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Technical feasibility of a sustainable Concentrated Solar Power in Morocco through an energy analysis
Renewable and Sustainable Energy Reviews 81 (2018) 1087–1095
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Technical feasibility of a sustainable Concentrated Solar Power in Morocco through an energy analysis
MARK
⁎
T. Bouhala,b, , Y. Agrouaza,b, T. Kousksoua, A. Allouhib, T. El Rhafikic, A. Jamilb, M. Bakkasc Laboratoire des Sciences de l’Ingénieur Appliquées à la Mécanique et au Génie Electrique (SIAME), Université de Pau et des Pays de l’Adour – IFR – A. Jules Ferry, 64000 Pau, France b Ecole Supérieure de Technologie de Fès, Université de Sidi Mohamed Ben Abdellah (USM.B.A), Route d’Imouzzer, BP 242, Fez Morocco c Ecole Nationale Supérieure d’Arts et Métiers ENSAM, Université Moulay Ismail (U.M.I), Marjane II, BP-4024 Meknès Ismailia, Morocco a
A R T I C L E I N F O
A BS T RAC T
Keywords: Concentrated solar power Parabolic trough solar collectors Physical modelling Heat transfer Site selection Morocco
Morocco imports about 96% of its required energy needs. Solar energy, as one of the most abundant and valuable renewable energy alternatives in the country, offers interesting opportunities for Morocco. In order to minimize its strong foreign energy dependence, Morocco hosts actually the largest Concentrated Solar Power (CSP) using parabolic trough collectors (PTC) as a technology for converting solar irradiation into thermal energy for electricity generation. The purpose of this paper is to assess the thermal performance of this technology and the potential projects concerned by Moroccan Solar Plan. A physical model is developed to determine flow parameters and heat transfer applied to PTC technology. Annual simulations in six climatic regions in Morocco were carried out. Several suggestions were drawn with regards to the design and parametric studies effectuated under Ouarzazate CSP project. It is found that the location and the climate are determinant parameters on the global performance of the parabolic trough solar collectors.
1. Introduction The rising economic competitiveness of Concentrated Solar Power (CSP) with fossil fuels will perform a determinant role in the future [1,2]. At present times, the most advanced technology for solar thermal power production is represented by CSP. One principal focus of interest, and a big contributor to the actual popularity of CSP, concerns the desert regions of North Africa [3,4]. The export of electricity to Europe from the desert locations of the Middle East and North Africa (MENA), which receive some of the highest solar irradiation in the world, is considered as the most encouraging near-term prospect for CSP expansion [5,6]. There have been various models and new methods published to optimize the Parabolic Trough Collectors (PTC) and solar air heaters to have better performance applying the method of Least Squares Support Vector Machine [7,8]. Many researchers all through the globe, have been interested by the technical feasibility of CSP plants in different countries. In Tunisia, a work based on the possibility to interconnect concentrating solar power technology with Europe was carried out by Balghouthi et al. [9]. Based on the results of this study, it was found that Tunisia is in an ideal location for the transfer of electricity produced by CSP plants in North Africa to Europe. This study also proposed the CSP Project to be
economically competitive provided that the majority of the plant equipments be manufactured locally in Tunisia. In the desert areas of the Middle East and North Africa (MENA), an optimization of drycooled parabolic trough (CSP) plants was conducted by Qoaider et al. [10]. Their simulation results show that plants with large thermal energy storage systems and oversized solar field have a better performance, as well as produce power at lower costs in comparison to smaller plants. A conclusion was made, that at a plant site with preeminent solar conditions available, the CSP dry-cooled power plants provide an alternative for green power production. The future economics of concentrating solar power (CSP) for electricity production in Egypt were investigated by Shouman et al. [11]. More specifically, a road map strategy for the launch of CSP in the Egyptian markets was presented by them, removing the main barriers for financing and starting market introduction in the peak and medium load segments of the power supply. The conclusion made was that Egypt is the suitable location for CSP projects worldwide. A feasibility investigation of a CSP power plant in Chile under a Power Purchase Agreements (PPA) model was conducted by Serverta et al. [12]. The roles of upfront and soft financing grants to narrow the gap for the first CSP projects in Northern Chile were highlighted. It was found that CSP technology associated with thermal energy storage suits
⁎ Corresponding author at: Laboratoire des Sciences de l’Ingénieur Appliquées à la Mécanique et au Génie Electrique (SIAME), Université de Pau et des Pays de l’Adour – IFR – A. Jules Ferry, 64000 Pau, France. E-mail address: [email protected] (T. Bouhal).
http://dx.doi.org/10.1016/j.rser.2017.08.056 Received 21 June 2016; Received in revised form 12 June 2017; Accepted 12 August 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Concentrated solar power using a parabolic trough collector: 160 MW power plant, Noor I in Ouarzazate solar complex, Morocco.
of the main renewable energy options for electricity production. The power plants based on PTC usually use a Heat Transfer Fluid (HTF) to collect heat energy which makes it possible to integrate this heat in a Rankine water/steam power cycle to generate electricity [19–21]. The PTCs are composed of parabolic trough-shaped mirrors, which reflect the incident radiation from the sun on the solar receiver tube. As illustrated in Fig. 3, the HTF flows across the receiver tube of PTC and is heated up under the effect of incident radiation. Then, the HTF is collected to be sent to the power block, where it passes through a set of heat exchangers and produces superheated steam at high temperatures. The superheated steam drives a steam turbine where rotational mechanical energy is then converted into electricity [22,7,23]. A thermal oil is commonly used as a working fluid that circulate through the absorber tube and transform the solar irradiation into thermal energy and carries heat to heat exchangers or analogous for driving a Rankine steam turbine. The Parabolic Trough Receiver (receiver tube) (see Fig. 4) comprises of a tube of stainless steel accompanied by a selective metalceramic coating enclosed by an evacuated anti-reflective glass tube. The vacuum envelope mainly enables to notably preserve the surface of the absorber from oxidation and to decrease thermal losses at increased operating temperatures. The vacuum in the receiver tube should be below the conduction band Knudsen gas to decrease convective losses in the annular space, which is naturally preserved near to 0.0001 mmHg. The metal-ceramic multilayer hedge is positioned over the steel tube to afford low thermal emissivity and optimum optical properties with significant absorptivity of direct solar radiation at the temperature of operation to minimize heat radiation. The glass envelope has an outer to minimize both antireflective coating and Fresnel reflective losses from the glass surface [7,24,25].
better to the demand profile of energy needs. Morocco has emerged as among the first in establishing regulatory frameworks and policies for promoting renewable energy sources and energy efficiency, especially in the solar and the wind energy sectors in the Mediterranean area [13,14]. Morocco has initiated the most ambitious strategy with the Moroccan Solar Plan (MSP) playing a fundamental role in the MENA region, demanding that 52% of the installed capacity for electricity production is from renewable energies (RCREEE, 2010 [15]). Hence, by 2020, the plan for the installation of 2 GW solar power production capacity using Concentrated Solar Power (CSP) has been adopted. The 160 MW CSP power station Noor I which is a large-gangway parabolic trough CSP plant was officially commissioned in the province of Ouarzazate in Morocco (see Fig. 1). The first phase encloses the construction of 300 MW plant, the second phase involves the construction of 200 MW and 150 MW plants for Noor II and Noor III respectively, and the third phase involves the construction of the Noor IV CSP plant with a capacity of 70 MW [16]. In accordance with the International Renewable Energy Agency (IRENA), the strategic geographical position offers Morocco to become a regional hub with network interconnections. A series of actions have been taken to follow this program, including legislations, the establishment of renewable energy and energy efficiency agencies and the engagement of different domestic and international stakeholders [13– 15]. Recently, the Moroccan Agency for Energy Efficiency (AMEE) in collaboration with the National Center of Meteorology has developed a new climatic zoning map for Morocco [17,18]. These new zones divide the climate of Morocco into six parts with the same solar irradiation, altitude and other significant indicators. Each zone is marked by a reference city (Fig. 2). Table 1 regroups the different cities of each climatic zone. The originality of the present work is to use the proposed zoning to predict the thermal behavior of the potential CSP projects based on PTC technology concerned by Moroccan Solar Plan. In this way, the current study proposes a detailed thermal analysis of PTC technology for concentrated solar power (CSP) under Moroccan conditions. A physical model is presented to analyze the flow parameters and heat transfer applied to parabolic trough solar collectors. A set of simulations in six chosen areas in Morocco was carried out.
2.2. Mathematical formulation In the following section, the mathematical model of the receiver tube is presented. This model was performed by dividing the receiver into “n” control volumes (CVs), with temperature continuity at the boundary surfaces (Fig. 5). A characteristic of the CV is shown graphically in Fig. 5, where ‘i’ and ‘i+1’ represent the inlet and the outlet sections, respectively. Taking into consideration the characteristic geometry of the receiver, the governing equations have been considered assuming the following assumptions:
2. Physical model 2.1. Parabolic Trough Collector (PTC)
– The radial heat fluxes are assumed uniform and normal to the surfaces for each CV and are evaluated at the average temperature between the outlet and the inlet sections (Ti + Ti+1) . 2 – Axial heat conduction inside the HTF is neglected.
The Parabolic Trough Collector (PTC) which is a sub-technology of the Concentrated Solar Power systems, is the lowest cost large-scale and most proven solar power alternative available today and is also one 1088
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Fig. 2. Climatic zoning of Morocco (Adapted from [18]).
The net heat flux q̇i′ ′ includes solar absorption and heat loss.
Table 1 Moroccan cities classification in accordance with climatic zoning.
qi̇ ′ ′ Ai = qsolarAbs ̇′′ ̇′′ , i Ai − q HeatLoss , i Ai
Climatic zone
Cities
Zone 1
Agadir, Tiznit, Sidi Ifni, Laayoune, Dakhla, Guelmim, Tan Tan, Kenitra, Rabat, Sale, Casablanca, El Jadida, Safi, Essaouira Tangier, Tetouane, Larache, Al Houceima, Nador Fez, Meknes, Sidi Slimane, Chefchaouen, Taza, Oujda, Berrechid, Settat, Fkih Ben Salah, Khouribga, Beni Mellal Ifrane, El Hajeb, Azzrou, Khenifra, Immouzzer Du Kandar, Midelt Marrakech, Benguerir, Kalaa Seraghna Errachidia, Taroudant, Ouarzazate, Smara, Bouarfa
Zone 2 Zone 3 Zone 4 Zone 5 Zone 6
The solar absorption term includes both the absorber tube ′ and the glass envelope (q5̇ ′SolAbs , i ).
′′ ′′ qSolarAbs ̇′′ , i Ai = q3̇ SolAbs, i Δx + q5̇ SolAbs, i Δx
(3)
′ ′′ The terms q̇3′SolAbs , i and q̇5SolAbs, i are treated as thermal fluxes and they are a function of optical losses that include shadowing and tracking efficiency [26]. To enhance the accuracy, a term of direct solar incidence angle Kθ is considered for cases in which the irradiation is not normal to the CV [27], with (Eq. 4) and without (Eq. 5) glass envelope:
– Potential energy is neglected. – Mass flow is constant.
Kθ = cos θ + 0.000884*θ − 0.00005369*θ 2
(4)
Kθ = cos θ + 0.0003512*θ − 0.00003137*θ 2
(5)
The value of θ must be expressed in degrees. The multiplication of these optical terms results in the total optical efficiency of the glass envelope (ηenv ) and the absorbed tube (ηabs ). The other parameters of the glass envelope and the selective coating on the absorber tube are the transmittance (τ ), emissivity (ε ) and absorptance (α ). For the glass envelope, these parameters are constants and independent of temperature, for instance, the absorptance of the selective coating while the emissivity of the selective coating is a function of temperature [26].
The steady-state energy balance over the above mentioned finite CV for the receiver can be estimated with the following equation [26]:
⎡⎛ ⎛ 1 ⎞ 1 ⎞ ⎤ 0 = qi̇ ′ ′ Ai + ṁ ⎢ ⎜h + u2⎟ − ⎜h + u2⎟ ⎥ ⎢⎣ ⎝ 2 ⎠i ⎝ 2 ⎠i +1⎥⎦
(2) ′ (q3̇ ′SolAbs ,i)
(1)
where q′̇ ′ + q′̇ ′
– q̇i′ ′ = i 2 i +1 : the arithmetic average of the net heat flux per unit area between “i” and ‘i+1″ (W/m2) – Ai: circumferential area of the CV (m2) – ṁ : mass flow rate (kg/s) – h : enthalpy (J/kg) – u : bulk fluid velocity (m/s).
′ q5̇ ′SolAbs = qsi̇ ′ ′ ηenv αenv
(6)
′ q3̇ ′SolAbs = qsi̇ ′ ′ ηabs αabs
(7) (q̇si′ ′)
The solar irradiation term in Eqs. (6) and (7) is determined by multiplying the Direct Normal solar Irradiation (DNI) by the projected 1089
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Fig. 3. Schematic of a PTC solar power plant (Adapted from [19]).
Fig. 4. Typical receiver tube of a Parabolic Trough Collector PTC (Adapted from [19]).
Fig. 5. Simplified schematic of a CV of the Parabolic Trough Receiver (Receiver tube).
Table 2 Receiver tube design conditions and parameter studies. Parameters design
Evaluation purposes
HTF flow rate
Determine sensitivity of trough performance to HTF flow rate Determine sensitivity of receiver performance to the type of heat transfer fluid. Evaluate the loss in receiver performance because of loss vacuum in annulus, or a broken glass envelope. Determine sensitivity of trough performance to the mirror reflectance. Determine sensitivity of trough performance to solar incident angle. Determine sensitivity of trough performance to solar insolation.
HTF type Receiver condition and wind speed Mirror reflectance Solar incident angle Solar insolation
Fig. 6. a: Efficiency versus the average HTF temperature for different HTF types (LS-2 Collector, Solel UVAC Cermet, Reflectivity = 0.935, Insolation = 950 W/m2). b: Heat loss versus the average HTF temperature for different HTF types (LS-2 Collector, Solel UVAC Cermet, Reflectivity = 0.935, Insolation = 950 W/m2).
normal reflective surface of the receiver i.e. aperture area and dividing it by the receiver length. ̇′′ The heat loss q HeatLoss , i Ai in Eq. (2) includes the radiation and convection heat losses from the glass envelope and the conductive 1090
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Fig. 7. a: Efficiency versus average HTF temperature for various wind speed and for different receiver tube conditions (LS-2 Collector, Solel UVAC Cermet, Therminol VP1 and V̇ = 8. 832l/s , Insolation = 950 W/m2). b: Heat loss versus average HTF temperature for various wind speed andfor different receiver tube conditions (LS-2 Collector, Solel UVAC Cermet, Therminol VP1 and V̇ = 8. 832l/s , Insolation = 950 W/m2).
Fig. 8. a: Efficiency versus average HTF temperature for different solar weighted reflectivities (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s , Insolation=950 W/m2). b: Heat loss versus average HTF temperature for different solar weighted reflectivities (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s , Insolation=950 W/m2).
losses across the support brackets as written in Eq. (8).
q HeatLoss ̇′′ , i Ai
= q34 ̇ rad , i Δx +
q34 ̇ ′ ′ conv, i Δx
+ qcond ̇ , bracket, total, i
annular space between horizontal cylinders, we can use the following correlation [26]:
(8)
1
where
q34, ̇ cv =
qcond ̇ , bracket, total, i = ni qcond ̇ , bracket , i ni is the number of brackets connected to the CV and is a function of the segment length Δx . The radiation heat transfer q̇34rad between the glass envelope and the absorber tube can be evaluated by assuming that the surface is gray, it diffuses reflections and irradiations, has no interference due to annulus gas and opaque glass on the infrared radiation which has expressed in Eq. (9):
q34 ̇ rad = σπD3
+
(1 − ε4) . D3 ε4 D4
5
3⎞4 ⎛ ⎜1 + ⎛⎜ D3 ⎞⎟ 5 ⎟ ⎜ ⎝ D4 ⎠ ⎟ ⎠ ⎝
(10)
where T4 is the glass envelope inner surface temperature, Pr is the Prandtl number of annulus gas, Ra D3 = number of annulus gas, β =
1 Tavg
gβ (T3 − T4) D33 αϑ
is the Rayleigh
(for an ideal gas), α is the thermal
diffusivity, ϑ is the kinematic viscosity, Tavg = T3 + T4 is the average 2 temperature and all physical properties are evaluated at this temperature and k34 is the thermal conductance of annulus gas at Tavg . The losses through the bracket is the last heat transfer term considered. In this work, the bracket is assumed to behave as infinite fin with base temperature Tb:
(T34 − T44 ) 1 ε3
⎛ PrRa D ⎞ 4 2.425k34 (T3 − T4 ) ⎜ (0, 861 + Pr3 ) ⎟ 34 ⎠ ⎝
(9)
where σ is expressed as Stefan-Boltzmann constant, T3 is the glass envelope inner surface temperature, T4 is outer surface temperature of the absorber pipe, ε3 is the emissivity of the absorber selective coating, ε4 the emissivity of the inner surface of glass envelope, D3 and D4 are the outside diameter of absorber pipe and the inner diameter of the glass envelope, respectively. The term for convection can be evaluated by the average longitudinal temperatures for each segment of receiver. The natural convection occurs between the absorber and glass envelope when the annulus vacuum losses in receiver tube (pressure >~133.322Pa ). In an
qcond ̇ , bracket =
hb Pb kb Acs, b
(Tb − T6 ) LHCE
(11)
where hb is the heat transfer coefficient by convection, Pb is the outer perimeter of the receiver support bracket, Acs,b is the minimum crosssectional area of bracket, kb is the thermal conductance of the bracket, Tb is temperature at base of bracket, T6 is the ambient temperature and LHCE is the receiver tube length. 1091
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assuming that the flow is fully developed and turbulent [26]:
Δpi =
⎛ ṁ ⎞ f . Δx. ⎜ A ⎟ ⎝ cs ⎠ 2.D2 . ρi
(15)
where D2 the inner diameter of absorber pipe and f is the Darcy friction parameter which can be evaluated using the Colebrook correlation [19]. In each CV, the values of the flow variables (temperature, velocity and pressure) at the outlet section of each CV are obtained through the equations Eqs. (13)–(15) from the known values at the inlet section and the boundary conditions. The procedure to obtain the solution is carried out in the manner, moving forward by step in the flow direction. Convergence is checked at each CV using the following condition expressed in Eq. (16):
1−
ϕi*+1 − ϕi Δϕ
≺10−3 (16)
where ϕ refers to the dependent variables (temperature, velocity and pressure); and ϕ* represents their values at the preceding iteration. The reference value Δϕ is locally evaluated as ϕi +1 − ϕi . Environmental data for different climatic sites in Morocco were taken from the meteorological software METEONORM. 3. Results and discussion 3.1. Performance of receiver tube under Ouarzazate CSP project The estimation of the total performance of the Parabolic Trough Receiver (receiver tube) in Ouarzazate region has been evaluated by testing various design conditions and parameters as shown in Table 2. Figs. 6(a) and (b) indicate that the type of the heat transfer fluid has little effect on receiver tube performance. Also, the receiver tube efficiency is maximum at 74% when the average heat transfer fluid temperature is under 100 °C and decreases with the increase in heat transfer fluid temperature to achieve 65% at a temperature of 400 °C. The amount of heat losses increases as the heat transfer fluid temperature increases. Hence, the operating temperature of the fluids should be lower which would improve the performance and efficiency of the heat collector element. It is interesting to note that each HTF type has a recommended range of operating temperatures and the HTF type to be used is only dependent on the cost and availability. Figs. 7(a) and (b) represent the variation of efficiency (heat loss) versus the average HTF temperature for three receiver conditions that are naturally possible (broken glass envelope, loss vacuum and vacuum) and for different wind speeds. This simulation gives us an idea about the amount of heat loss if the receiver tube is broken or the vacuum in the annulus is lost as compared to the ideal conditions when the vacuum is maintained. According to Fig. 7a, one can see that the efficiency of the receiver tube drops dramatically from 65% to zero when the wind speed increases from 0 m/s to 20 m/s. These results indicate the significance of the wind speed on the efficiency of heat collecting element. In addition, Fig. 7b shows that the heat loss increased inside the Parabolic Trough Receiver which gives a better explanation of the efficiency drop. The figure Fig. 8(a) shows the influence of mirror reflectance on the efficiency of the HCE. It is clearly seen that the reflectance of mirror has a strong dependence on the efficiency. As we can see, a small fluctuation in the mirror reflectance from 0.8 to 0.85 increases the efficiency by 5%. These results indicate that it is necessary to schedule a weekly cleaning of this mirrors for maintaining a better efficiency of receiver tube. On the other hand, Fig. 8(b) represents the heat loss across the Parabolic Trough Receiver. One can see weak impact of the mirror
Fig. 9. a: Efficiency versus different solar insolation values (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s ). b: Heat Loss versus different solar insolation values (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s ).
The coefficient hb used (Eq. 11) depends on the wind speed. If there is no wind (≤ 0.1 m/s) hb is estimated by the Churchill and Chu corelationco-relation [26] for natural convection in a long isothermal horizontal cylinder. If there is wind (> 0.1 m/s), hb is calculated using Zhukauskas’Zhukauskas’ correlation [26]. Assuming the heat transfer fluid density is related as a function of temperature, the variation of the specific enthalpy through the CV can be approximated with the following equation:
Δhii +1 ≈ ci . (Ti +1 − Ti )
(12)
where ci is the specific heat of the HTF at the average temperature of the CV. Substituting all the above results into Eq. (1) gives:
Ti +1 =
′ ′′ ̇ rad , i − q34 ̇ conv, i ) . Δx − qcond ̇ , bracket, total, i] [(q3̇ ′SolAbs , i + q5̇ SolAbs, i − q34
ṁ *ci +
1 (u 2 2 i
− ui2+1) ci
+ Ti
(13)
In the above equation, the inlet velocity ui is derived from the absorber volumetric flow rate and cross-sectional area, both of which are inputs. The remaining velocities are predicted from conservation of mass through the CV.
ui +1 =
ṁ ρi Acs
(14)
where ρi is the density of the HTF which is calculated at an average temperature of CV and Acs is the cross-sectional area of the absorber pipe. The variation of the pressure (Δpi ) through CV is evaluated by 1092
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Fig. 10. 1: Energy rate and efficiency in Zone 1 (Agadir). 2: Energy rate and efficiency in Zone 2 (Tangier). 3: Energy rate and efficiency in Zone 3 (Fez). 4: Energy rate and efficiency in Zone 4 (Ifrane). 5: Energy rate and efficiency in Zone 5 (Marrakech). 6: Energy rate and efficiency in Zone 6 (Ouarzazate).
(b). As shown in Fig. 9(a), the different values of solar insolation are in the range from 300 W/m2 to 1000 W/m2 were used in the simulations so as to analyze their effect on the efficiency. Indeed, the increase of the insolation enhances the efficiency since it directly affects the heat gain collected by the receiver tube. Therefore, besides clouds, sites with fragments in the air such as sand, dust and other pollutants would have a high impact on receiver tube performance. Fig. 9(b), represents the heat loss inside the receiver tube for the same values of solar insolation. Thus, we observe that the solar insolation has a heavy influence on the heat loss. In fact, the collector
reflectance on the heat loss due to some parameters that held constant in this simulation [26]. Therefore, the mirror reflectance affects directly the optical losses, the heat gain and the efficiency of the receiver tube. 3.2. Energy analysis of CSP for different sites in Morocco Solar insolation is a key parameter in evaluating the effect of site selection on the receiver tube performance for a CSP plant. The impact of solar irradiation on efficiency and heat loss through the collector versus the average HTF temperature are presented in Figs. 9(a) and 1093
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for improving the performance of potential CSP plants because, in addition to technical parameters related to PTC technology, other aspects are taken into account in the model of solar irradiance, ambient temperature and site characteristics. However, this model has numerous limitations and defects. Among these limitations are:
• • • • •
The model neglects the effects of the no-uniformity in the solar insolation; The wind effects are simplified; The model neglects the effects of radiation heat loss from the receiver; The model does not do optimization or exergy analysis; The model uses only an energy balance and not a detailed study based on Navier-Stokes equations.
Absolutely, the receiver performance model can be improved for more accuracy by reduction of some of these defects. Despite these limitations, the model is more than enough to get an idea about the heat transfers and heat losses in the Parabolic Trough Receiver.
Fig. 11. Monthly efficiency for various climatic zones in Morocco.
used has a strong dependence on heat loss, heat transfer fluid and the flow rate. Thus, these results hint the importance of choosing a suitable site to implement the potential CSP plants. Moreover, there are other aspects that must be considered for the overall project development process like site characteristics, water availability, cost of installation, and infrastructure connections. From an energetic point of view, the concentrated solar power (CSP) using PTC technology and its technical feasibility under the Moroccan conditions, annual simulations of six climatic regions were carried out. Fig. 10.1 to 6 represent the annual energy rates i.e. optical loss, heat loss and heat gain for the six climatic zones in Morocco. From these figures, we can implicate that only heat loss held constant throughout the year while the heat gain and optical losses increase significantly during the sunniest months and decrease during the less sunny months in all climatic zones. The heat gain and optical losses are lower in the season of winter than summer because the plants basically work at lower HTF temperatures during the winter when solar energy requirement is lower. The plots also reveal the optical losses which are much larger than the heat losses. As we can also observe that Ouarzazate (Zone 6, see Fig. 10.6) allows annual energy rates much higher than other zones followed by Ifrane (Zone 4, see Fig. 10.4) due to the change in the climatic conditions (wind speed, ambient temperature, humidity,…). These results reveal that solar irradiation and location are determinant parameters for implementing a thermal solar plant for the production of electricity. Fig. 11 represents the efficiency of the receiver tube for six climatic sites in Morocco. This figure indicates that maximal values of efficiency were attained (approximately 70%) in Ouarzazate site (Zone 6). This result can be explained by the huge solar irradiation and the ideal climatic conditions in this region. In addition, Ifrane (Zone 4) represents the most favorable location of CSP plant, especially during the season of winter. It can also be seen that the change of the efficiency during the year in Tangier (Zone 2) is approximately similar to that observed in the Fez city (Zone 3) with a slight advantage of Fez especially during the summer season where the efficiency can budge the limit of 68%. In Agadir and Marrakech sites (referring to Zones 1 and 5 respectively), the efficiencies have the undermost values compared to those attained in other locations, especially during summer. The efficiency has values around 65% and 67% for Agadir and Marrakech respectively. The solar receiver software model which has been proposed in this work can serve as a tool to improvise and optimize the performance of the Parabolic Trough Receiver in other countries. Indeed, several suggestions and recommendations were drawn with regards to the design and parameter studies effectuated under Ouarzazate CSP project. The applicability of model remains valid for other countries
4. Conclusion A physical model is presented to investigate the technical feasibility of Concentrated Solar Power (CSP) using a parabolic trough collector as a technology under Moroccan conditions. The suggested model is used to examine the influence of different parameters on the Parabolic Trough Receiver performance. Annual simulations in six climatic regions in Morocco were carried out. It is found that maximal values of efficiency were obtained in Ouarzazate site (Zone 6) compared to studied potential sites. The obtained results show also that the selection of thermal heat transfer fluid is a crucial point for increasing CSP efficiency and cost effectiveness. Other aspects must be considered for the global project development process like water availability, site characteristics, infrastructure connections, the cost of installation and market and political environment. The proposed model is more than enough to evaluate heat transfer and heat losses inside the Parabolic Trough Receiver and can be used as a tool to improve and optimize the performance of the CSP projects under different climatic conditions. Acknowledgments The authors acknowledge the support provided by the “Institut de Recherche en Energie Solaire et Energies Nouvelles (IRESENMorocco)” under the project of Solar Cooling Process in Morocco (SCPM). References [1] Aidroos BD, Abdul RH, Wan Z, Wan O, Obaid FS. Historical development of concentrating solar power technologies to generate clean electricity efficiently – a review. Renew Sustain Energy Rev 2015;41:996–1027. [2] Esen M, Yuksel T. Experimental evaluation of using various renewable energy sources for heating a greenhouse. Energy Build 2013;65:340–51. [3] Mahia R, de Arce R, Medina E. Assessing the future of a CSP industry in Morocco. Energy Policy 2014;69:586–97. [4] Brand B, Stambouli AB, Zejli D. The value of dispatchability of CSP plants in the electricity systems of Morocco and Algeria. Energy Policy 2012;47:321–31. [5] ESTELA. Solar power from Europe’s sun belt – a European solar thermo-electric industry initiative contributing to the European Commission Strategic Energy Technology Plan. Brussels: European Solar Thermal Electricity Association; 2009. [6] Komendantova N, Patt A. Employment under vertical and horizontal transfer of concentrated solar power technology to North African countries. Renew Sustain Energy Rev 2014;40:1192–201. [7] Liu Q, Yang M, Lei J, Jin H, Gao Z, Wang Y. Modeling and optimizing parabolic trough solar collector systems using the least squares support vector machine method. Sol Energy 2012;86:1973–80. [8] Esen H, Ozgen F, Esen M, Sengur A. Modelling of a new solar air heater through least-squares support vector machines. Expert Syst Appl 2009;36(7):10673–82. [9] Balghouthi M, Trabelsi SE, Amara MB, Bel Hadj Ali A, Guizani A. Potential of concentrating solar power (CSP) technology in Tunisia and the possibility of interconnection with Europe. Renew Sustain Energy Rev 2016;56:1227–48.
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analysis of parabolic trough solar receiver. Appl Energy 2011;88:5097–110. [20] Hachicha AA, Rodríguez I, Castro J, Oliva A. Numerical simulation of wind flow around a parabolic trough solar collector, Appl. Energy, 107, pp. 426–437. [21] Esen H, Ozgen F, Esen M, Sengur A. Artificial neural network and wavelet neural network approaches for modelling of a solar air heater. Expert Syst Appl 2009;36(8):11240–8. [22] Siqueira AMO, Gomes PEN, Torrezani L, Lucas EO, Pereira GMC. Heat transfer analysis and modeling of a parabolic trough solar collector: an analysis. Energy Procedia 2014;57:401–10. [23] Ozgen F, Esen M, Esen H. Experimental investigation of thermal performance of a double-flow solar air heater having aluminium cans. Renew Energy 2009;34(11):2391–8. [24] Yanjuan W, Qibin L, Jing L, Hongguang J. Performance analysis of a parabolic trough solar collector with non-uniform solar flux conditions. Int J Heat Mass Transf 2015;82:236–49. [25] Esen H, Esen M, Ozsolak O. Modelling and experimental performance analysis of solar-assisted ground source heat pump system. J Exp Theor Artif Intell 2017;29(1):1–17. [26] Forristal RE. Heat Transfer Analysis and Modeling of a Parabolic Trough Solar Receiver Implemented in Engineering Equations Solver. Golden: National Renewable Energy Laboratory; 2003. [27] Gnielinski V. New equations for heat and mass transfer in turbulent pipe and channel flow. Int Chem Eng 1976;16:359–63.
[10] Qoaider L, Liqreina A. Optimization of dry cooled parabolic trough (CSP) plants for the desert regions of the Middle East and North Africa (MENA). Sol Energy 2015;122:976–85. [11] Shouman ER, Khattab NM. Future economic of concentrating solar power (CSP) for electricity generation in Egypt. Renew Sustain Energy Rev 2015;41:1119–27. [12] Serverta JF, Cerrajerob E, Fuentealba EL, Greos S. Feasibility of a CSP power plant in Chile under a PPA model, the role of soft financing and upfront grant. Energy Procedia 2015;69:1704–10. [13] Kousksou T, Allouhi A, Belattar M, Jamil A, El Rhafiki T, Arid A, et al. Renewable energy potential and national policy directions for sustainable development in Morocco. Renew Sustain Energy Rev 2015;47:46–57. [14] Ouammi A, Zejli D, Dagdougui H, Benchrifa R. Artificial neural network analysis of Moroccan solar potential. Renew Sustain Energy Rev 2012;16:4876–89. [15] Kousksou T, Allouhi A, Belattar M, Jamil A, El Rhafiki T, Zeraouli Y. Morocco's strategy for energy security and low-carbon growth. Energy 2015;84:98–105. [16] Moroccan Agency for Solar Energy (MASEN), 〈http://www.masen.ma〉. [17] Allouhi A, Kousksou T, Jamil A, El Rhafiki T, Mourad Y, Zeraouli Y. Economic and environmental assessment of solar air-conditioning systems in Morocco. Renew Sustain Energy Rev 2015;50:770–81. [18] Allouhi A, Jamil A, Kousksou T, El Rhafiki T, Mourad Y, Zeraouli Y. Solar domestic heating water systems in Morocco: an energy analysis. Energy Convers Manag 2015;92:105–13. [19] Padilla RV, Demirkaya G, Goswami DY, Stefanakos E, Rahman MM. Heat transfer
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Technical feasibility of a sustainable Concentrated Solar Power in Morocco through an energy analysis
MARK
⁎
T. Bouhala,b, , Y. Agrouaza,b, T. Kousksoua, A. Allouhib, T. El Rhafikic, A. Jamilb, M. Bakkasc Laboratoire des Sciences de l’Ingénieur Appliquées à la Mécanique et au Génie Electrique (SIAME), Université de Pau et des Pays de l’Adour – IFR – A. Jules Ferry, 64000 Pau, France b Ecole Supérieure de Technologie de Fès, Université de Sidi Mohamed Ben Abdellah (USM.B.A), Route d’Imouzzer, BP 242, Fez Morocco c Ecole Nationale Supérieure d’Arts et Métiers ENSAM, Université Moulay Ismail (U.M.I), Marjane II, BP-4024 Meknès Ismailia, Morocco a
A R T I C L E I N F O
A BS T RAC T
Keywords: Concentrated solar power Parabolic trough solar collectors Physical modelling Heat transfer Site selection Morocco
Morocco imports about 96% of its required energy needs. Solar energy, as one of the most abundant and valuable renewable energy alternatives in the country, offers interesting opportunities for Morocco. In order to minimize its strong foreign energy dependence, Morocco hosts actually the largest Concentrated Solar Power (CSP) using parabolic trough collectors (PTC) as a technology for converting solar irradiation into thermal energy for electricity generation. The purpose of this paper is to assess the thermal performance of this technology and the potential projects concerned by Moroccan Solar Plan. A physical model is developed to determine flow parameters and heat transfer applied to PTC technology. Annual simulations in six climatic regions in Morocco were carried out. Several suggestions were drawn with regards to the design and parametric studies effectuated under Ouarzazate CSP project. It is found that the location and the climate are determinant parameters on the global performance of the parabolic trough solar collectors.
1. Introduction The rising economic competitiveness of Concentrated Solar Power (CSP) with fossil fuels will perform a determinant role in the future [1,2]. At present times, the most advanced technology for solar thermal power production is represented by CSP. One principal focus of interest, and a big contributor to the actual popularity of CSP, concerns the desert regions of North Africa [3,4]. The export of electricity to Europe from the desert locations of the Middle East and North Africa (MENA), which receive some of the highest solar irradiation in the world, is considered as the most encouraging near-term prospect for CSP expansion [5,6]. There have been various models and new methods published to optimize the Parabolic Trough Collectors (PTC) and solar air heaters to have better performance applying the method of Least Squares Support Vector Machine [7,8]. Many researchers all through the globe, have been interested by the technical feasibility of CSP plants in different countries. In Tunisia, a work based on the possibility to interconnect concentrating solar power technology with Europe was carried out by Balghouthi et al. [9]. Based on the results of this study, it was found that Tunisia is in an ideal location for the transfer of electricity produced by CSP plants in North Africa to Europe. This study also proposed the CSP Project to be
economically competitive provided that the majority of the plant equipments be manufactured locally in Tunisia. In the desert areas of the Middle East and North Africa (MENA), an optimization of drycooled parabolic trough (CSP) plants was conducted by Qoaider et al. [10]. Their simulation results show that plants with large thermal energy storage systems and oversized solar field have a better performance, as well as produce power at lower costs in comparison to smaller plants. A conclusion was made, that at a plant site with preeminent solar conditions available, the CSP dry-cooled power plants provide an alternative for green power production. The future economics of concentrating solar power (CSP) for electricity production in Egypt were investigated by Shouman et al. [11]. More specifically, a road map strategy for the launch of CSP in the Egyptian markets was presented by them, removing the main barriers for financing and starting market introduction in the peak and medium load segments of the power supply. The conclusion made was that Egypt is the suitable location for CSP projects worldwide. A feasibility investigation of a CSP power plant in Chile under a Power Purchase Agreements (PPA) model was conducted by Serverta et al. [12]. The roles of upfront and soft financing grants to narrow the gap for the first CSP projects in Northern Chile were highlighted. It was found that CSP technology associated with thermal energy storage suits
⁎ Corresponding author at: Laboratoire des Sciences de l’Ingénieur Appliquées à la Mécanique et au Génie Electrique (SIAME), Université de Pau et des Pays de l’Adour – IFR – A. Jules Ferry, 64000 Pau, France. E-mail address: [email protected] (T. Bouhal).
http://dx.doi.org/10.1016/j.rser.2017.08.056 Received 21 June 2016; Received in revised form 12 June 2017; Accepted 12 August 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Concentrated solar power using a parabolic trough collector: 160 MW power plant, Noor I in Ouarzazate solar complex, Morocco.
of the main renewable energy options for electricity production. The power plants based on PTC usually use a Heat Transfer Fluid (HTF) to collect heat energy which makes it possible to integrate this heat in a Rankine water/steam power cycle to generate electricity [19–21]. The PTCs are composed of parabolic trough-shaped mirrors, which reflect the incident radiation from the sun on the solar receiver tube. As illustrated in Fig. 3, the HTF flows across the receiver tube of PTC and is heated up under the effect of incident radiation. Then, the HTF is collected to be sent to the power block, where it passes through a set of heat exchangers and produces superheated steam at high temperatures. The superheated steam drives a steam turbine where rotational mechanical energy is then converted into electricity [22,7,23]. A thermal oil is commonly used as a working fluid that circulate through the absorber tube and transform the solar irradiation into thermal energy and carries heat to heat exchangers or analogous for driving a Rankine steam turbine. The Parabolic Trough Receiver (receiver tube) (see Fig. 4) comprises of a tube of stainless steel accompanied by a selective metalceramic coating enclosed by an evacuated anti-reflective glass tube. The vacuum envelope mainly enables to notably preserve the surface of the absorber from oxidation and to decrease thermal losses at increased operating temperatures. The vacuum in the receiver tube should be below the conduction band Knudsen gas to decrease convective losses in the annular space, which is naturally preserved near to 0.0001 mmHg. The metal-ceramic multilayer hedge is positioned over the steel tube to afford low thermal emissivity and optimum optical properties with significant absorptivity of direct solar radiation at the temperature of operation to minimize heat radiation. The glass envelope has an outer to minimize both antireflective coating and Fresnel reflective losses from the glass surface [7,24,25].
better to the demand profile of energy needs. Morocco has emerged as among the first in establishing regulatory frameworks and policies for promoting renewable energy sources and energy efficiency, especially in the solar and the wind energy sectors in the Mediterranean area [13,14]. Morocco has initiated the most ambitious strategy with the Moroccan Solar Plan (MSP) playing a fundamental role in the MENA region, demanding that 52% of the installed capacity for electricity production is from renewable energies (RCREEE, 2010 [15]). Hence, by 2020, the plan for the installation of 2 GW solar power production capacity using Concentrated Solar Power (CSP) has been adopted. The 160 MW CSP power station Noor I which is a large-gangway parabolic trough CSP plant was officially commissioned in the province of Ouarzazate in Morocco (see Fig. 1). The first phase encloses the construction of 300 MW plant, the second phase involves the construction of 200 MW and 150 MW plants for Noor II and Noor III respectively, and the third phase involves the construction of the Noor IV CSP plant with a capacity of 70 MW [16]. In accordance with the International Renewable Energy Agency (IRENA), the strategic geographical position offers Morocco to become a regional hub with network interconnections. A series of actions have been taken to follow this program, including legislations, the establishment of renewable energy and energy efficiency agencies and the engagement of different domestic and international stakeholders [13– 15]. Recently, the Moroccan Agency for Energy Efficiency (AMEE) in collaboration with the National Center of Meteorology has developed a new climatic zoning map for Morocco [17,18]. These new zones divide the climate of Morocco into six parts with the same solar irradiation, altitude and other significant indicators. Each zone is marked by a reference city (Fig. 2). Table 1 regroups the different cities of each climatic zone. The originality of the present work is to use the proposed zoning to predict the thermal behavior of the potential CSP projects based on PTC technology concerned by Moroccan Solar Plan. In this way, the current study proposes a detailed thermal analysis of PTC technology for concentrated solar power (CSP) under Moroccan conditions. A physical model is presented to analyze the flow parameters and heat transfer applied to parabolic trough solar collectors. A set of simulations in six chosen areas in Morocco was carried out.
2.2. Mathematical formulation In the following section, the mathematical model of the receiver tube is presented. This model was performed by dividing the receiver into “n” control volumes (CVs), with temperature continuity at the boundary surfaces (Fig. 5). A characteristic of the CV is shown graphically in Fig. 5, where ‘i’ and ‘i+1’ represent the inlet and the outlet sections, respectively. Taking into consideration the characteristic geometry of the receiver, the governing equations have been considered assuming the following assumptions:
2. Physical model 2.1. Parabolic Trough Collector (PTC)
– The radial heat fluxes are assumed uniform and normal to the surfaces for each CV and are evaluated at the average temperature between the outlet and the inlet sections (Ti + Ti+1) . 2 – Axial heat conduction inside the HTF is neglected.
The Parabolic Trough Collector (PTC) which is a sub-technology of the Concentrated Solar Power systems, is the lowest cost large-scale and most proven solar power alternative available today and is also one 1088
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Fig. 2. Climatic zoning of Morocco (Adapted from [18]).
The net heat flux q̇i′ ′ includes solar absorption and heat loss.
Table 1 Moroccan cities classification in accordance with climatic zoning.
qi̇ ′ ′ Ai = qsolarAbs ̇′′ ̇′′ , i Ai − q HeatLoss , i Ai
Climatic zone
Cities
Zone 1
Agadir, Tiznit, Sidi Ifni, Laayoune, Dakhla, Guelmim, Tan Tan, Kenitra, Rabat, Sale, Casablanca, El Jadida, Safi, Essaouira Tangier, Tetouane, Larache, Al Houceima, Nador Fez, Meknes, Sidi Slimane, Chefchaouen, Taza, Oujda, Berrechid, Settat, Fkih Ben Salah, Khouribga, Beni Mellal Ifrane, El Hajeb, Azzrou, Khenifra, Immouzzer Du Kandar, Midelt Marrakech, Benguerir, Kalaa Seraghna Errachidia, Taroudant, Ouarzazate, Smara, Bouarfa
Zone 2 Zone 3 Zone 4 Zone 5 Zone 6
The solar absorption term includes both the absorber tube ′ and the glass envelope (q5̇ ′SolAbs , i ).
′′ ′′ qSolarAbs ̇′′ , i Ai = q3̇ SolAbs, i Δx + q5̇ SolAbs, i Δx
(3)
′ ′′ The terms q̇3′SolAbs , i and q̇5SolAbs, i are treated as thermal fluxes and they are a function of optical losses that include shadowing and tracking efficiency [26]. To enhance the accuracy, a term of direct solar incidence angle Kθ is considered for cases in which the irradiation is not normal to the CV [27], with (Eq. 4) and without (Eq. 5) glass envelope:
– Potential energy is neglected. – Mass flow is constant.
Kθ = cos θ + 0.000884*θ − 0.00005369*θ 2
(4)
Kθ = cos θ + 0.0003512*θ − 0.00003137*θ 2
(5)
The value of θ must be expressed in degrees. The multiplication of these optical terms results in the total optical efficiency of the glass envelope (ηenv ) and the absorbed tube (ηabs ). The other parameters of the glass envelope and the selective coating on the absorber tube are the transmittance (τ ), emissivity (ε ) and absorptance (α ). For the glass envelope, these parameters are constants and independent of temperature, for instance, the absorptance of the selective coating while the emissivity of the selective coating is a function of temperature [26].
The steady-state energy balance over the above mentioned finite CV for the receiver can be estimated with the following equation [26]:
⎡⎛ ⎛ 1 ⎞ 1 ⎞ ⎤ 0 = qi̇ ′ ′ Ai + ṁ ⎢ ⎜h + u2⎟ − ⎜h + u2⎟ ⎥ ⎢⎣ ⎝ 2 ⎠i ⎝ 2 ⎠i +1⎥⎦
(2) ′ (q3̇ ′SolAbs ,i)
(1)
where q′̇ ′ + q′̇ ′
– q̇i′ ′ = i 2 i +1 : the arithmetic average of the net heat flux per unit area between “i” and ‘i+1″ (W/m2) – Ai: circumferential area of the CV (m2) – ṁ : mass flow rate (kg/s) – h : enthalpy (J/kg) – u : bulk fluid velocity (m/s).
′ q5̇ ′SolAbs = qsi̇ ′ ′ ηenv αenv
(6)
′ q3̇ ′SolAbs = qsi̇ ′ ′ ηabs αabs
(7) (q̇si′ ′)
The solar irradiation term in Eqs. (6) and (7) is determined by multiplying the Direct Normal solar Irradiation (DNI) by the projected 1089
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Fig. 3. Schematic of a PTC solar power plant (Adapted from [19]).
Fig. 4. Typical receiver tube of a Parabolic Trough Collector PTC (Adapted from [19]).
Fig. 5. Simplified schematic of a CV of the Parabolic Trough Receiver (Receiver tube).
Table 2 Receiver tube design conditions and parameter studies. Parameters design
Evaluation purposes
HTF flow rate
Determine sensitivity of trough performance to HTF flow rate Determine sensitivity of receiver performance to the type of heat transfer fluid. Evaluate the loss in receiver performance because of loss vacuum in annulus, or a broken glass envelope. Determine sensitivity of trough performance to the mirror reflectance. Determine sensitivity of trough performance to solar incident angle. Determine sensitivity of trough performance to solar insolation.
HTF type Receiver condition and wind speed Mirror reflectance Solar incident angle Solar insolation
Fig. 6. a: Efficiency versus the average HTF temperature for different HTF types (LS-2 Collector, Solel UVAC Cermet, Reflectivity = 0.935, Insolation = 950 W/m2). b: Heat loss versus the average HTF temperature for different HTF types (LS-2 Collector, Solel UVAC Cermet, Reflectivity = 0.935, Insolation = 950 W/m2).
normal reflective surface of the receiver i.e. aperture area and dividing it by the receiver length. ̇′′ The heat loss q HeatLoss , i Ai in Eq. (2) includes the radiation and convection heat losses from the glass envelope and the conductive 1090
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Fig. 7. a: Efficiency versus average HTF temperature for various wind speed and for different receiver tube conditions (LS-2 Collector, Solel UVAC Cermet, Therminol VP1 and V̇ = 8. 832l/s , Insolation = 950 W/m2). b: Heat loss versus average HTF temperature for various wind speed andfor different receiver tube conditions (LS-2 Collector, Solel UVAC Cermet, Therminol VP1 and V̇ = 8. 832l/s , Insolation = 950 W/m2).
Fig. 8. a: Efficiency versus average HTF temperature for different solar weighted reflectivities (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s , Insolation=950 W/m2). b: Heat loss versus average HTF temperature for different solar weighted reflectivities (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s , Insolation=950 W/m2).
losses across the support brackets as written in Eq. (8).
q HeatLoss ̇′′ , i Ai
= q34 ̇ rad , i Δx +
q34 ̇ ′ ′ conv, i Δx
+ qcond ̇ , bracket, total, i
annular space between horizontal cylinders, we can use the following correlation [26]:
(8)
1
where
q34, ̇ cv =
qcond ̇ , bracket, total, i = ni qcond ̇ , bracket , i ni is the number of brackets connected to the CV and is a function of the segment length Δx . The radiation heat transfer q̇34rad between the glass envelope and the absorber tube can be evaluated by assuming that the surface is gray, it diffuses reflections and irradiations, has no interference due to annulus gas and opaque glass on the infrared radiation which has expressed in Eq. (9):
q34 ̇ rad = σπD3
+
(1 − ε4) . D3 ε4 D4
5
3⎞4 ⎛ ⎜1 + ⎛⎜ D3 ⎞⎟ 5 ⎟ ⎜ ⎝ D4 ⎠ ⎟ ⎠ ⎝
(10)
where T4 is the glass envelope inner surface temperature, Pr is the Prandtl number of annulus gas, Ra D3 = number of annulus gas, β =
1 Tavg
gβ (T3 − T4) D33 αϑ
is the Rayleigh
(for an ideal gas), α is the thermal
diffusivity, ϑ is the kinematic viscosity, Tavg = T3 + T4 is the average 2 temperature and all physical properties are evaluated at this temperature and k34 is the thermal conductance of annulus gas at Tavg . The losses through the bracket is the last heat transfer term considered. In this work, the bracket is assumed to behave as infinite fin with base temperature Tb:
(T34 − T44 ) 1 ε3
⎛ PrRa D ⎞ 4 2.425k34 (T3 − T4 ) ⎜ (0, 861 + Pr3 ) ⎟ 34 ⎠ ⎝
(9)
where σ is expressed as Stefan-Boltzmann constant, T3 is the glass envelope inner surface temperature, T4 is outer surface temperature of the absorber pipe, ε3 is the emissivity of the absorber selective coating, ε4 the emissivity of the inner surface of glass envelope, D3 and D4 are the outside diameter of absorber pipe and the inner diameter of the glass envelope, respectively. The term for convection can be evaluated by the average longitudinal temperatures for each segment of receiver. The natural convection occurs between the absorber and glass envelope when the annulus vacuum losses in receiver tube (pressure >~133.322Pa ). In an
qcond ̇ , bracket =
hb Pb kb Acs, b
(Tb − T6 ) LHCE
(11)
where hb is the heat transfer coefficient by convection, Pb is the outer perimeter of the receiver support bracket, Acs,b is the minimum crosssectional area of bracket, kb is the thermal conductance of the bracket, Tb is temperature at base of bracket, T6 is the ambient temperature and LHCE is the receiver tube length. 1091
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assuming that the flow is fully developed and turbulent [26]:
Δpi =
⎛ ṁ ⎞ f . Δx. ⎜ A ⎟ ⎝ cs ⎠ 2.D2 . ρi
(15)
where D2 the inner diameter of absorber pipe and f is the Darcy friction parameter which can be evaluated using the Colebrook correlation [19]. In each CV, the values of the flow variables (temperature, velocity and pressure) at the outlet section of each CV are obtained through the equations Eqs. (13)–(15) from the known values at the inlet section and the boundary conditions. The procedure to obtain the solution is carried out in the manner, moving forward by step in the flow direction. Convergence is checked at each CV using the following condition expressed in Eq. (16):
1−
ϕi*+1 − ϕi Δϕ
≺10−3 (16)
where ϕ refers to the dependent variables (temperature, velocity and pressure); and ϕ* represents their values at the preceding iteration. The reference value Δϕ is locally evaluated as ϕi +1 − ϕi . Environmental data for different climatic sites in Morocco were taken from the meteorological software METEONORM. 3. Results and discussion 3.1. Performance of receiver tube under Ouarzazate CSP project The estimation of the total performance of the Parabolic Trough Receiver (receiver tube) in Ouarzazate region has been evaluated by testing various design conditions and parameters as shown in Table 2. Figs. 6(a) and (b) indicate that the type of the heat transfer fluid has little effect on receiver tube performance. Also, the receiver tube efficiency is maximum at 74% when the average heat transfer fluid temperature is under 100 °C and decreases with the increase in heat transfer fluid temperature to achieve 65% at a temperature of 400 °C. The amount of heat losses increases as the heat transfer fluid temperature increases. Hence, the operating temperature of the fluids should be lower which would improve the performance and efficiency of the heat collector element. It is interesting to note that each HTF type has a recommended range of operating temperatures and the HTF type to be used is only dependent on the cost and availability. Figs. 7(a) and (b) represent the variation of efficiency (heat loss) versus the average HTF temperature for three receiver conditions that are naturally possible (broken glass envelope, loss vacuum and vacuum) and for different wind speeds. This simulation gives us an idea about the amount of heat loss if the receiver tube is broken or the vacuum in the annulus is lost as compared to the ideal conditions when the vacuum is maintained. According to Fig. 7a, one can see that the efficiency of the receiver tube drops dramatically from 65% to zero when the wind speed increases from 0 m/s to 20 m/s. These results indicate the significance of the wind speed on the efficiency of heat collecting element. In addition, Fig. 7b shows that the heat loss increased inside the Parabolic Trough Receiver which gives a better explanation of the efficiency drop. The figure Fig. 8(a) shows the influence of mirror reflectance on the efficiency of the HCE. It is clearly seen that the reflectance of mirror has a strong dependence on the efficiency. As we can see, a small fluctuation in the mirror reflectance from 0.8 to 0.85 increases the efficiency by 5%. These results indicate that it is necessary to schedule a weekly cleaning of this mirrors for maintaining a better efficiency of receiver tube. On the other hand, Fig. 8(b) represents the heat loss across the Parabolic Trough Receiver. One can see weak impact of the mirror
Fig. 9. a: Efficiency versus different solar insolation values (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s ). b: Heat Loss versus different solar insolation values (LS-2 Collector, Solel UVAC Cermet, Therminol VP1, V̇ = 8. 832l/s ).
The coefficient hb used (Eq. 11) depends on the wind speed. If there is no wind (≤ 0.1 m/s) hb is estimated by the Churchill and Chu corelationco-relation [26] for natural convection in a long isothermal horizontal cylinder. If there is wind (> 0.1 m/s), hb is calculated using Zhukauskas’Zhukauskas’ correlation [26]. Assuming the heat transfer fluid density is related as a function of temperature, the variation of the specific enthalpy through the CV can be approximated with the following equation:
Δhii +1 ≈ ci . (Ti +1 − Ti )
(12)
where ci is the specific heat of the HTF at the average temperature of the CV. Substituting all the above results into Eq. (1) gives:
Ti +1 =
′ ′′ ̇ rad , i − q34 ̇ conv, i ) . Δx − qcond ̇ , bracket, total, i] [(q3̇ ′SolAbs , i + q5̇ SolAbs, i − q34
ṁ *ci +
1 (u 2 2 i
− ui2+1) ci
+ Ti
(13)
In the above equation, the inlet velocity ui is derived from the absorber volumetric flow rate and cross-sectional area, both of which are inputs. The remaining velocities are predicted from conservation of mass through the CV.
ui +1 =
ṁ ρi Acs
(14)
where ρi is the density of the HTF which is calculated at an average temperature of CV and Acs is the cross-sectional area of the absorber pipe. The variation of the pressure (Δpi ) through CV is evaluated by 1092
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Fig. 10. 1: Energy rate and efficiency in Zone 1 (Agadir). 2: Energy rate and efficiency in Zone 2 (Tangier). 3: Energy rate and efficiency in Zone 3 (Fez). 4: Energy rate and efficiency in Zone 4 (Ifrane). 5: Energy rate and efficiency in Zone 5 (Marrakech). 6: Energy rate and efficiency in Zone 6 (Ouarzazate).
(b). As shown in Fig. 9(a), the different values of solar insolation are in the range from 300 W/m2 to 1000 W/m2 were used in the simulations so as to analyze their effect on the efficiency. Indeed, the increase of the insolation enhances the efficiency since it directly affects the heat gain collected by the receiver tube. Therefore, besides clouds, sites with fragments in the air such as sand, dust and other pollutants would have a high impact on receiver tube performance. Fig. 9(b), represents the heat loss inside the receiver tube for the same values of solar insolation. Thus, we observe that the solar insolation has a heavy influence on the heat loss. In fact, the collector
reflectance on the heat loss due to some parameters that held constant in this simulation [26]. Therefore, the mirror reflectance affects directly the optical losses, the heat gain and the efficiency of the receiver tube. 3.2. Energy analysis of CSP for different sites in Morocco Solar insolation is a key parameter in evaluating the effect of site selection on the receiver tube performance for a CSP plant. The impact of solar irradiation on efficiency and heat loss through the collector versus the average HTF temperature are presented in Figs. 9(a) and 1093
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for improving the performance of potential CSP plants because, in addition to technical parameters related to PTC technology, other aspects are taken into account in the model of solar irradiance, ambient temperature and site characteristics. However, this model has numerous limitations and defects. Among these limitations are:
• • • • •
The model neglects the effects of the no-uniformity in the solar insolation; The wind effects are simplified; The model neglects the effects of radiation heat loss from the receiver; The model does not do optimization or exergy analysis; The model uses only an energy balance and not a detailed study based on Navier-Stokes equations.
Absolutely, the receiver performance model can be improved for more accuracy by reduction of some of these defects. Despite these limitations, the model is more than enough to get an idea about the heat transfers and heat losses in the Parabolic Trough Receiver.
Fig. 11. Monthly efficiency for various climatic zones in Morocco.
used has a strong dependence on heat loss, heat transfer fluid and the flow rate. Thus, these results hint the importance of choosing a suitable site to implement the potential CSP plants. Moreover, there are other aspects that must be considered for the overall project development process like site characteristics, water availability, cost of installation, and infrastructure connections. From an energetic point of view, the concentrated solar power (CSP) using PTC technology and its technical feasibility under the Moroccan conditions, annual simulations of six climatic regions were carried out. Fig. 10.1 to 6 represent the annual energy rates i.e. optical loss, heat loss and heat gain for the six climatic zones in Morocco. From these figures, we can implicate that only heat loss held constant throughout the year while the heat gain and optical losses increase significantly during the sunniest months and decrease during the less sunny months in all climatic zones. The heat gain and optical losses are lower in the season of winter than summer because the plants basically work at lower HTF temperatures during the winter when solar energy requirement is lower. The plots also reveal the optical losses which are much larger than the heat losses. As we can also observe that Ouarzazate (Zone 6, see Fig. 10.6) allows annual energy rates much higher than other zones followed by Ifrane (Zone 4, see Fig. 10.4) due to the change in the climatic conditions (wind speed, ambient temperature, humidity,…). These results reveal that solar irradiation and location are determinant parameters for implementing a thermal solar plant for the production of electricity. Fig. 11 represents the efficiency of the receiver tube for six climatic sites in Morocco. This figure indicates that maximal values of efficiency were attained (approximately 70%) in Ouarzazate site (Zone 6). This result can be explained by the huge solar irradiation and the ideal climatic conditions in this region. In addition, Ifrane (Zone 4) represents the most favorable location of CSP plant, especially during the season of winter. It can also be seen that the change of the efficiency during the year in Tangier (Zone 2) is approximately similar to that observed in the Fez city (Zone 3) with a slight advantage of Fez especially during the summer season where the efficiency can budge the limit of 68%. In Agadir and Marrakech sites (referring to Zones 1 and 5 respectively), the efficiencies have the undermost values compared to those attained in other locations, especially during summer. The efficiency has values around 65% and 67% for Agadir and Marrakech respectively. The solar receiver software model which has been proposed in this work can serve as a tool to improvise and optimize the performance of the Parabolic Trough Receiver in other countries. Indeed, several suggestions and recommendations were drawn with regards to the design and parameter studies effectuated under Ouarzazate CSP project. The applicability of model remains valid for other countries
4. Conclusion A physical model is presented to investigate the technical feasibility of Concentrated Solar Power (CSP) using a parabolic trough collector as a technology under Moroccan conditions. The suggested model is used to examine the influence of different parameters on the Parabolic Trough Receiver performance. Annual simulations in six climatic regions in Morocco were carried out. It is found that maximal values of efficiency were obtained in Ouarzazate site (Zone 6) compared to studied potential sites. The obtained results show also that the selection of thermal heat transfer fluid is a crucial point for increasing CSP efficiency and cost effectiveness. Other aspects must be considered for the global project development process like water availability, site characteristics, infrastructure connections, the cost of installation and market and political environment. The proposed model is more than enough to evaluate heat transfer and heat losses inside the Parabolic Trough Receiver and can be used as a tool to improve and optimize the performance of the CSP projects under different climatic conditions. Acknowledgments The authors acknowledge the support provided by the “Institut de Recherche en Energie Solaire et Energies Nouvelles (IRESENMorocco)” under the project of Solar Cooling Process in Morocco (SCPM). References [1] Aidroos BD, Abdul RH, Wan Z, Wan O, Obaid FS. Historical development of concentrating solar power technologies to generate clean electricity efficiently – a review. Renew Sustain Energy Rev 2015;41:996–1027. [2] Esen M, Yuksel T. Experimental evaluation of using various renewable energy sources for heating a greenhouse. Energy Build 2013;65:340–51. [3] Mahia R, de Arce R, Medina E. Assessing the future of a CSP industry in Morocco. Energy Policy 2014;69:586–97. [4] Brand B, Stambouli AB, Zejli D. The value of dispatchability of CSP plants in the electricity systems of Morocco and Algeria. Energy Policy 2012;47:321–31. [5] ESTELA. Solar power from Europe’s sun belt – a European solar thermo-electric industry initiative contributing to the European Commission Strategic Energy Technology Plan. Brussels: European Solar Thermal Electricity Association; 2009. [6] Komendantova N, Patt A. Employment under vertical and horizontal transfer of concentrated solar power technology to North African countries. Renew Sustain Energy Rev 2014;40:1192–201. [7] Liu Q, Yang M, Lei J, Jin H, Gao Z, Wang Y. Modeling and optimizing parabolic trough solar collector systems using the least squares support vector machine method. Sol Energy 2012;86:1973–80. [8] Esen H, Ozgen F, Esen M, Sengur A. Modelling of a new solar air heater through least-squares support vector machines. Expert Syst Appl 2009;36(7):10673–82. [9] Balghouthi M, Trabelsi SE, Amara MB, Bel Hadj Ali A, Guizani A. Potential of concentrating solar power (CSP) technology in Tunisia and the possibility of interconnection with Europe. Renew Sustain Energy Rev 2016;56:1227–48.
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