Category Archives: EuroSun2008-2

Experimental Evaluation of an Indirect Solar Assisted Heat Pump. System for Domestic Water Heating

A. Bridgeman* and S. J. Harrison

Solar Calorimetry Laboratory, Queen’s University, Department of Mechanical and Materials Engineering,

130 Stuart Street, Kingston, ON, K7L 3N6, Canada

Corresponding Author, Bridgeman@me. queensu. ca

Abstract

An indirect solar assisted heat pump (ISAHP) system for heating domestic hot water has shown promise as an alternative to conventional electric or natural gas water heaters. In a previously conducted theoretical study, it was concluded that an ISAHP could operate with a lower life-cycle cost than a conventional solar domestic hot water (SDHW) system. Therefore, to further investigate the feasibility of the proposed system, an experimental study was conducted on a prototype (ISAHP) system. To undertake the study, a fully instrumented heat pump water heater was assembled in a laboratory environment and connected to a simulated “solar heat” input. The “solar” input was provided by an electrically heated circulation loop that delivered temperature-controlled fluid to the heat pump evaporator. This allowed repeatable test sequences to be performed in the laboratory regardless of weather conditions. A simulated solar profile ranging from 750 — 1500 W was delivered to the heater throughout the test. The corresponding fluid temperature ranged from 15 — 37°C, and the results indicated coefficient of performance (COP) values ranging from 2.4 to 3.2. These results, while in close agreement, are approximately 12% lower than those predicted from previous theoretical values.

Keywords: Solar assisted heat pumps, Heat pumps, Domestic water heating, Canada

1. Introduction

In Canada, water heating is the second most energy intensive end use in the residential sector, accounting for 22% of the consumed energy [1]. Due to growing concern for depleting fuel supplies, higher fuel prices and greenhouse gas emissions, alternatives to the conventional water heating methods such as electric and natural gas water heaters are being investigated. Two systems currently receiving considerable attention worldwide are Solar Domestic Water Heaters (SDWHs) and heat pump systems that source energy from the ambient air, or geothermal energy.

While each of these systems may operate with lower energy consumption than a typical electric water heater, both systems have performance limitations. Air-source heat pump water heaters are attractive in temperate regions, but lack popularity in Canada due to the warm temperatures needed for their proper function [2]. Geothermal heat pumps demonstrate improved performance over air source heat pumps because the heat is drawn from the earth, which is much warmer than ambient in the winter. However, due to the ground loops necessary for these types of heat pumps to function, property alterations and high initial costs have made them less practical for existing homes [2]. Solar Domestic Water Heaters have been increasing in popularity in Canada, and can decrease the energy consumption of an electric water heater by up to 90% in the summer [3], but large temperature differences between the collector and ambient air during the winter months lower the collector efficiency significantly, limiting the seasonal performance.

A combined system, known as a Solar Assisted Heat Pump (SAHP) could be used to alleviate many of the disadvantages of either system operating independently. The advantage to the heat pump cycle, by coupling it with a solar thermal collector, is an increase in evaporator temperature over either air-source or ground-source heat pumps. This increase in temperature results in an improved heat pump coefficient of performance (COP). From the solar collector point of view, the use of the heat pump lowers the fluid temperature returning to the collector near or below ambient. This lower temperature increases the collector efficiency, and allows for substantial heat gains with low cost unglazed solar absorber panels, even under marginal conditions [4, 5, 6]. The combined system allows for efficient operation over a wider range of seasons and weather conditions, and for more hours throughout the day.

The concept of a SAHP dates back to 1955 when it was first proposed by Sporn and Ambrose [7]. Numerous studies took place in the 1980s and early 90s examining the feasibility of SAHP systems for either space or water heating. Most of these systems were classified as Direct Expansion Solar Assisted Heat Pumps (DX-SAHP), in which the refrigerant would flow through the solar collector directly, which doubled as the evaporator for the heat pump. Chaturvedi [5, 8] found that collector efficiencies between 40 — 70% were feasible with bare collectors operating under ambient condition in winter, and found heat pump COPs ranging from 2 — 3, which was 30 — 50% higher than air source heat pumps. In the mid 90s Morrison [6] stated that the majority of previous systems proposed had not achieved commercial success due to the complexity of the combination of heat pump and solar collector components, and high installation costs due to the need for plumbing, electrical and refrigeration connections during installation. He then proposed an integral design, in which the collector and heat pump unit were incorporated as an integral part of the storage tank, which must be installed outside. Morrison found only a slight decrease in performance compared to a typical system in Sydney, Australia, but predicted a substantial reduction in cost, and simplification of installation. Huang and Chyng [9, 10] have recently investigated similar integral DX-SAHP systems in Taiwan. They found COPs reaching up to 3.83 during a long term performance test, in which the system was run for 13,000 hours continuously.

Although the integral DX-SAHP overcame installation complexities of SAHP systems and achieved commercial success in some parts of the world, installing the water storage tank outdoors introduces another problem in the Canadian environment. The cold conditions in the winter months increase the heat loss from the tank decreasing the system’s performance. To avoid this problem, an Indirect Solar Assisted Heat Pump (ISAHP) is under investigation at the Queen’s Solar Laboratory in Ontario, Canada. A schematic of an ISAHP is shown in Figure 1. This system differs from a direct solar assisted heat pump in that the heat pump collects energy via a heat exchanger connected to the collector anti-freeze loop, rather than flowing through the collector itself. This eliminates the need for long refrigeration lines and costly refrigeration fittings on the collector, but allows for the heat pump unit and storage tank to be installed inside the residence. Another feature of this system is the external side-arm natural convection heat exchanger, which acts as the heat pump’s condenser. As the heat exchanger transfers energy from the refrigerant to the potable water, the water increases in temperature causing its density to decrease. This induces buoyancy driven natural convection, circulating the water through the heat exchanger and eliminating the need for a pump. Due to the relatively low flow of the natural convection loop, this configuration has the potential for increasing thermal stratification in the storage tank. The benefit of stratification is that it delivers cool fluid from the bottom of the tank to the heat pump condenser, while maintaining hot water at the top of the storage for distribution to the load. This improves the overall system performance [11].

1st Internatio /

Подпись:Подпись: Natural Convection Loop Подпись: Electric PumpПодпись: Expansion ValveПодпись:Подпись: Water Mains SupplyПодпись: Fig. 1. Schematic of an ISAHPimage149To Load

Introduction and Objectives

The application of renewable energies such as solar energy to produce fresh water is receiving increased interest due to the need for solving the water shortage problems in various areas of the world at the same time as conventional energy sources used for obtaining water in different scenarios become depleted.

The world desalination installed capacity has increased from a bit more than 10 million cubic meter per day in 1986 to more than 42 million in 2006. Only in Spain the capacity of newly commissioned plants in 2006 was higher than 400 000 m3/day [1]. So it is not surprising that the use of renewable energy sources in water desalination is of high interest, especially for remote areas where a conventional energy supply is not easily available. This application is, however, still not well developed and it has only been tested in pilot plants and at a few demonstration sites. Numerous examples of research work on renewable energy desalination systems can be found in [2-5] as well as in other reviews in the literature. There are various desalination methods available on the market that use mainly thermal or mechanical energy in their fundamental separation processes. Among them,

Reverse Osmosis (RO) is quite suitable for small to medium capacity systems and also has good perspectives for cost reduction and improvement in efficiency in the near future [6]. In RO, pressure applied to the saline solution forces pure water through a semipermeable membrane. The membrane is selective and allows the passage of water but is impermeable to other substances.

The energy to produce the required pressure for RO can be generated with renewable energy sources such as wind energy, dish-Stirling systems, solar thermoelectrical plants or photovoltaic solar electrical generation. Solar thermal energy coupled to a power cycle by using direct mechanical power can also be employed.

Water is commonly used in Rankine power cycles, although other types of inorganic (ammonia, ammonia/water, …) and organic fluids (hydrocarbons, fluorocarbons, siloxanes, …) can be used. The main advantage of organic working fluids in Rankine cycles (ORC) is that they can be driven at lower temperatures than similar cycles using water and also in many cases superheating is not necessary.

The research on ORC has been focused in the production of electricity, mainly related to recovery of low temperature waste heat, geothermal heat, biomass, or solar energy. Many references to these applications are available in the literature. A couple of facilities with ORC plants using solar thermal energy were constructed in 1978 in Cadarache, France and in 1981 at El Hamrawin, Egypt, but unfortunately there has been little information published about them [7]. A commercial parabolic trough ORC power plant (Saguaro plant) completed in 2006 in Arizona is of particular interest. It is a 1 MWe plant using n-pentane as the working fluid for the ORC, and is based on plants used in geothermal applications having 10340 m2 of parabolic trough collectors [8]. Studies on ORC applications for desalination are very scarce although a few projects exist and some studies are available. Burgess and Lovegrove [9] discussed the application of solar thermal powered desalination using membrane and distillation technologies. One of their conclusions was that more detailed analyses of solar driven RO are required to determine its costs and applicability. In [10] it is proposed a solar ORC system using R-134a and evacuated tube collectors. The system efficiency is low (7%) but the authors considered it comparable to equivalent photovoltaic desalination systems. The first laboratory test simulating the heat provided by solar collectors has been given by Manolakos et al [11]. An screening and performance assessment of working fluids for RO solar thermal desalination can be found in Delgado [12]. In Bruno et al [13] it is presented a model optimising the solar field/ORC global efficiency using a process simulator and calculating the required solar field area for several types of collectors and selecting the most appropriate fluid for each one. Also a technical and economical comparison with a photovoltaic/RO system is presented. The main conclusion was that solar driven systems specially those using medium-high temperature collectors can compete favourably in terms of specific annual cost €/m3 with the PV/RO system.

Using medium-high temperature solar collectors e. g. trough collectors, the use of water or organic fluids is possible, e. g. the Solar-Thermal power plant Andasol (Spain) using water or the Saguaro solar plant (USA) using n-pentane. In summary, existing research on solar ORC for desalination is very limited, and few efforts have been reported on determining the most useful working fluids for this application and their optimal operation conditions.

In this paper it is presented a rigorous model developed in Trnsys/Trnopt [14] linked with EES [15] for the optimisation of the operating temperature of solar Rankine cycles connected to RO desalination

plants to maximise the desalted water production. Two cases are presented and compared to provide the mechanical energy required for the RO system: a steam Rankine cycle and an organic Rankine cycle using n-pentane. The selected thermal solar field consist of trough solar collectors because in previous preliminary studies made by the authors [13] it was concluded that they provided the higher global efficiency. The complete solar field/organic Rankine cycle is modelled using Trnsys in the case of steam and Trnsys for the solar field and an EES model for the ORC system linked with Trnsys in the case of the organic fluid. The performance of the RO system is calculated using the ROSA software [16] and the modelling parameters used in [13].

Cam Mechanism Design

The variable solar day length is the result of three factors listed in decreasing order of importance: (1).The Earth’s orbit around the sun is not a perfect circle but elliptical, so the earth travels faster

when it is nearer the sun than when it is farther away; (2).The Earth’s axis is tilted to the plan containing its orbit around the sun; (3).The Earth spins at an irregular rate around its axis of rotation.

The monthly average of daylight for Brasov area is presented in Fig. 3.

image107

Fig. 3 Monthly average of daylight for Brasov area

 

Month

In order to determine the kinematics conditions for the tracking system, a math example is considered for the month of March when the average day time is of 12 hours. Thus, the reflector should rotate 1800 during the 12 hours (12 hours x 60 minutes = 720 minutes, average speed 150/hour). The maximization of the system efficiency is obtained by step operating the collector at specific time steps, the energy spent for collector orientation being consequently lowered.

Considering that the tracking mechanism is driven by a gear box at each 18 minutes, 40 steps per day are necessary (720min / 18 min = 40 steps). The collector should rotate 1800 in 12 hours, which means 4,5 deg/step (180° / 40 =4,5 deg/step).

Based on the above example, the control program has to guarantee 40 steps during 12 hours of daylight, every 18 minutes, to rotate the reflector by 1800. In order to transmit the movement, two opposite cams with cardioids shape are used.

In Fig. 4 there is presented the tracking mechanism for a parabolic trough collector. The mechanism driving is performed using a gear box, equipped with an control system.

Number of bolts “n” corresponding to a cam, is calculated considering the condition.

2* ж * r2

n = — = Integer no. (1)

P2

Based on these conditions, the radius r2 is chosen. If the generator radius is r2 = 600 mm (centroid corresponding to wheel with bolts) the results is n=40 bolts/cam. Thus, the tracking cam mechanism has the following dimensions:

— Centroid radia: rj =15 mm (cams centroid), r2 =600 mm (wheel centroid);

— Speed ratio: ii2=40;

— The angular step between bolts for one cam: 90.

image108

Number the sections and sub-sections. Please do not use automatic paragraph numbering.

The following steps should be achieved for the analysis of the tracking system as a multi-body system (MBS): (1) defining the mechanism as a multi-body system (bodies and geometrical constraints); (2) establishing the reference systems attached to each body (local systems for the mobile bodies plus the global system attached to the fixed body); (3) geometrical definition of the

odies on their own reference systems; (4) conversion of the coordinates from the local reference system to the global system; (5) defining the analytical equations of the global coordinates for the interesting points of the model; (6) defining the analytical equations of the geometrical and kinematical constraints; (7) formulating the differentially motion equations using different calculus formalisms (e. g. Newton-Euler, Lagrange); (8) defining the bodies mass properties and the forces/reaction torques from the tracking system; (9) solving the mixed algebraic and differentia equations system.

The analysis algorithm of the tracking system supposes the development of two specific models using MBS:

a) kinematical model — composed by the bodies connected by of kinematical joints and the geometrical parameters of the mechanism (joint placement); the input is generated using kinematical constraints (driving movements) applied in the rotational/translational joints of the driving elements which usually control the displacement and angular velocity;

b) forward dynamic model — composed by the kinematical model and the external/internal forces that act on the tracking system (e. g. mass forces, wind, etc.). The model is used to determine the driving torque (for operating with a rotational engine) or the driving force (for operating with a linear actuator) which generates the kinematical movement of the tracking system;

On the dynamic model of the system, only the inertial effect of the collector was taken into account and the wind speed was considered neglect.

The tracking system analyzed as a multi-body system method is consists of nc=3 bodies (1-fixed body, 2- cam, 3- follower).

Подпись: Fig. 5. Virtual prototype of trough collector

The degree of freedom of the mechanism, according to the structural model is M= 3*(nc-1)-Erg =3(3-1)-5=1. In Fig. 5 there is presented the virtual prototype of the tracking system mechanisms, obtained using ADAMS software.

The inertial-mass properties (mass, location of mass center, inertial torques) of the tracking system bodies necessary for the dynamic model were established by means of a solid 3D modeling using Solid Works software, Fig. 4. The transfer between the CAD and MBS software (ADAMS/View) was performed using of a STEP format file (Standard for the Exchange of Product Model Data).

Using ADAMS, materials were associated for each body, the software automatically generating the inertial-mass properties of the bodies (e. g. Fig. 6 presents the properties of a parabolic trough collector). The total angle reached by the collector is 1800 (-90°…+90°), with the zero position at noon (« 12 o’clock), the return to the initial position (sunrise) being performed at 21 o’clock. Simulation is performed considering the specific conditions of the Spring Equinox (sunrise hour — 5.99 solar time, sunset hour -18.08) and the latitude of Brasov area.

In the virtual MBS ADAMS model, the collector displacement function was modeled by summarizing a series of STEP type time functions as follows:

STEP(time, 5.99, 0.0, 8.95, 0.0) + STEP(time, 8.95, 0.0, 9.05, 15.0d) + STEP(time, 9.95, 0.0, 10.05, 15.0d) + STEP(time, 10.95, 0.0, 11.05, 15.0d) + STEP(time, 11.95, 0.0, 12.05, 15.0d) + STEP(time, 12.95, 0.0, 13.05, 15.0d) + STEP(time, 13.95, 0.0, 14.05, 15.0d) + STEP(time, 14.95,

0. 0, 15.05, 15.0d) +STEP(time, 15.95, 0.0, 16.05, 15.0d) + STEP(time, 16.95, 0.0, 17.05, 15.0d) + STEP(time, 17.95, 0.0, 18.05, 15.0d) + STEP(time, 21.0, 0.0d, 21.2, -180.0d).

With these values, the collector displacement function is presented in Fig. 7.

image110

Fig. 6 Inertial-mass properties of the trough collector

In order to evaluate the tracking system efficiency, the amount of direct radiation which reaches the receiver was calculated, using literature equations correlating the sun movement on the sky and the collector rotational angle [5, 11, 12, 13].

In Fig. 8 the diagrams obtained for the radiation collected on the trough receiver are presented for the following cases: (1) the trough collector is continuously orientated; (2) the trough collector is rotated using the displacement function as presented in Fig. 7; (3) the trough collector is fixed being orientated South.

As Fig. 8 shows, the step — wise tracking proposed leads to efficiency almost equal, (98%) with those obtained by continuously tracking.

The efficiency increase due to tracking is over 35%.

2. Conclusion

Tacking step — wise mechanisms based on double cams and multiple followers are presented as a solution for parabolic trough collector. In comparison with other tracking systems, this system presents the following advantages:

— perform high gear transmission ratio and efficiency having reduced dimensions;

— the continuous rotational movement from the double cam to the bolts wheel is performed (the two cams gear on the same time with the opposite rollers for a cam rotation of ф2=180°…254°);

— high loading capacity due to a high contact ratio;

— low costs due to a simple design;

image111

simple maintenance.

Based on the dynamic model, the torque required for the collector orientation was obtained. The collector daily motion was performed in steps, the displacement function being established considering a minimum number of actions. By using this tracking system the amount of direct radiation captured by the parabolic trough receiver raises with about 35% than in the case of fix collectors orientated toward to the South.

The designed tracking system achieves the imposed movement functions and meets the desired placement of the collector along the operating cycle. Starting from this virtual prototype both the mechanism and the tracking algorithm will be tested in laboratory and field operating conditions. Based on the tests, the tracking algorithm is optimized targeting a minimum number of steps thus, an increased overall energy efficiency.

References

[1] http://www. powerfromthesun. net/chapter1/Chapter1.htm.

[2] http://www. solarpaces. org/csp_technology. htm.

[3] Ciobanu, D., Visa, I., Tracking systems for parabolic trough collectors, Bulletin of the Transilvania University of Brasov, vol. 12-series A, ISSN 1223-9631, Brasov, 2005, pg. 29-36.

[4] Ciobanu, D., VISA., I., Tracking system type cam mechanisms for parabolic trough collector, Acta Techica Napocensis, vol. II (50), ISSN 1221-5872,auj Napoca, 2007, pg. 115-120.

[5] Duffie, J. A., Beckman, W. A., Solar engineering of thermal processes, Second edition, A Wiley — Interscience Publication, John Wiley & Sons, 1991, ISBN 0-471-51056-4.

[6] ***The World Patent No: WO 0310 1471A.

[7] ***The US Patent No: US 446938.

[8] *** The Russian Patent No: 2105935.

[9] ***The US Patent No: US 5798517.

[10] Dudifa, Fl., §.a.,., Mechanisms course. Gear pairs. Cam mechanisms. Kinematics, Transilvania University of Bra§ov, 1989.

[11] Burduhos B., Visa I., Rusu C., Diaconescu D.: On the Orientation Cycle Optimization of a PV Testing Tracked Platform, CSE 2008 — 2nd Conference on Sustainable Energy, Brasov, Romania, 3-5 Iulie 2008, ISBN: 978-973-598-316-1, pg. 85-92.

[12] Diaconescu, D., Visa, I., Burduhos, B., Saulescu, R., Orientation data needed in the design of the pseudo-equatorial tracker’s control program, OPTIM 2008-11* International Conference on Optimization of Electrical and Electronic Equipment, Basov,2008, Romania, ISBN 978-973-131-030-5, pg. 449-454.

[13] Diaconescu, D., Visa, I., Burduhos, B.: On the Received Direct Solar Radiance of the PV Panel Orientated by Pseudoequatorial Tracker, COMEC — The 2nd International Conference Computational Mechanics and Virtual Engineering2, Brasov, Romania, 11 — 13 Octombrie 2007, ISBN 978-973-598-117­4, pg. 43-48.

Acknowledgements

The authors wish to thank the Spanish Ministry for Education and Science for funding received through research projects REN2003-09247-C04-01/TECNO and ENE2006-13267-C05-01/ALT, under the National R&D Program, and 2003-2004 Technical and Scientific Infrastructure Program (FEDER CIEM-E008).

The authors would also like to acknowledge the technical support of the PSA and ITC staff involved in this project.

References

[1] Martinez, D. ‘The Facilities for Thermal Testing of Materials at Plataforma Solar de Almeria’. Advanced Thermal Technologies and Materials. ISBN: 5-7038-1416-2. Vol. II. P.153-159. 1999. MIR, Moscow (Russia).

[2] Martinez, D., Rodriguez, J. ‘Surface Treatment by Concentrated Solar Energy: the Solar Furnace at the Plataforma Solar de Almeria’. Surface Modification Technologies XI. ISBN: 1-86125-055-X.. P. 441­447. 1998. Institute of Materials. London (UK).

[3] K. H. Funken, M. Roeb, P. Schwarzboezl y H. Warnecke ‘Aluminum remelting using directly solar — heated rotary kilns’, ASME’s ‘Journal of Solar Energy Engineering’, ISSN 0199-6231, volume 123, number 2, may 2001.

[4] S. Moller, R. Palumbo. ‘The development of a solar chemical reactor for the direct thermal dissociation of zinc oxide’, ASME’s ‘Journal of Solar Energy Engineering’, ISSN 0199-6231, volume 123, number 2, may 2001.

[5] ; T. Guillard, G. Flamant y D. Laplaze. ‘Heat, mass and fluid flow in a solar reactor for fullerene synthesis ’, ASME’s ‘Journal of Solar Energy Engineering’, ISSN 0199-6231, volume 123, number 2, may 2001.

[6] Mallol, G.; Llorens, D.; Beltran, G.; Horrillo, S.; Gimenez, S. ‘Reproduccion de las condiciones industriales de coccion de las baldosas ceramicas en un horno de rodillos piloto’. Tecnica Ceramica,

276, 855-864, 1999

[7] Pitz-Paal, R.; Fiebig, M.; Cordes, S. ‘First Experimental Results from the Test of a Selective Volumetric Air Receiver’. Proceedings of the 6th International Symposium on Solar Thermal Concentrating Technologies. (ISBN: 84-7834-163-3). P. 277-290. Editorial CIEMAT (Madrid).

[8] Hoffschmidt, B.; Tellez, F. M.; Valverde, A.; Fernandez, J.; Fernandez, V.; ‘Performance evaluation of a 200 kW(th) HiTRec-II open volumetric air receiver’. ‘Journal of Solar Energy Engineering’, ISSN 0199­6231, volume 125, number 1, february 2003.

[9] Hoffschmidt, B.; Fernandez, V.; Konstandopoulos, A. G.; Mavroidis, I.; Romero, M.; Stobbe, P.; Tellez,

F. ‘Development of Ceramic Volumetric Receiver Technology ’. 5th Cologne Solar Symposium. June,

2001. K. H. Funken and W. Bucher, eds. Forschungsbericht 2001-10, DLR-Cologne, Germany, pp. 51-61.

MEDESOL PROJECT: SEAWATER DESALINATION BY SOLAR. DRIVEN MEMBRANE DISTILLATION

E. Guillen1* , W. Gernjak1, D. Alarcon1 and J. Blanco1

1 CIEMAT-Plataforma Solar de Almeria, Ctra. de Senes s/n, 04280 Tabernas, Almeria, Spain.
Corresponding Author, slena. guillen@psa. es

Abstract

Freshwater shortage difficulties make it necessary to find new sources of supply. Nowadays desalination seems to be the solution adopted in many countries to solve this problem. All around the planet, regions with a lack of freshwater match up with those with large amounts of available solar radiation. Therefore, solar desalination can be a suitable option to tackle these water scarcity problems in those particular areas, especially in the coastal ones. This paper will describe the first results of MEDESOL Project (Seawater Desalination by Innovative Solar — Powered Membrane Distillation System) funded by the European Commission within the 6th Programme Framework. Main objective of the project is to develop an environmentally friendly cost-improved desalination technology to supply fresh water in arid and semi-arid regions in EU and developing countries based on Solar Membrane Distillation (SMD). The system to be developed is intended to be a stand-alone technically simple to operate arrangement, and to have a capacity between 0.5 to 50 m3/day. SMD is a solar thermally driven process differs from other membrane technologies in that its driving force is the difference in water vapour pressure across the membrane, caused in turn by the temperature difference between the cold and hot side of it, rather than the total pressure. Feed water is heated by solar energy and comes into direct contact with the hydrophobic membrane, which allows only the vapour to cross it. Despite its advantages SMD has been developed to a lesser extent, compared with other solar technologies like Solar PV-driven RO or Solar Distillation. Amongst these advantages, its low operating temperatures which make it possible to use low-grade heat as the only thermal supply (that is the case of energy delivered by static solar collectors) and its low operational pressure and footprint, make SMD a promising technology. There are several configurations to create the vapour pressure difference and to try to deal with the main issues of this technology which are the heat losses and the mass transfer concerns. In the case of MEDESOL project, the configuration chosen has been Air Gap Membrane Distillation (AGMD) in order to lessen heat losses. The project introduces a novel scheme based on multistage-concept in order to achieve maximum heat recovery, minimum membrane area required and substantially reduce brine generation. Along the lines of that concept, several configurations will be assessed. The heat supply will come from an innovative compound parabolic solar concentrator, specifically developed for the intended range of temperatures (90° C) the same way an advanced non-fouling surface coating for the seawater heat exchanger will be devised and checked. As said before, this paper will show the results obtained so far.

Keywords: Solar membrane distillation, AGMD, desalination.

1. Introduction

The confluence of fresh water shortage difficulties and solar radiation in isolated regions constitutes the perfect framework to apply simple technologies like solar membrane distillation (SMD) to supply the shortfalls. Nowadays renewable energy driven desalination technologies are considered suitable for decentralized systems where water demand is lower than 20 m3/d (water demand in shortage conditions is normally of about 55 l/inhabitant/day). The lack of a centralized energy supply and established infrastructures and the low water demand that characterizes these kinds of regions make it worthless to scale-down larger desalination plants technologies, speaking for the use of solar or whichever renewable energy driven process available, for water desalination systems. Moreover, the selection of an appropriate desalination system must take into account the special requirements of these regions that are basically:

• Hard climate and environmental conditions of these regions make it necessary to have a robust technology which could stand for example all types of raw water and be practically maintenance-free.

• Lack of technicians, thus sophisticated technologies that imply complex pre-treatments or intricate maintenance procedures are contraindicated. Stand-alone technologies are therefore required.

• Necessity of high quality water production, as the water is for human consumption.

The solutions given to tackle this problematic are basically two, photovoltaic energy coupled with reverse osmosis systems (PV-RO), and solar thermally driven distillation systems (STD). There are difficulties when operating small scale PV-driven stand alone systems [1] and it is also well-know that RO systems are easily affected by raw water conditions (in many cases pre-treatment is required) and energy input varies as a function of salt concentration. Moreover, RO units have the requirements of a continuous process [2] and solar energy is intrinsically discontinuous. Amongst thermally driven systems, solar stills are the most common, because their practically non-existing operation and maintenance requirements. But their main drawbacks are a low yield (the production capacity of a simple type still is in the range of 2-5 l/m2/day [3]) low thermal efficiency and large specific collector area per cubic meter (Energy efficiency is a crucial factor when designing solar-powered systems, since the largest investments costs come from the collector area needed)[1]. Solar Membrane Distillation complies with all the requirements specified above and combines the advantages of a membrane-based technology, smaller installation areas, and those of the thermal-driven processes, low operational and maintenance costs [4] and has also a low environmental impact as it uses solar energy instead of conventional fossil fuels.

In another vein, the low temperature heat demand of membrane distillation technology make it also possible to use other energy sources such as waste heat to drive the process and thus use it for larger applications.

The Virtual Prototype of the Tracking System

The literature presents some constructive solutions of tracking mechanisms [6-10], but a general approaching for the conceptual design and the structural synthesis of these mechanisms is missing. Thus rises the necessity of a unitary modelling method of mechanisms, and in our opinion this method is based on the Multi Body Systems (MBS) theory, which may facilitates the self- formulating algorithms, having as main goal the reducing of the processing time. According with the MBS theory, a mechanical system is defined as a collection of bodies with large translational and rotational motions, linked by simple or composite joints. In this way, the structural design of the tracking systems consists in the following stages [11]: identifying all possible graphs, taking into account the space motion of the system, the type ofjoints, the number of bodies, and the degree of mobility of the multibody system; selecting the graphs that are admitting supplementary conditions imposed by the specific utilization field; transforming the selected graphs into mechanisms by mentioning the fixed body and the function of the other bodies, identifying the distinct graphs versions based on the preceding particularizations, transforming these graphs versions into mechanisms by mentioning the types of geometric constraints.

Подпись: Fig. 1. The virtual prototype of the solar tracking system. In the structural synthesis, there can be taken considered general criteria, for example the degree of mobility of the mechanism (M=1 for the single-axis trackers, and M=2 for the dual­axis trackers), the number of bodies, and the motion space (S=3 in the planar space, and S=6 in the general spatial case), as well as specific criteria, for example the type of the joint between the base and the input/output body. In this way, the structural synthesis method was applied and a collection of possible structural schemes were obtained.

The solution for system used in the study was selected from the multitude of the structural solution by using of the Multi Criteria Analysis. The evaluation criteria of the solutions were referring to the tracking precision, the amplitude of the motion, the manufacturing and implementation. The solution corresponds to an equatorial (polar) tracking system, at which the daily motion is directly driven by a rotary motor (fig. 1). The solar collector is rotated relative to a support on which the motor is disposed. The support can be rotated relative to the sustaining frame for the seasonal tilt angle adjustment, but the seasonal motion is not considered in paper.

For blocking the system in the stationary positions between actuatings, when the motor is stopped, the model includes an irreversible transmission, and in this way there is no energy consumption in these positions. The solid model of the tracking system was realized using the CAD environment CATIA. The geometries of the parts were transferred to the MBS environment ADAMS using the STEP file format (via ADAMS/Exchange Interface). The virtual model of the tracking system takes into consideration the mass forces, the reaction in joints, and the joint frictions, which are modelled by the coefficient of dynamic friction, the friction arm, the bending reaction arm, the radius of the pin, the stiction transition velocity, the maximum stiction deformation, and the preload friction torque.

For simulating the real behaviour of the tracking system, we developed the control system in the concurrent engineering concept, using ADAMS/Controls and EASY5. For connecting the mechanical model and the control system, the input & output parameters have been defined. The control torque represents the input parameter in the mechanical model. The output transmitted to the controller is the daily angle of the solar collector (in fact, the angular position of the rotor). For the input state variable, the run-time function is 0.0 during each step of the simulation, because the control torque will get the value from the control system. The run-time function for the input variable is defined using a specific ADAMS function that returns the value of the given variable: VARVAL(control_torque).For the output state variable, the run-time function returns the angle about the revolution axis: daily_ angle — AZ(collector. MAR_1, support. MAR_2), which returns the rotational displacement of one coordinate system marker attached to collector about the Z-axis of another marker attached to support.

The next step is facilitating the exporting of the ADAMS plant files for the control application. The input and output information are saved in a specific file for EASY5 (*.inf); the export also generates a command file (*.cmd) and a dataset file (*.adm) that are used during simulation. With these files, the control system diagram was created in EASY5 (fig. 2). The input signal block represents the database with the daily angles of the solar panel (i. e. the imposed motion law); this subject is described in the next section of the paper (optimizing the mechatronic tracking system from the motion/control law point of view).

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Fig. 2. The control system diagram.

From the controller point of view, for obtaining reduced transitory period and small errors, we used a general PID controller. The specific parameters of the controller have been established having in view the following conditions: the increasing of the proportional term generates the decreasing of the transitory period from the dynamic response of the system, and of the position error, respectively; the integral term generates a class of dynamic responses and attenuates the error history; the derivative term generates a class of dynamic responses and amortizes the error; the system is considered with a critical amortization. In the mechatronic model, ADAMS accepts the control torque from EASY5 and integrates the mechanical model in response to them. At the same time, ADAMS provides the current daily angle for EASY5 to integrate the control system model.

Placement and orientation of solar troughs

P. Sansoni 1 , D. Fontani 1, F. Francini 1, L. Mercatelli 1, G. Chiani 2, M. De Lucia 2

1 CNR-INOA Istituto Nazionale di Ottica Applicata, Largo E. Fermi 6 — 50125 Firenze — Italy
2 Dip. Energetica — CREAR, Univ. di Firenze, Via Santa Marta, 3 — 50139 Firenze — Italy
* Corresponding Author, paola. sansoni@inoa. it

Abstract

The advantage of trough collectors is to require tracking the sun only in one direction, because in the other direction they do not perform sun concentration. The solar trough axis can either be placed parallel to the North-South direction or parallel to the East-West direction. In North-South positioning the tracking system follows the sun in its daily excursion. In East-West placement the tracking system follows the displacements in sun’s altitude occurring every day of the year. For the studies presented in this paper the North — South positioning is preferred to the East-West placement.

The paper examines the effects of an incorrect placement of collector axis, proposing an empirical correction procedure to recover the lost energy. Considering North-South positioning with errors up to 15° of misalignment, the study evidences that they cause significant losses of collected energy. Moreover the amount of missing energy always depends on sun’s altitude. Using the tracking system to compensate misalignment errors it is possible to recuperate most of the lost energy. This recovering procedure is applied to a trough concentrator with parabolic profile.

Keywords: sun tracking, simulations.

THE 500 kW SYNPET SOLAR GASIFICATION PLANT

The installation of the 500 kW Solar Gasification Plant on the top of the SSPS/CRS tower at the Plataforma Solar de Almeria is foreseen for the end of 2008. Manufacture of the solar-receiver was finished at the beginning of this year following the design of ETH. Rest of the components, including coke pneumatic transport, heat exchanger, torch, etc for upstream and downstream systems were defined by CIEMAT.

A gasification plant, by their nature, involves the processing of flammable gases, mists or vapors or by combustible dusts. In Europe, certification standards are now being put in place for these risk areas as part of the ATEX Directive. Under the associated ATEX 94/9/EC Directive limitations are dictated by the type of equipment that can be installed within the classified hazardous areas. Once an area has been identified as hazardous it should be classified into zones based on the frequency and persistence of the potentially explosive atmosphere. Equipment is categorised depending on the level of zone (0, 1 and 2) where it is intended to be used. A certification procedure following ATEX Directive was included in our installation for the purpose of advancing the technology to commercialization. As a result, a classification is given according to our preliminary layout.

The solar gasification plant is designed to utilize about 50 kg/h of petcoke (dry basis). A special unit based on pneumatic conveyor to feed finely ground coke (particle size less than 100 pm) from the ground has been installed. It is expected a coke consumption between 30 and 50 kg/h and a water consumption between 60 and 100 kg/h. This installation includes a multi-screw feeder and a mixing tank, as well as valves, flow meters and pumps. The multi-screw feeder comprises a weighing hopper for metering and weighing pulverized coke. It runs with high accuracy with a capacity of 50 kg/h. In the mixing tank, coke is mixed with water to form the slurry.

Finally, slurry prepared in the tank is conveyed to the entrance of the reactor by a pump. An electrical evaporator with a capacity of about 60 kg/h will supply the extra steam flow to keep the window cool and clean from particles. The coke reacts with steam at a temperature of 1100 — 1400°C to produce a raw fuel gas and ungasified material. After having left the reactor, the product gas is simply cooled down by water in a standard TEMA type BEU heat-exchanger to reduce the temperature enough to accomplish the requirements of the torch. In this regard, the requirements of heat exchanger for a heat recovery gasification system are more stringent than a cooler. Major difficulties encountered in the use of heat recovery systems for these technologies are fouling and pressure and temperature limitations of present materials and designs. An important effort has been initiated in our group for the demonstration of advanced heat exchangers and cleaning systems for gasification applications based on industrial experience.

In a final separator tank, the remaining water content of the product gas is separated from the syngas. Most of the residual solids from gasification fall into this device or in a water trap placed after. These residual solids generally consist of slag (the inert material from the feed coke) and some unreacted carbon. A small amount of the gas is separated for chemical analysis in a gas chromatograph and/or infrared gas measurement system. For this plant a syngas production of around 100 to 180 kg/h has been estimated. The schematic layout of the system is showed in Fig.

2.

In a future step, it is envisaged to use the remaining sensible heat of the exhaust gases to preheat the reacting steam before entering the reactor. Furthermore, a gas cleaning system is also foreseen, this includes a separation unit for particles and NH3 using water as washing medium, a H2S removal unit working with a NaOH solution, and finally the synthetic gas would be stored in a conventional refinery gas deposit system.

For evaluation of SYNPET solar receiver and plant performance the system is equipped with the following sensors:

• Gas chromatograph and IR gas analyzer connected to the exit for online measurement of the H2, CO2, CO and CH4 mole fractions

• Thermocouples to monitor temperatures at several locations (receiver, inlet and outlet, insulation, vessel, heat exchangers, etc.)

• Mass flow meter to measure syngas quantity

• Pressure transducers at several locations (receiver pressure, receiver pressure drop, etc). All the data will be recorded by the control system located on the ground.

Previous Studies

Two previous studies have been undertaken investigating ISAHP systems. The first study involved developing a model in TRNSYS [12], a transient simulation program, to investigate the feasibility of the system, performing both a performance and cost analysis. The second study involved building a prototype of the system, and performing a range of constant temperature input tests, comparing the results with the simulated results. A brief description of each previous study is provided below.

1.1. Numerical Analysis

The first study was conducted in the Solar Calorimetry Lab by Freeman [3] and simulated the performance of indirect solar assisted heat pump using TRNSYS. The program used mostly component models developed with the TRNSYS software, but models were created for the heat pump, the natural convection heat exchanger, and the heat pump controller. A detailed description of the TRNSYS model, and a theoretical analysis and derivation of the steady-state vapour compression heat pump model is given by Freeman [3], and is briefly summarized in a previous paper by the authors [13].

Results of these previous studies predicted higher seasonal solar fractions than conventional Solar Domestic Hot Water (SDHW) systems. It was concluded that the ISAHP gathered more energy from the environment during marginal weather conditions, as well as during the winter when compared to either an SDHW or air-to-water heat pump systems. The study also found that the life cycle cost of the ISAHP system showed up to 29% savings over the SDHW system for major cities across Canada.

Description of the Power Cycle — Reverse Osmosis Desalination Plant

The system consists of three main subsystems: the Rankine cycle, RO and solar plants. The Rankine cycle subsystem is a thermodynamic power cycle that uses water or an organic working fluid. The working fluid is heated to boiling, and the expanding vapour is used to drive a turbine and more generally any expander (Figure 1). This expander provides all the mechanical energy required to drive the high-pressure RO pump and the high pressure ORC and solar plant circulation pumps. In this process, after its expansion, the working fluid vapour is condensed back into the liquid state using the raw water feed for the desalination subsystem as a coolant. This water is in turn preheated to increase the performance of the RO. The condensed working fluid is recycled back through the system to again produce work. A regenerator is included to increase energy efficiency in the case of using an organic fluid. The heat required to preheat, evaporate and superheat the working fluid is obtained in this case from a thermal solar plant using thermal oil as a heat transfer fluid.

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1. Modelling

The TRNSYS simulation environment for thermal systems [15] was selected to model the thermal solar plant and calculate the desalted water production based on its completeness regarding meteorological and solar plant component libraries. In this study the steam Rankine cycle is modelled using the same Trnsys environment. However, it is very difficult to implement rigorous models to predict the thermodynamic properties of other working fluids. To solve this problem in the literature has been proposed to use transfer functions to import and export variables between Trnsys and EES [17, 18]. In the present study the strategy selected was to connect directly both programs. This option is more time consuming than the use of correlated functions but if the analysed time period is not too long the running time is reasonable.