Category Archives: EuroSun2008-2

Validation of the global model of the cycle

Once the models of each component of the circuit are validated, these models are interconnected to simulate the whole system, as shown in Figure 8.

image125

Fig. 8 Block diagram of the global model of the cycle

 

The following “causalities” are observed:

• the pump imposes the refrigerant flow rate,

• the expander imposes its supply pressure and exhaust temperature,

• the evaporator imposes the refrigerant superheating and the pump exhaust pressure,

• the condenser imposes the expander exhaust pressure and the pump supply pressure.

This simulation model is not fully predictive, because the subcooling at the exhaust of the condenser is defined as a model input. In order to predict this subcooling, a refrigerant charge model would have to be included.

Подпись: Fig. 9 Predicted vs measured output power with the global model

Figure 9 shows the prediction of the output shaft power for the global cycle model. All measurements are predicted within a 10 % accuracy. Errors of each model are indeed cumulated, which leads to a lower accuracy for the global model than for the individual components.

4. Conclusion

This paper proposes a semi-empirical model of an ORC involving a relatively limited number of parameters. The comparison between predicted values and experimental results show a fairly good agreement (for the cycle model as well as for the different sub-models).

The experimental study carried out shows a good expander isentropic effectiveness and demonstrates the viability of utilizing a mass-produced compressor as an expander in a small scale ORC. This represents an important step towards realizing the cost reductions that would make a kilowatts-sized Solar ORC economical for developing countries.

That the overall cycle efficiency remains limited is partly explained by the low temperature of the heat source and by a low pump efficiency. The former can be rectified by using higher temperature heat sources, while the latter can be addressed by selecting of a pump optimized for the pressure ratio and flow rate of the ORC.

Future work will focus on the integration of the ORC model into a global model including the solar collector in order to size the system, define a control strategy, and optimize the working conditions and the components.

Nomenclature Subscripts

A

area

(m2)

a

air

AU

heat exchange coefficient

(W/K)

amb

ambient

F

objective function

(-)

cd

condenser

h

convective heat transfer coefficient

(W/m2K)

ex

exhaust

M

mass flow rate

(kg/s)

exp

expander

N

rotational speed

(rpm)

f

working fluid

p

pressure

(Pa)

hx

heat exchanger

Q

heat flux

(W)

meas

measured

T

temperature

(°C)

n

nominal

T

torque

(N. m)

pp

pump

Vs

swept volume

(m3)

pred

predicted

v

specific volume

(m3/kg)

r

refrigerant

Vs

swept volume

(m3)

s

isentropic

V

volume flow rate

(m3/s)

sf

secondary fluid

W

power

(W)

sh

shaft

X

pump capacity ratio

(-)

su

supply

w

water

References

[1] Hung, T.-C., Waste heat recovery of organic Rankine cycle using dry fluids, Energy Conversion and Management (2001), Vol. 42, pp. 539-553..

[2] Incropera F. P., D. P. DeWitt, Fundamentals of Heat and Mass Transfer (2002), John Wiley & Sons

[3] International Energy Agency (2006), World Energy Outlook 2006, Paris

[4] E Kane, M., Larrain, D., Favrat, D., Allani, Y., 2003, Small hybrid solar power system, Energy (2003), Vol. 28, pp. 1427-1443

[5] Klein S., Beckman W. (2008), Engineering Equation Solver (EES), University of Wisconsin, Madison

[6] Liu, B.-T., Chien, K.-H., Wang, C.-C., Effect of working fluids on organic Rankine cycle for waste heat recovery, Energy (2004), Vol. 29, pp. 1207-1217.

[7] Lemort, V., Teodorese, I. V., Lebrun, J., Experimental study of the integration of a scroll expander into a heat recovery Rankine cylce, Proc. 18th Int. Compressor Engineering Conference at Purdue (2006), Purdue University, West Lafayette, C105.

[8] Maizza, V., Maizza, A., Unconventional working fluids in organic Rankine cycles for waste energy recovery systems, Applied Thermal Engineering (2001), Vol. 21, pp. 381-390.

[9] Price, et. al. Advances in Parabolic Trough Solar Power Technology, Journal of Solar Energy Engineering (2002), Volume 124, Issue 2, pg. 123

[10] Wei D., Lu X., Lu Z., Gu, J., Dynamic modeling and simulation of an Organic Rankine Cycle (ORC) system for waste heat recovery, Applied Thermal Engineering (2008), Volume 28, pp. 1216-1224

[11] Wenqiang, Liu, et. al. Techno-economic assessment for off-grid hybrid generation systems and the application prospects in China, World Energy Council (2007)

[12] Yamamoto, T., Furuhata, T., Arai, N., Mori, K., 2001, Design and testing of the Organic Rankine Cycle, Energy (2001), Vol. 26, pp. 239-251.

Reflector construction

Another critical point for the outcome of the project was the reflector manufacturing. Despite the low geometrical concentration ratio of the device (about 11), due to the large focus distance, the optical precision required it is similar to parabolic troughs that work with concentration ratios several times higher. On the other hand, in order to integrate the collector in building roofs, weight must be kept as low as possible.

For the reflecting surface, thick glass mirrors with permanent curvature were discarded almost from the beginning due to their high weight, high production costs for small manufacturing series and the low curvatures required. Therefore the two main alternatives considered were:

1. Glass on Metal Laminates (GOML)

2. Solar grade specular aluminium sheets

Подпись: Fig. 6. Sandwich panel

Eventually the second option was chosen mainly because of weight considerations. Aluminium sheets can be used in sandwich structures filled with polyurethane foam. In this process polyurethane is injected between two metal sheets inside a mould and the pressure due to expansion of the foam forces the metal sheets to take the shape of the mould (Fig. 6).

Подпись: 7 b) Fig. 7. Reflector construction

With this method small reflector pieces (1 x 1.5 m approx.) can be easily produced (Fig 7.a) and then this pieces are assembled to the reflector frame (see Fig. 5. and 7.b) to form the complete reflector of 4.5 x 6 m.

7.a)

An accurate control of the reaction conditions of the foam is of critical importance for the optical quality of the obtained surface because incomplete reactions lead to surface distortions. Nevertheless, under controlled conditions the optical quality obtained is excellent. The figure 8 shows a comparison between a theoretical radiation distribution on the focus, estimated with ray tracing, for a prefect geometry and assuming a material dispersion of о = 7.5 mrad, and the experimental curve obtained from a photograph of the focus for normal incidence.

image030

Despite the importance of this component for the performance of the collector, at the beginning of the project it was decided to try to use market available solutions whenever possible, instead of developing a new component.

Nevertheless, as can be seen in figure 2, at different times of the day the solar radiation reaches the receiver from a different direction. Therefore either the absorbent surface should be cylindrical or it should be oriented to the centre of the reflector for each incident angle.

The orientation of the receiver surface could be an interesting option, particularly if the concentrator were to be used in combined thermal/PV applications, but it was decided not to implement it in the first prototypes in order to reduce the amount of mechanisms.

Therefore a receiver with a cylindrical absorber with a diameter between 45 and 55 mm was required. The only type of absorber available with this geometry were the Sydney evacuated collectors. In this kind of tubes, the heat is primarily absorbed by the inner glass layer and then transferred to a U shaped copper tube or to a “heatpipe” through an aluminium fin. In the figure 9.a a section of the tube with a U-pipe is shown.

The main advantages of this kind of collector are its low cost and a very good sealing of the evacuated volume. On the other hand, the low conductivity of the glass absorber requires high temperature increments in order to evacuate the incident radiation. This problem is increased by the fact that it is very difficult to ensure a perfect contact between the glass and the aluminium fin, and between the aluminium fin and the copper tube. In the figure 9.b the simulated temperature distribution is shown for the case of the concentrated radiation reaching the tube laterally. In this model, only conduction between the different elements has been taken into account, and an air filled gap of 0.1 mm is assumed at both the glass-aluminium and the aluminium-copper interfaces. Although a more complex model would be required in order to accurately predict the absolute temperature values, the results obtained are representative of the relative temperature gradients between the glass zone and the metal parts.

Those relatively large temperature gradients can produce a low efficiency of the collector and breaking of the glass tube due to thermal stresses. Nevertheless it is difficult to theoretically predict those problems because they are highly sensitive to factors, such as the perfect contact between the different parts, that are very difficult to control. Therefore it was decided to experimentally evaluate the collectors for normal incidence (Fig. 10).

For an input fluid at a ambient temperature (21°) and a normal direct radiation of 750 W/m2, the efficiency obtained for the reflector-receiver system was about 70 %, and the stagnation

image031 Подпись: 9.b Temperature distribution

temperature of the tubes was 297 °С. Due to time issues no other efficiency measurements were carried out, and the obtained values were considered to be sufficient for the first prototype.

Fig. 10. Experimental setting

Regarding to the breaking of the tubes, during the stagnation tests no breaking of the tubes was observed. During the efficiency tests, only two failures were observed, and both of them were produced during manipulations or sudden changes in the fluid regime that could be easily avoided during normal operation. Therefore it was decided to use standard Sydney evacuated tubes for the receiver of the collector.

Подпись: Fig. 11. Evacuated tube failure

Nevertheless, during the first tests of the first collector prototype several tube failures have been observed. Most of them have been originated at either one of the tube tips (Fig. 11). Although a complete study of the causes of the breaking is still not completed, it is likely that most of the failures were related to the thermal stresses induced at the tube tips. Whether it is possible to reduce those thermal stresses with an improved fin design is still under analysis.

SSPS CENTRAL RECEIVER PLANT

The SSPS-CRS plant was inaugurated as part of the International Energy Agency’s SSPS project (Small Solar Power Systems) en September 1981. At the present time, as with the CESA-I plant, it is a test facility devoted mainly to testing small solar receivers in the 200 to 350kW thermal capacity range. The heliostat field is made up of 92 39.6 m2 first generation units. Under typical conditions of 950 W/m2, total thermal capacity of the field is 2.7 MW and peak flux obtained is above 2.0 MW/m2. 99% of the power is collected in a 2.5m dia. circumference and 90% in a 1.8m circumference.

For these experiments, two height levels at the tower were taken into consideration to find the optimum location. For this experiment, the receiver reactor will be placed at the 40m-level of the SSPS-tower. The performance that can be expected from the receiver was presented in a previous paper [4].

Solar field freeze protection

At the given site, ambient temperatures fall below 2 °C at 20% of the year [4]. But even other regions for future applications of this technology may require freeze protection for the solar field. The intention to feed the solar steam directly into the existing distribution precludes the options to operate the solar field with refrigerants or antifreeze additives to the water. Another option is to decommission the plant during winter. Draining and refilling the system requires hardly any additional equipment installation. However, maintenance effort will be increased, and annual performance reduced. The latter could be avoided by an automatic draining and refilling system, at the expense of increased complexity and cost.

The proposed solution is circulation heating during times with danger of frost. First estimates indicate that the solar gain during winter will over-compensate the heating demand for freezing protection, subject to verification during the system operation and monitoring phase. Heat sources

for the freeze protection could be either steam or condensate, but also waste heat utilization or electric heating may be considered.

3. Conclusions and outlook

The layout and integration of direct solar steam generation for a process heat application with parabolic trough collectors has been planned for a demonstration plant, which is under construction and will start operation late 2008. This will be the first installation which allows test and evaluation of direct steam generation in an industrial environment. A monitoring program will be performed to validate the design assumptions and simulation models. The integration of a solar steam generator into the steam distribution of existing conventional installations can be a cost effective alternative to the retrofit of solar steam or hot water systems supplying individual low to medium temperature processes.

4. Acknowledgements

The authors gratefully acknowledge the financial support given to the P3 project by the Federal German Ministry for the Environment, Nature Conservation and Nuclear Safety (contracts No. 0329609A, 0329609B, and 0329609C).

References

[1] C. Vannoni, R. Battisti, S. Drigo (Eds.), (2008). Potential for Solar Heat in Industrial Processes, Task 33/IV booklet by IEA SHC and SolarPACES, published by CIEMAT, Madrid, Spain.

[2] C. Brunner, C. Slawitsch, K. Giannakopoulou, H. Schnitzer, (2008). Industrial Process Indicators and Heat Integration in Industries, Task 33/IV booklet by IEA SHC and SolarPACES, published by Joanneum Research, Graz, Austria.

[3] W. Weiss, M. Rommel (Eds), (2008). Process Heat Collectors — State of the Art within Task 33/IV, Task 33/IV booklet by IEA SHC and SolarPACES, published by AEE INTEC, Gleisdorf, Austria.

[4] T. Hirsch, K. Hennecke, D. Kruger, A. Lokurlu, M. Walder, (2008). The P3 Demonstration Plant: Direct Steam Generation for Process Heat Applications, Proceedings of 14th Biennial CSP SolarPACES Symposium, Las Vegas, Nevada, USA, March 4-7, 2008.

[5] K. Hennecke, J. Kotter, O. Michel, D. Peric, D. (2002): Solar Process Steam Generation for the Production of Porous Concrete. 11th SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy Technologies, Zurich, Switzerland, September 4-6, 2002.

Modelling of Solar Rankine Cycle Engines for Reverse Osmosis Desalination with Trnsys

Javiera Valdivia, Jesfis Lopez-Villada, Joan Carles Bruno*, Alberto Coronas

Universitat Rovira i Virgili, CREVER — Group of Applied Thermal Engineering, Mechanical Engineering
Department, Avda. Paisos Catalans, 26, 43007-Tarragona, Spain

Corresponding Author, iuancarlos. bruno@urv. cat

Abstract

The use of power cycles driven by solar energy to provide the required mechanical energy to drive the high-pressure pump of Reverse Osmosis systems for desalination is an interesting alternative to the conventional electric systems. In this paper it is presented a model developed in Trnsys/Trnopt for the optimisation of the operating temperature in these systems to maximise the desalted water production. Two systems are studied and compared to provide the mechanical energy required for the Reverse Osmosis system: a steam Rankine and an organic Rankine cycle using n-pentane. The solar field consist of solar trough collectors. The complete solar field/organic Rankine cycle is modelled using Trnsys in the case of steam and an EES model linked with Trnsys in the case of the organic 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. In the future the developed model will be extended to study also other types of solar collectors and working fluids.

Keywords: Reverse osmosis desalination, solar thermal energy, organic Rankine cycle

Optimizing of a new tracking systems for small parabolic trough collectors

D. Ciobanu1*, I. Visa1* and D. Diaconesc1*

1 Transilvania University of Brasov, Centre for Sustainable Development, Eroilor Street, Brasov, Romania

* Corresponding Author, daniela. ciobanu@unitbv. ro

Abstract

Concentrator collectors are generally used in order to increase the amount of energy absorbed from the Sun. These collectors use only the direct radiation, being equipped usually with a single axis tracking system. The paper refers to parabolic trough collectors, whose geometry imposes one axis tracking system. In order to set the kinematic and dynamic condition necessary for the tracking system, it is considered as a math example the month of March when the average day time is of 12 hours. Thus, the reflector should rotate 1800 during the 12 hours. The maximization of system efficiency is obtained by operating the collector at specific moments aiming to minimize the energy consumption during the tracking generation.

The dynamic behavior of the tracking system is obtained based on the modeling and simulation of the mechanism by using ADAMS software.

Keywords: solar-thermal energy conversion, tracking system, cam mechanism, multibody system method

1. Introduction

The function of a solar collector is simple; it intercepts incoming insolation and turns it into thermal energy that can be applied to meet a specific demand.

image105

Solar collectors are classified depending on the working principle in plate and concentrator. Plate collectors use the global radiation while the concentrator collectors use only direct radiation. Most common concentrator collectors are parabolic trough, Fig.1, a, central receiver and parabolic dish, Fig. 1, b, [1].

A parabolic trough concentrates the incoming solar radiation into a line running along the length of the trough, Fig. 2. A tube (receiver) carrying a transferring heat fluid, is placed along this line absorbing the concentrated solar radiation and heating the inside fluid [2, 3, 4].

image106

Fig.2 Operating principle for parabolic trough collector

 

For a better concentration of the solar radiation, due to the change of sun position on the sky, these collectors are equipped with tracking systems.

Tracking systems are classified by their motions. Rotation can be round a single axis (which could have any orientation but which is usually horizontal East-West, horizontal North-South, vertical, or parallel to the earth axis) or can be round two axes. The parabolic trough collectors are designed to operate with tracking rotation round one axis due to its geometry [1, 5].

Most common tracking systems use a gear box and a belt, [6, 7], rope or chain transmission. Collector trackers also use actuators or systems based on “hydro-mechanic”, [8] or gravitation, [9] principle.

Belt and rope transmission tracking systems require accurate maintenance and could generate errors; chain transmission cannot be used for large dimension collectors; systems based on “hydro­mechanic” principle have large dimension and those based on gravitation principle require a daily human intervention and are influenced by the environment temperature.

The proposed tracking system derives from a rotational (cardioids), [10] cam mechanism and oscillating role follower. A complex mechanism is generated by this, similar with a bolts gear. The mechanism uses two opposite cams and multi-followers generated by a wheel with two rows of bolts with half step delay. The advantages of this mechanism are: reduced dimensions by use of a two teeth gear — cams, lustiness, low costs due to a simple design, smooth motion and high efficiency due to the cycloidal gear, generating a more accurate operation comparing with other tracking systems. Equipped with a servo-engine and an adequate temporization, this mechanism can generate a controllable collector orientation.

Methodology

First system tests determined the maximum achievable air temperature in the treatment chamber. Later testing verified that the solar system was able to match required heating/cooling ramps. Some typical T-t curves used in the ceramics industry were chosen as an example.

3. Results and discussion

The results of the most representative ‘maximum temperature test’ can be seen in Figure 4#, where the maximum achievable temperature with a solar irradiance of 980 W/m2 is 1000°C. It is worth mentioning that this limit is set by the receiver material, as its 1180°C safety limit was measured with the solar furnace shutter open only 85%.

The absorber material is a ‘SiSiC’ ceramic, chosen because of its proven availability and good performance in volumetric receivers at the PSA9.

When upper system limits had been determined, testing for facility suitability for complex T-t cycles was begun.

SolarPRO Test 060130

 

120

 

110

 

100

 

90

 

80

 

Подпись: SHUTTER (%)

70

 

60

 

50

 

40

 

30

 

20

 

10

 

Fig. 4. Maximum temperature test

 

image005

Подпись: T ■ receive Tref Tcycle Подпись:Temperature on receiver’s surface (°С)

Temperature in the center of the treating chamber (°С) Temperature of the theoretical T-t cycle (°C)

Direct solar radiation (W/m2)

Opening percentage of power attenuator (%)

The theoretical ceramic drying cycle T-t curve, in which a maximum temperature of 150°C must be achieved in 40 minutes, followed by a steep cooling ramp down to ambient temperature is shown in Figure 6.

As seen in Figure 5, there was no problem in following the required heating ramp. On the contrary, difficulties arose in accomplishing the desired cooling ramp. The air-cooling system may have to be specifically modified to increase the inlet cooling-air flow when necessary to solve this problem.

image008 Подпись: Temperature on receiver’s surface (°С) Temperature in the center of the treating chamber (°С) Temperature of the theoretical T-t cycle (°C) Direct solar radiation (W/m2) Opening percentage of power attenuator (%)

Fig. 5. Results of a drying test

Finally, ceramic baking was also tested in a higher temperature range, in which a maximum temperature of 800°C must be reached in 10 minutes, as seen in Figure 6. Test results were quite satisfactory.

Treceiver Temperature on receiver’s surface (°С)

Tref Temperature in the center of the treating chamber (°С)

Tcycle Temperature of the theoretical T-t cycle (°С)

Radiation Direct solar radiation (W/m2)

Shutter Opening percentage of power attenuator (%)

4. Conclusions

A project has been launched to study the feasibility of using concentrated solar energy for high — temperature industrial processes, such as ceramics manufacturing. A solar device (see Figure 7), based on the well-proven volumetric receiver technology has been designed, manufactured and assembled in the PSA Solar Furnace.

image010

Preliminary testing has been performed to:

— Determine the maximum achievable air stream temperature.

— Study the feasibility of performing a ceramic drying cycle to industry requirements

— Study the feasibility of performing a ceramic baking cycle to industry requirements

Results so far are encouraging, as a maximum temperature over 1000°C has been reached in the air stream, and the system has been proven able to match the heating ramps quite well, which will allow all the selected processes to be studied.

Nevertheless, we are working in several aspects that still can be improved, and new results will be presented in future works.

Economic Analysis

The solar assisted heat pump desalination system, which has been constructed, is an experimental setup, thus it produces a small amount of desalinated water. In order to make it economically feasible, it is necessary to scale up the system into a higher production capacity, one that is able to provide water for domestic needs, or to supply water in remote areas. From Table 1, we can see that to supply the water requirement for a single family we need to produce at least more than 150 l of water per day. With a production rate of 150 //day, the system will also be able to supply

potable water to a remote elementary school of 60 students. However, we must always take into account the amount of investment needed and the payback period of the system to make it economically feasible.

Table 1 Water requirement of commercial and institutional buildings [9]

Type of Building

Average water consumption per day

Men’s dormitories

50 //student

Women’s dormitories

45 //student

Motels:

60 units

55 //unit

100 units or more

40 //unit

Office buildings

4 //person

Nursing homes

70 //bed

Apartment houses:

50 units

152 //units

100 units

140 //units

200 units

133 //units

Schools:

Elementary

2.5 //student

Junior and senior

7 //student

A high price of solar liquid collector will cause the solar energy savings to be less competitive compared to conventional fuel systems. Therefore, it is necessary to make use of the heating and cooling provided by the heat pump efficiently. In order to achieve this, a new setup is proposed as shown in Figure 7. In the new setup, the refrigerant is allowed to condense fully through the distillation chamber and water tank, thus heating the feed water. Therefore, the need of liquid solar collector to preheat the feed water may be eliminated from the system. In the proposed new design, the distillation chamber will maintain the same vacuum pressure of 0.14 bar, thus water saturation temperature will still be at 52 °C. Refrigerant leaving the desalination chamber will preheat the feed water close to a temperature of 50 °C and feed water will be further heated by the electrical heater to 70 °C.

image075

Figure 7: Proposed solar heat pump desalination setup

The proposed setup produces distillate water at different production rates, and thus different component sizes and investments are needed. A comparison of the production rates, 225 //day, 450 //day, and 900 //day showed that at lower production rate, the payback period increases. At low production rate, as shown in Figure 8, less solar energy is used in the process, thus the fuel savings is reduced, resulting in higher payback period. At 225 //day the payback period is close to 4 years, while at 450 and 900 //day the optimum payback period close to 3.5 years. At 900 //day, the operation and maintenance cost of the system is high, thus reducing the savings acquired from solar energy usage, causing a similar payback period to 450 //day. At 225 l/day the optimum collector area is 20 m2, while for 450 and 900 l/day it is 34 and 67 m2, respectively. For production of 450 // day, it is achieved with a compressor power input of 15 kW, and 30 kW for 900 // day.

image076

Figure 8: Comparison of payback period for different production rates

image077

Figure 9: Comparison of payback period with the usage of liquid solar collector

As shown in Figure 9, installation of solar water collector to the system with a production rate of 900 //day will increase its optimum payback period to more than 4 years. A system with the same production rate will have a lower payback period without utilizing the solar water collector.

6. Conclusions

A series of experiments has been conducted with the solar assisted heat pump desalination system. A liquid solar collector was added to the system to preheat the feed water before entering the distillation chamber. Experimental results showed that the system could reach a Coefficient of Performance (COP) of 10, and at a relatively stable meteorological conditions, the evaporator collector has an efficiency value between 80 and 90%, whereas the liquid solar collector has an
efficiency value between 50 and 60%. The water production rate is generally close to 1 l/hr for the system, with a Performance Ratio (PR) close to 1.5.

An economic analysis was conducted to determine the feasibility of the system. It was found out that in order to be feasible, the system’s production capacity must be increased. However, the increase of production rate will also increase the investment cost of the system. With the high price of liquid solar collector, the payback period of the system becomes less feasible. A new design of the system, one that does not use liquid solar collector, was proposed. With the new design, the payback period of the system becomes much more attractive. A high production rate will require more investment cost, and it is shown that removing the liquid solar collector will reduce the payback period significantly. The optimum payback period of the system with a 900 l/day production is close to 3.5 years with a 67 m2 evaporator collector. Analysis of the economic parameters showed that the oil price is very influential in determining the competitiveness of the system.

In conclusion, it was found that the solar assisted heat pump desalination system exhibits great potential for future developments. With the increase of oil fuel prices, solar energy will likely to be more economically feasible as a source of clean energy.

Nomenclature:

I Solar irradiation, W/m2

image078 Подпись: Useful energy gained by collector, W Fuel price, S$/MJ Maintenance cost, S$ Operation cost, S$ Capital recovery factor Collector investment cost, S$ Fuel escalation rate Discount rate System life cycle, years Solar energy input, MJ

AC Collector area, m2

Reference:

[1] Renato Lazzarin (2001), Ground as a possible heat pump source, Geothermische Energie, 32/33, 9, http://www. geothermie. de/gte/gte32-33/gte32-33index. htm

[2] T. N. Anderson, G. L. Morrison, M. Behnia (2002), Experimental analysis of a solar-boosted heat pump water heater with integral condenser, Proceedings of Solar 2002, Australian New Zealand Solar Energy Society

[3] J. Siqueiros & F. A. Holland (2000), Water desalination using heat pumps, Energy, 25, 717-729

[4] Onder Ozgener, Arif Hepbasli, A review on the energy and exergy analysis of solar assisted heat pump systems, Renewable & Sustainable Energy Review, 2005, pp.1-16

[5] Torres Reyes E, Cervantes de Gortari J., Optimal performance of an irreversible solar-assisted heat pump, Energy Internal Journal, 1(2), 2001, pp.107-111

[6] Hawlader, M. N.A., K. C. Ng, T. T. Chandratilleke, D. Sharma and Kelvin Koay H. L., 1987, Economic Evaluation of A Solar Water Heating System. Energy Conversion Management, vol. 27, pp 197 — 204

[7] Kreider, Jan F., Charles J. Hoogendoorn, and Frank Keith, 1989, Solar Design: Components, Systems, Economics, Hemisphere Publishing Corporation

[8] Hawlader, M. N.A., Prasanta K. Dey, Sufyan Diab and Chan Ying Chung, 2004, Solar Assisted Heat Pump Desalination. Desalination, vol. 168, pp 49 — 54

[9] Stein, Benjamin, John S. Reynolds, and William J. McGuinness, 1986, Mechanical and Electrical Equipment for Buildings 7th Edition, John Wiley and Sons

The Optimization Of The Single-Axis Tracking System. Used For A Solar Collector

C. Alexandru*, M. Com^if and I. Vi§a

Transilvania University of Bra§ov, Product Design and Robotics Department, 29 Bd. Eroilor, 500036,

Bra§ov, Romania
* calex@unitbv. ro

Abstract

This paper presents researches on increasing the efficiency of the solar energy conversion, by designing and optimizing a single-axis tracking mechanism, which changes the daily position of the collector. The main task in optimizing the active component (the tracked solar collector) is to maximize the energetic gain by increasing the solar input and minimizing the energy consumption for tracking. The tracking system is approached in mechatronic concept, by integrating the electronic control system in the mechanical structure of the solar tracker at the virtual prototype level. Thus, the physical testing process is greatly simplified, and the risk of the control law being poorly matched to the real system is eliminated.

Keywords: solar panel, tracking mechanism, control system, virtual prototype

1. Introduction

There is a fact that the fossil fuels (gas, oil, coal) are limited and hand strong pollutants. In the last 15 years, the price of petroleum had tripled and the previsions on the medium term there are not quite encouraging. The increase of the emissions of carbon dioxide, responsible for the global warming and for the greenhouse effect, may have devastating consequences on the environment. The solution to the previously highlighted problems is the renewable energy, including the energy efficiency, the energy saving and systems based on clean renewable energy sources, like sun, wind and water. The solar energy conversion is one of the most addressed topics in the fields of renewable energy systems. The sun is a giant nuclear fusion reactor and the energy it supplies is equivalent of about 27,000 times the total amount of energy presently produced from all other sources. The present-day techniques allow converting the solar radiation in two basic forms of energy: thermal and electric energy. The technical solution for converting the solar energy in thermal energy is well-known: the solar collectors [1].

The efficiency of the thermal solar systems depends on the degree of use and conversion of the solar radiation. The energy balance refers to the surface that absorbs the incoming radiation and to the balance between energy inflow and energy outflow. The rate of useful energy leaving the absorber is given by the difference between the rate of incident radiation on absorber and the rate of energy loss from the absorber. In literature, the increasing of the efficiency of the solar collectors is approached mainly through the optimization of the conversion to the absorber level. On other hand, the degree of use of the solar radiation can be maximized by use of tracking systems for the orientation of the solar panels in accordance with the paths of the Sun. Basically the tracking systems are mechanical systems that integrate mechanics, electronics, and information

technology. These mechanisms are driven by rotary motors or linear actuators, which are controlled in order to ensure the optimal positioning of the collector relatively to the Sun position.

The orientation principle of the solar collectors is based on the input data referring to the position of the Sun on the sky dome. For the highest conversion efficiency, the sunrays have to fall normal on the receiver so the system must periodically modify its position in order to maintain this relation between the sunrays and the collector. The positions of the Sun on its path along the year represent input data for the design process of the tracking systems. The Earth describes along the year a rotational motion following an elliptical path around the sun. During one day, the Earth also spins around its own axis describing a complete rotation that generates the sunrises and the sunsets. The variation of the altitude of the sun on the celestial sphere during one year is determined by the precession motion, responsible for a declination of the Earth axis in consideration with the plane of the elliptic yearly path. In these conditions, for the design process of the tracking systems there are considered two rotational motions: the daily motion, and the yearly precession motion.

Consequently, there are two basic types of tracking systems: single-axis tracking systems, and dual-axis tracing systems. The single-axis tracking systems spins on their axis to track the sun, facing east in the morning and west in the afternoon. The tilt angle of this axis equals the latitude angle of the loco because this axis has to be always parallel with the polar axis; in consequence for this type of tracking system is necessary a seasonal tilt angle adjustment. The two-axis tracking systems combine two motions, so that they are able to follow very precisely the Sun path along the period of one year; that’s why dual axis tracking systems are more efficient than the single one, but also more expensive because they are using electrical and mechanical parts that determines the usage of expensive components in the system. Generally, for the orientation of the solar collectors, single-axis tracking systems are used.

Determining the real behaviour of the tracking systems is a priority in the design stage since the emergence of the computer graphic simulation. Important publications reveal a growing interest on analysis methods for Multi Body Systems — MBS [2, 3]. In the last decade, a new type of studies was defined through the utilization of the MBS software: Virtual Prototyping [4, 5]. This technique consists mainly in conceiving a detailed model and using it in a virtual experiment, in a similar way with the real case. One of the most important advantages of this kind of simulation is the possibility to perform virtual measurements in any point or area, and for any parameter. Thus, the behavioural performance predictions are obtained much earlier in the design cycle of the tracking systems, thereby allowing more effective and cost efficient design changes and reducing overall risk substantially.

In these conditions, our paper presents researches on increasing the efficiency of the solar collectors by designing and optimizing a single-axis tracking mechanism, which changes the daily position of the collector. The main task in optimizing the tracked solar collector is to maximize the energetic gain by increasing the solar input, and minimizing the energy consumption for tracking. The design is made by developing the virtual prototype of the mechatronic tracking system, which is a complex dynamical model. In fact, the virtual prototype is a control loop composed by the multi-body mechanical model connected with the dynamic model of the motor, and with the controller dynamical model.

For developing the virtual prototype, we used a digital prototyping platform, which includes CAD (CATIA), MBS (ADAMS/View), and C&C (ADAMS/Controls & EASY5) software solutions.

The approach is made in the concurrent engineering concept, by integrating the mechanical device model and the control system model at the virtual prototype level. In this way, the physical testing

process is greatly simplified, and the risk of the control law being poorly matched to the real (hardware) system is eliminated.

First experimental results, conclusions and further steps

A solar concentrator with a fixed reflector has been developed. The first prototype of the collector consists in a 4.5 x 6 m reflector that concentrates the sun radiation on 8 rows of evacuated tubes (Fig. 12).

The experimental characterization of the collector is still under progress. By now the highest efficiency obtained for an inlet temperature of 100°C is 43% for a direct incident radiation of 890

Подпись: Fig. 12. CCStaR prototype

W/m2. Although this thermal efficiency is low compared to the 70% of maximum optical efficiency obtained for normal incidence, it should be taken into account that it has been obtained for an incidence angle close to, but not coincident, with the normal, that the reflector was not cleaned, and that in the evaluation of the optical efficiency, structure shadows and edge effects were not taken into account. Therefore there is still room to optimize the design in order to approach to the theoretical limits, particularly in the areas of the upper structure design and the fine adjustment of the tracking. In the next months it is planed to further evaluate the collector under different radiation and weather conditions and to numerically simulate a detailed collector model using ray-tracing techniques. This will allow the identification of the primary sources of losses and will provide the fundamental guidelines to optimize the prototype.

5. Acknowledgements

This project has been funded by the CIDEM, an agency of the Generalitat de Catalunya (regional

government from Catalonia), and by a PROFIT credit of the Spanish Ministry of Industry, Tourism and

Commerce.

References

[1] European Commission, (2001) Green Paper — Towards a European strategy for the security of energy supply. Luxembourg: Office for Official Publications of the European Communities.

[2] POSHIP The Potential of Solar Heat for Industrial Processes Project No. NNE5-1999-0308. European Commission Directorate, General Energy and Transport.

[3] Russell, J. L, Jr. DePlomb E. DP. Gulf Oil Corporation. Concentrator, method, and system for utilizing radiant energy. US Pa. 3868823. 1975

[4] Pujol R. Martinez V. Moia A. Schwiger H. Analysis of stationary Fresnel like concentrator with tracking absorber. 13th International Symposium on concentrating solr power and chemical energy technology. 2006.

[5] Ravinder Kumar Bansal. Thoretical Analysis of fixed mirror solar concentrator. A thesis presented in partial fulfillment of the requeriments for the degree master of science in engineering. Arizona State University. May 1974

[6] Russell, J. L, Jr. Principles of the fixed mirror concentrator. General Atomic Company. San Diego, California. SPIE Vol. 85 Optics in Solar Energy Utilization. 1976.

[7] Russell, J. L. Jr.; Schuster, J. R.; Eggers, G. H. Development status of the fixed mirror solar concentrador. Proceedings of the 12th intersociety energy conversion engineering conference. 1141-1146. 1977.

[8] Pujol R. Analisis de un concentrador solar lineal con reflector fijo y foco movil siguiendo una trayectoria circular. A final paper presented for the degree of Master of Physics. Universitat de les Illes Balears. September 1974.

[9] Martinez V. Pujol R., Moia A. Schwiger H. Analysis of a stationary parabolic linear concentrator with tracking absorber. 13th International Symposium on concentrating solar power and chemical energy technology. 2006.