Category Archives: EuroSun2008-2

Collector design

Once the geometrical relations of the reflector were established the design process started. It is difficult to divide this process into separate tasks because each particular solution strongly influences the range of the possible solutions for the whole system.

The main design challenge is how to manufacture and assemble a high precision optical device that could be easily integrated into building covers, with minimal or none on-site adjustments, maintaining manufacturing costs per square meter (or better per kWh generated) as low as possible.

To meet those requirements three elements are of critical importance:

1. The positioning system of the receiver

2. The reflector manufacturing and assembly process

3. The receiver performance.

Salt water experiments

For the salt water experiments, the same factorial design was intended to be implemented, but the results (a significant raise in conductivity, with values ranging from 40-60 pS/cm) when working with low fluxes on both cold and hot channels, and regarding the leakage problems we already had in the first stage, made us decided to evaluate the performance at the best conditions that were settled on the first stage, those are 20 l/min on both channels, and thus only the effect of temperatures were checked. Generally, smaller distillate fluxes and higher conductivities were observed when working with salt water, about 20% less distillate production and regarding conductivity, the values for fresh water experiments were never above 10 pS/cm (average value of 3.97 pS/cm), while for salt water ones were never below 12 pS/cm with an average value of 61.7 pS/cm.

image082

image083 image084

Fig.3. a) Comparison between fresh and salt water experiments of distillate conductivity versus hot inlet temperature, working at 20 l/min of flow rate. b) Comparison between fresh and salt water experiments of distillate production versus hot inlet temperature, working at 20 l/min of flow rate.

image085

 

image086

Fig.4. AGMD modules at PSA.

5. Conclusions

Main conclusions of the experimental campaign were the expected ones:

• The variable with higher contribution to the distillate production is the hot feed temperature as reported in literature [6] (increasing feed temperature makes distillate production higher due to the exponential increase of vapour partial pressure), followed by the hot flow rate and their interaction.

• Rising hot feed flow rate increases the heat and mass transfer coefficients in the boundary layer on the membrane surface, thereby reducing the temperature and concentration polarization effects, and as a result increasing the distillate flow [9].

• Cold side temperature and flow rate have a lower effect on the production than the hot side, for the case of flow rate the effect is almost negligible.

• Although MD is claimed to be not affected by salt concentration of the feed inlet, the results of the experiments reveal that not only the conductivity of the distillate but the production is negatively influenced by salt concentration.

References

[1] J. Koschikowski, M. Wieghaus, M. Rommel, Solar thermal-driven desalination plants based on membrane distillation, Desalination, 156 (2003) 486-587.

[2] E. Tzen et al., Desing of a stand alone PV-desalination system for rural areas, Desalination, 119 (1998) 327­334.

[3] K. Kalidasa, Kn. K.S. K. Chockalingam, K. Srithar, Progresses in improving the effectiveness of he single basin passive solar still, Desalination, 220 (2008) 677-686.

[4] Z. Ding et al., Analysis of a solar-powered membrane distillation system, Desalination, 172 (2005) 27-40.

[5] M. S. El-Bourawi et al., A framework for better understanding membrane distillation separation process, Journal of membrane science, 285 (2006) 4 -29.

[6] A. M. Alklaibi, N. Lior, Membrane-distillation desalination: status and potential, Desalination, 171 (2004) 111-131.

[7] R. Chouikh, S. Bouguecha, M. Dhabbi, Modelling of a modified air gap distillation membrane for the desalination of seawater, Desalination, 181 (2005) 257-265.

[8] M. N. Chernyshov, G. W. Meindersma, A. B. De-Haan, Modelling of a temperature and salt concentration distribution in membrane distillation feed channel, Desalination, 157 (2003) 315-324.

[9] F. Banat, R. Jumah and M. Garaibeh, Exploitation of a solar energy collected by solar stills from desalination by membrane distillation, Renewable Energy, 25 (2002) 293-305.

Solar steam integration into existing distribution

Integrating the solar generated steam directly into the existing steam line is a more promising option. In order to fit the solar boiler into the system identical technical standards as for the production steam line should be applied. This means that, for example, the piping materials used in the solar field have to be the same as in the production line, the chemical properties of the condensate should be the same, etc. In any case, due to safety regulations, the solar field and all the steam devices (valves, flexible hoses, steam drum, etc.) should have the “CE” mark and all welding must be done by classified welder for pressure equipment to pass the performance and acceptance test of the technical inspection agency.

Подпись: Fig, 3: Hydraulic scheme of solar steam integration into existing distribution

If these basic requirements are fulfilled, the solar steam could directly feed into the production line by means of an overpressure valve (>4 bara), with the feed water to the solar steam generator provided from the industrial steam system. Condensate from the solar system can be tracked back by the condensate line of the existing system. The feed water pump for the solar field will be controlled by a level measurement in the steam drum. Figure 3 shows the system layout for this configuration. The steam drum is operated at constant pressure of about 4.3 bara.

During strong transients, that might have negative impact on the stability of the steam line, the generated steam can be blown off through a waste steam line above the roof. In this case, the production line is not affected. If the solar field is out of operation for an extended period, all condensate will be removed into a waste reservoir and the plant refilled with fresh feed water to avoid condensate degradation by corrosion and aging.

This option allows a very simple and compact balance of plant for the solar steam generator, and avoids cost and losses associated with additional piping and controls for the distribution. The solar steam generator is simply treated like any conventional supplementary boiler which might be retrofitted to an existing system.

Solar potential for industrial processes

The integration of solar heat has a large potential in industrial applications, as the industrial sector covers about 28% of the total primary energy consumption for final uses in EU25. The recent study “ECOHEATCOOL” reports that about 30% of the total industrial heat demand is required at temperatures below 100 °C and 57% at temperatures below 400 °C [1]. As a matter of fact, in several industrial sectors, such as food, wine and beverage, transport equipment, machinery, textile, pulp and paper, the share of heat demand at low and medium temperature (below 250 °C) is about (or even above) 60% of the total figure [2].

image044100%

80%

60%

40%

20%

Подпись: Figure 1: Processes on different temperature levels in different industry sectors; Data for 2003, 32 Countries: EU25 + Bulgaria, Romania, Turkey, Croatia, Iceland, Norway and Switzerland.

0%

In the framework of the IEA Task 33 SHIP a solar potential study was carried out that surveyed all data available for solar thermal potential studies for industrial applications. This study showed that the figures on temperature levels applied in different sectors that are obtained from industry statistics are fully confirmed by the outcomes of the estimates done in the reported potential studies for solar process heat [3]. The result of this study shows the potential of solar heat (based on potential studies of selected countries) for EU25. Solar process heat could cover 3,8% of the industrial heat demand, corresponding to 100 — 125 GWth.

Country

Industrial final energy consumption

Industrial heat demand (Final energy to heat demand conversion factor: 0.75)

Solar process heat potential at low & medium temperature

Solar process heat/ Industrial heat demand

Potential in terms of capacity

Potential in terms of collector area

Source of the data used for calculation

n

[PJ/year]

[PJ/year]

[PJ/year]

[GW*]

[МІО 1112]

Austria

264*

137

5.4

3.9%

3

4.3

Eurostat energy balances, year 1999; PROMISE project

SCcin

.

493*

17.0

3.4%

5.5 — 7

6 — 10

POSHIP project

Portugal

_

90“

4.0

4.4%

1.3 — 1.7

1.9-2.5

POSHIP project

Italy

1,653*

857

31,8

3.7%

10

14,3

Eurostat energy balances, year 2000

Netherlands

89“

46

1.95

3.2%

0.5 — 0.7

0.8 — 1

Onderzoek naar het potentieel van

zonthermische energie in de inustrie. (FEC for 12 branches only)

EU 25

12,994*

6,881

258.2

3.8%

100 — 125

143 — 180

Eurostat energy balances, year 2002

Table 1: Industrial heat demand and solar heat potential for selected countries and the EU 25 [3]

Stvrian Potential Study 2006

For Styria, a detailed potential study for the industrial sector was carried out in 2006 in the framework of the Styrian Promise project. Based on a questionnaire and via telephone calls over 470 companies were contacted to gather energy demand data and required temperature levels for processes. Based on the information acquired a statistical analysis was done to calculate the total energy demand of all Styrian companies in the respective sectors. To calculate the solar potential the following criteria were taken into account:

Process technical potential (improvement of technologies for low temperature applications)

Solar technical potential (efficiency of solar technology, available roof area)

Ecological potential (CO2/SO2 emission limits)

Social potential (awareness of companies)

Economical potential (investment costs, funding, conventional fuel and biogenic fuel prices)

Quantitatively it was only possible to account for the process technical and the solar technical potential, as the other factors are underlying regulatory agreements that rely on the current political framework.

For the process technical potential the following figures were assumed: (a) 100% for the food sector (all processes are low temperature processes), (b) 20% for the metal sector (conservative assumption, and strong reduction as low temperature process steps (phosphating, pickling etc.) may only account to a part of the overall energy demand of metal companies) and (c) 31% for the paper industry (reduction due to short residence times and partly necessary steam applications).

The solar potential was fixed with 20% for the whole industrial sector [4].

The results show that the total potential in the industry sector amounts to approximately 0,618 PJ/a. This equals an installed collector-area of 480.000 m2. It has to be considered that the potential in the textile and chemical industry was not included due to missing data. The largest potential in industrial companies was found in the sectors of food (0,2 PJ/a) and paper (0,28 PJ/a).

For trade companies a similar approach was used, however the process technical potential was always set to 100% as only low temperature is used in the relevant trade companies. Among commercial enterprises sports facilities (0,31 PJ/a), garden markets (0,13 PJ/a) and hospitals (0,12 PJ/a) have the highest potentials. The total potential in the commercial sector amounts to 0,587 PJ/a, which equals an installed collector-area of approximately 460.000 m2.

In total, the potential study for Styria shows that by installing a collector-area of approximately 1 Mio. m2, 68.000 t of CO2 per year could be saved.

Optimum Orientation of the Mini-Mirror Array

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The box of the MMA can be oriented freely in two dimensions, i. e. azimuth and elevation angle. For each position of the MMA in a heliostat field, a specific combination of these angles yields the best performance on an annual basis.

As an example,

Figure 7 shows the influence of the box orientation on the annual performance for a heliostat that is located 71m north and 71m east of the tower. The annual reflection efficiency shows a maximum at an azimuth angle of 251.5° and an elevation angle of 36°. This in a reference system where an azimuth angle of 270° corresponds to the box oriented south (east = 0°), and an elevation angle of 0° indicates a non-tilted horizontal box.

In the following calculations, the heliostat under consideration is always oriented to achieve optimum annual reflection efficiency.

Characterization results of a new volumetric receiver for high-. temperature industrial process heat in a solar furnace

I. Canadas 1, D. Martinez1*, F. Tellez1, J. Rodriguez1, G. Mallol2

1 Plataforma Solar de Almeria-CIEMAT. P. O. Box 22; 04200-Tabemas; SPAIN 2

Instituto de Tecnologia Ceramica, Castellon (SPAIN)

* Corresponding Author: diego. martinez@psa. es

Abstract

The Spanish-fUnded ‘Solar PRO’ project is assessing the suitability of the ceramics manufacturing industrial process-heat applications.

An experimental setup has been erected and characterized in the Plataforma Solar de Almeria’s Solar Furnace. This setup is based on an open volumetric receiver, heating an air current up to 1100°C in a sample processing chamber.

In a first stage, this system has been optimized and characterized and further its suitability for ceramics manufacturing processes has been studied. Nevertheless, it’s a multi-process device able to work at any high-temperature industrial process within its temperature limits.

Keywords: solar; process heat, high temperature, solar furnace, volumetric receiver, ceramics manufacturing

1. Introduction

Solar thermal energy is the renewable energy which, because of its characteristics, must take on a relevant role in industry, as it provides, either directly or through transfer to a fluid or absorber material, the thermal energy necessary for many industrial processes, and can supply solar process heat at different temperatures.

The industrial processes that usually require the largest energy share are those that take place at high temperatures. For the future implantation of the solar thermal concentrating technology in high-temperature industrial processes, a strong boost for research is required and for each particular process, its technological feasibility must be demonstrated, adapting the design and production parameters.

The SolarPRO project, funded by Spanish Ministry for Education and Science, opens a new line of research, by demonstrating the technological feasibility of using solar thermal energy to supply high-temperature industrial processes other than electricity generation. The combined experience and knowledge from the many projects in central receiver technology and materials treatment in the Solar Furnace are made use of for that purpose.

The relatively small and very versatile Solar Furnace3,4,5 (figure 1) is used as a test bench, as it allows a broad range of experiments in which cost and conditions, control and monitoring can all be optimized.

The processes studied in this project are classified in two basic groups:

• Industrial production processes

• Waste treatment processes

2. Experimental

Desalination with a solar-assisted heat pump: an experimental and analytical study

M N A Hawlader, Tobias Bestari Tjandra and Zakaria Mohd. Amin

Dept. of Mechanical Engineering,

National University of Singapore
9 Engineering Drive 1
Singapore 117576

Abstract

The Solar Assisted Heat Pump (SAHP) desalination, based on the Rankin cycle operates in low temperature and utilizes both solar and ambient energy. An experimental SAHP desalination system has been constructed at the National University of Singapore (NUS). The system consisted of two main sections: a solar assisted heat pump and a water distillation section. Experiments were carried out under the different metrological condition of Singapore and results showed that the system had a performance ratio close to 1.3. The heat pump has a Coefficient of Performance of about 10, with solar collector efficiencies of 80 and 60% for evaporator and liquid collectors, respectively. Economic analysis shows that to achieve a high production rate while maintaining a low investment cost, a system, without using liquid solar collector, is preferred. This system, at a production rate of 900 liter/day with an evaporator collector area of around 70 m[1] [2], will have a payback period of about 3.5 years.

Keywords: Desalination, heat pump, solar collector, evaporator collector, economic analyses, payback period.

applications at temperatures less than 100oC but the most promising source is the solar energy [2]. Experimental work on heat pump assisted water purification has been carried out in Mexico since 1981 [3], where electrically driven mechanical vapour compression pumps were first used. Absorption heat pumps were then tested in large-scale purposes and Siqueiros and Holland [3] found that the cost for desalination to produce potable water for cities was competitive to that of RO and ED.

Ozgener and Hepbasli [4] has performed energy and exergy analysis on solar assisted heat pump (SAHP) systems. Torres-Reyes and Cervantes [5] studied both theoretically and experimentally on a SAHP with direct expansion of the refrigerant within the solar collector and performed a thermodynamic optimization. The maximum exergy efficiency was determined by taking into account the typical parameters and performance coefficients.

The feasibility of a solar energy system is determined not only from its performance but also from an economic analysis, which must be carried out to evaluate its performance. Usually, solar energy systems require an initial high investment followed by a low maintenance and operation costs [6]. The economic Figure of merit used in the economic optimization is the payback period, as it shows how soon the initial investment can be returned by accumulated fuel savings [7].

At National University of Singapore (NUS), a direct expansion solar assisted heat pump (SAHP) system was designed and built,[8]. Studies performed on the system indicated the effectiveness of small-scale application. Modifications were made to incorporate the SAHP into a single effect MED desalination system and a series of experiments were performed. In this paper, experiments and economic analyses performed on a novel solar assisted heat pump desalination system is presented and discussed. [3] water tank. A thermostatic expansion valve regulates the refrigerant’s mass flow rate. After passing through the expansion valve, the refrigerant is divided into two branches, one through the evaporator-collector, and the other to a cooling coil located at the top of the desalination chamber to condense water vapors. These two streams are then mixed before entering the compressor.

In the desalination section, a commercial solar collector is used to preheat incoming feed water. An electrical heater is positioned at the outlet of this solar collector to provide auxiliary heating to ensure the feed water to maintain the desired temperature, when solar radiation is inadequate. The electrical heater will maintain the water temperature to be not less than 70°C. After passing through the electrical heater, feed water enters the desalination chamber. The chamber is evacuated to a pressure of 0.14 bar and at this pressure the corresponding saturation temperature for water is 52.6°C. Thus, feed water entering the chamber will undergo thermodynamic flashing. The remaining part of water that does not evaporate will flow down to the bottom of the chamber, where it will be heated further by the heat pump’s condenser coil, thus evaporating the water. Vapors generated from flashing and evaporation will be condensed at the top section of the chamber by a cooling coil of the heat pump. Distillate water produced will flow down to a collection tray.

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Heat exchanger model validation

In order to tune the heat exchanger model with the experimental data, the heat transfer coefficients need to be identified. For one exchanger, four heat transfer coefficients are defined: one for each zone on the refrigerant side, and one for the secondary fluid.

In the evaporator, two different heat exchangers were used. In total, eight parameters were therefore necessary to describe the set of exchangers in series.

image124

 

For given supply and exhaust temperature conditions, the condenser model predicts its pressure. Figure 7 shows that this pressure is predicted with a relative error of about 3%.

In the evaporator, the pressure is imposed by the expander and the feed pump. Given its supply temperature and the saturation pressure, the model predicts the heat flow and the exhaust temperature. Figure 8 shows that the heat flow is predicted with an error lower than 2%.

Positioning system

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The positioning system can be viewed as a chain of referencing elements that starts at the sun position and ends at the collector receiver. In fixed mirror concentrators this chain is different from moving reflector systems such as parabolic dishes or parabolic troughs. In those systems the receiver is connected directly to the reflector through a fixed structure. Therefore, using closed loop control systems, the chain can be reduced to:

image026

The precision of the relative position of the sun to the reflector is mainly determined by the precision of the sensor used. Thus, in the manufacturing process it is only necessary to reference accurately the reflector to the receiver, two elements that have no relative motion. In fixed mirror concentrators the former reference chain is slightly different:

In this case, the reflector and the receiver have relative motion and therefore should be linked through the tracking mechanism. Then, it is necessary to precisely reference the tracking mechanism to the reflector and the receiver to the tracking mechanism.

Another particular characteristic of the fixed mirror concentrators is that the average focus distance for a given reflector width is higher than in moving reflector systems. This fact increases the precision requirements of both the tracking mechanism and the mechanical referencing system. Furthermore, this large focus distance could be a handicap to building integration in those cases where the visibility of the collector system is not desired.

Given those special characteristics two main design choices were made:

1. The width of each reflector was reduced as much as possible (546 mm in the current prototype)

2. In order to reduce the cost of the tracking mechanism the reflectors and the receivers were arranged in arrays of eight lines of evacuated tubes sharing the same tracking and positioning structure.

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The figure 5 shows a drawing of the final design of the current prototype. The tracking mechanism consists of four rotating arms that support and reference eight receiver rows. The receivers are standard “Sydney” evacuated tubes of 1.5 m in length and an absorber 47 mm in diameter.

Upscaling of a 500 kW Solar Powered Reactor. for steam Gasification of Petroleum Coke

A. Vidal*1, T. Denk2, A. Valverde2, A. Steinfeld3, L. Zacarfas4, J. C. de Jesus4 and

M. Romero1

1 CIEMAT, 28040 Madrid, Spain

2 PSA-CIEMAT, 04200 Tabernas (Almeria), Spain

3 PDVSA INTEVEP, 1070-A Caracas, Venezuela

4 ETH, 8092 Zurich, Switzerland Corresponding Author, alfonso. vidal@ciemat. es

Abstract

Подпись: as the as the createHybrid solar/fossil endothermic processes, in which fossil fuels are used exclusively

chemical source for H2 production and concentrated solar power is used exclusively

energy source of process heat, offer a viable route for fossil fuel decarbonization and a transition path towards solar hydrogen. Research in recent years has demonstrated the efficient use of solar thermal energy for driving endothermic chemical reforming reactions in which hydrocarbons are reacted to form syngas. This process produces not only a highly useful and transportable end product, but also results in the storage of a significant fraction of solar energy in the chemical bonds of the fuel molecules.

The steam-gasification of petroleum derivatives and residues using concentrated solar radiation is proposed as a viable alternative to solar hydrogen production. Therefore, PDVSA, CIEMAT and ETH started a joint project with the goal to develop and test a 500 kW plant for steam gasification of petcoke. This report summari zes the major accomplishments and challenges of upscaling the installation at the SSPS — tower of the Plataforma Solar de Almeria.

Keywords: Solar Chemistry, Gasification, Central Receiver, Petroleum Coke, Slurry

1. Introduction

Gasification, which is a means to convert fossil fuels, biomass and wastes into either a combustible gas or a synthesis gas for subsequent utilization. The feedstocks include coal, natural gas (for reforming applications), refinery residues and biomass/wastes in combination with coal.

The use of high temperature solar heat to drive the endothermic reaction associated with coal gasification has been suggested and investigated in the last 20 years. The advantages of supplying solar energy for process heat are three-fold:

• Calorific value of the feedstock is upgraded

• Gaseous products are not contaminated by the by products of combustion; and

• Discharge of pollutants to the environment is avoided.

An important example of such hybridization is the endothermic steam-gasification of petroleum derivatives and residues (petcoke) to synthesis gas (syngas), represented by the simplified net reaction:

x

CHxOy + (1 — y)H2O ^ (2 +1 — y)H2 + CO

where x andy are the elemental molar ratios of H/C and O/C in petroleum tar, respectively. In a previous paper, the chemical thermodynamics and reaction kinetics of reaction were examined [1].

With regard to refinery residues (bottoms), these can take several forms depending on the design on the refineries and their products. In particular, our study comprises one type of these refinery residues: solid materials such as coke which is a by-product from the processing of heavy and extra-heavy oils using delay-coking technology. Application of these technologies is resulting in increased yields of low value refinery residues such as residual fuel oil, coke and petroleum tar, furthermore stringent environmental regulations appears to be reducing the markets for these residues — particularly petroleum coke [2].

The project of solar petroleum coke gasification is a joint cooperation between the company Petroleos de Venezuela (PDVSA), the Eidgenossische Technische Hochschule (ETH) in Zurich / Switzerland, and the Centro de Investigaciones Energeticas, MedioAmbientales y Tecnologicas (Ciemat) in Spain. The primary goal is to develop a clean technology for the solar gasification of petroleum coke and other heavy hydrocarbons.

The project is divided into three phases. In a first step, after performing in-depth studies of the thermodynamic and kinetic behaviour, a small 5 kW prototype was tested in the Solar Furnace of PSI / Switzerland [1]. Goal was to demonstrate the feasibility of the solar gasification, to determine critical process parameters, to identify possible difficulties, and finally to get a solid data base for the scale up step in phase 2. One important result was the decision to use slurry for the feeding of the reactor [3].

In phase 2, the design, construction, and operation of a 500 kW reactor are foreseen. The design of the reactor itself was done by ETH and upstream and downstream system by Ciemat. Construction is managed by Ciemat, and operation will be done at the SSPS-tower at the Plataforma Solar de Almeria during 2008. In phase 3 finally, a 50 MW solar gasification plant located in Venezuela will be designed.