume ratio of the expander (su,1 ^ ad);

• Expansion at a fixed volume from the adapted pressure to the exhaust pressure (ad ^ ex,2);

• Mixing between suction flow and leakage flow (ex,2 ^ ex,1) and

image115

Cooling-down or heating-up in the exhaust port (ex,1 ^ ex).

The model requires only nine parameters (heat transfer coefficients, friction torque, leakage area, pressure drop equivalent diameter). Those nine parameters, defined for a specific type of expander and for a specific working fluid, are determined on the basis of experimental data.