Solar Thermal System Analysis for Northern Cyprus

SOLAR THERMAL SYSTEM ANALYSIS OF NORTHERN CYPRUS Mehmet Yenen Furkan Ercan Murat Fahrioglu Sustainable Environment and Energy Systems Middle East Technical University NCC TRNC [email protected] Sustainable Environment and Energy Systems Middle East Technical University NCC TRNC [email protected] Department of Electrical and Electronics Engineering Middle East Technical University NCC TRNC [email protected] Abstract— In recent years, the world as a whole tend to extract energy from renewable energy resources and fit it to the supply-demand chain due to the current resources coming to an end and harming the environment with CO2 emissions. In this paper, we present the solar opportunity as a renewable energy resource for Northern Cyprus, and based on measured data we compare two different solar energy collectors: Parabolic Trough and Fresnel systems. Depending on the beam and diffuse insolation and the topography, these systems’ advantages and disadvantages will be discussed, and a cost analysis will be presented throughout this paper. Keywords: Renewable energy, Concentrated Solar Power, Solar Thermal Energy, Fresnel Systems, Parabolic trough collector, Northern Cyprus. 1. INTRODUCTION Strategically positioned at the crossroads of Asia, Europe and the Middle East, Cyprus has a significant potential of solar energy harvesting. It is a fact that other than isolated individual cases, electrical energy demand of the island is met almost entirely with nonrenewable energy resources. Most of the residences are equipped with solar water heating systems. Out of all sorts of energy types, 4% of Cyprus’ energy consumption is met by solar energy, of which almost entirely used for heating water [25]. Such a solar energy potential may also be employed to produce electrical energy. Among EU countries, Cyprus is considered to have one of the top solar energy potential. Northern Cyprus depends heavily on imported energy sources. No oil or gas reserves are found on or around Northern Cyprus until now, yet the energy consumption depends entirely on oil and petroleum products. For Northern Cyprus, Cyprus Turkish Electricity Authority (KIB-TEK) generates, distributes and sells power. It has two 60 MW fuel oil fired generators that carry the base load and six 17.5 MW diesel generators for support [1]. Another energy company in Northern Cyprus, AKSA, provides 92 MW of capacity to the system by using diesel generators [1]. Lastly, there is a solar photovoltaic (PV) power plant in Serhatköy which has a capacity of 1.27 MW. Total electrical energy capacity of Northern Cyprus is 313 MW, about 99 % of which depends significantly on imported energy resources. Among these resources, fuel oil has the major share. Another major problem with using such resources to produce electrical energy is CO2 emissions, a major factor of Greenhouse gases that threatens the entire planet. It may be considered to be an issue of the overall planet, yet it can be emphasized as a national issue once it is highlighted with measurements. In Cyprus, the CO2 emission level in 2005 is measured to be 63.7% more than of 1990 [10]. Using renewable energy resources to meet the island’s demand may stop this growth and may even help to reduce it. Due to these facts and reasons, in this paper we present the analysis and evaluation of using solar panels in terms of their efficiency and cost. Fresnel systems and parabolic trough collector systems are within the scope of this paper. A comprehensive study was carried out for evaluating the position of the sun for designing and building a sun-tracking system, which measures direct solar radiation with a pyrheliometer [2]. Another valuable study indicates the two axes tracking on the solar energy collection [3]. According to the study, the measured solar energy collected on the tracking surface has approximately 40% higher efficiency than that on a fixed surface. . In [4] the authors carried out for various wind velocities and different collector orientations with respect to wind direction effect to the Parabolic through Collectors (PTC) system. Furthermore, in the study of Padilla et. al. [5], it is indicated that heat transfer and optical analysis of the PTC is significant to identify the efficiency and performance. We use hourly direct and diffuse solar insolation data in order to develop a simple mathematical model for analyzing solar irradiation. The appropriate data is obtained from Middle East Technical University Northern Cyprus Campus (METU NCC) solar data observation station over a two year period. The period is chosen based on data availability in particular, so as to obtain a comparable sample period for all energy series. The rest of this paper is organized as follows: In Section 2, we provide a general background regarding PTC and Fresnel System (FS) collector types. We detail system modeling in Section 3, and introduce our evaluation and results in Section 4. We conclude our work in Section 5 and discuss the future work in Section 6. 2. BACKGROUND 2.1. Motivation Understanding the global energy problems and its influences to the environment may create better estimation of solution for future generations. Hourly direct and diffuse irradiance solar energy has a significant role for green buildings, photovoltaic systems and concentrating solar power applications. Based on these data, we compare different types of solar energy harvesting systems, to be described in detail in the Background section. There are maps of world solar radiation data, yet they do not include enough detailed description about determination of solar energy and its concentration. We need such data for smaller scale systems as well as larger scales, thus we do not rely on such maps. Amount of the power depends on the amount of exposed sunlight on a system which performs solar insulation. Nonconcentrating systems are easier and cheaper to construct but sun tracking systems usually provides better results. On the other hand, these systems depend on the beam and diffused solar insulation. If two solar systems with different technologies have about the same amount of efficiency and output, in order to be cost-efficient and to encourage the enterpriser, the system with the lower cost is preferred. 2.2. Beam and Diffuse Insolation There are two main types of solar irradiation, called as beam insolation and diffuse insolation (or irradiation). Beam insolation is typically the insolation arrives directly from the sun, e.g. a type of irradiation has a direct path from the sun to the target. Diffuse insolation is the type of solar irradiation that is reflected from other objects, e.g. does not have a direct path from the sun and arrives through surrounding objects. One of the common objects that convert beam insolation to diffuse insolation on its way to target is the clouds. Density of beam or diffuse depends heavily on the topography; that is, based on the weather characteristics, diffuse or beam irradiation ratio may differ. To illustrate, depending on the high cloud density and humidity over the year, diffuse irradiation is dominant over the topography of many European countries. On the other hand, in Middle East and North Africa (MENA) regions, beam irradiation is dominant over diffuse irradiation due to the general weather properties over a year. Both beam and diffuse irradiations are collectible insolation types, and they are efficiently collected via different solar panels. Parabolic trough collector systems are efficient at collecting beam irradiation but are incapable of collecting diffuse irradiation due to their mechanism. Fresnel systems on the other hand are capable of collecting both types of irradiation. Yet they are not as efficient as PTC systems in terms of absorbing beam irradiation. Figure 1 presents both types of solar irradiations. Figure 1. Beam and diffuse irradiation types emerge due to weather conditions [11]. Depending on the reasons encountered, the main question is which type of concentrating solar panel system is more efficient in total in electricity generation for Northern Cyprus. Thus, a general weather condition approach is vital for our analysis. 2.3. Rankine Cycle Rankine Cycle is a basic, most commonly used steam-operated energy production system. A cycling fluid (i.e. water) is heated and turned into steam. It is then transferred to a turbine where the flow of the steam forces the turbine to rotate. The energy produced in this closed loop system generates electricity. Four subsystems are employed in a typical Rankine Cycle: pump motor, heating system, turbine, and cooling system. A detailed description of Rankine Cycle is depicted in Figure 2. Figure 2. A representative Rankine Cycle flow for Solar Thermal Energy Systems i. Pump motor is used for continuous fluid cycle. This equipment is significantly important in order to increase the pressure of the closed loop system. ii. Heating system is another important part of Rankine Cycle which helps fluid heat up to a critical point. It may consist of different kinds of combustion technologies. Thermal Power Plants (TPP) use coal and fuel oil types systems in general. Recent technological developments support various techniques for these applications. Including solar thermal energy systems (STE) or geothermal applications to assist main thermal plant are two examples of state-of-the-art. iii. iv. Rankine turbine is a machine that converts kinetic power of the moving fluid to mechanical power. Applying the mechanical power to a generator, mechanical power is converted to electrical power. For this analysis, Carnot efficiency is applied and used in this work. This efficiency describes the conversion efficiency factor of the closed loop system for thermal analysis. Second law of the thermodynamics identify that even an ideal engine is not able to convert 100 % of its input to energy. Because the most important limiting factors are hot temperature at which the heat enters the engine, (Thot) and the cold temperature of the environment into which the engine exhausts its waste heat (Tcold). Cooling System is the last part of defining a Rankine Cycle. It simply transforms moving fluid from gas phase back to fluid phase in order to obtain cold temperature of Carnot efficiency. 2.4. Energy Demand As we need to choose a base model for our study, we consider METU NCC as an investor to solar energy and as a demander for electrical energy. The primary reason why we choose METU NCC is the availability of data that we need throughout our work. In our model, we assume that METU NCC would want to maximize the electricity it uses generated by solar energy; and minimize the electrical energy used which is produced from propane gas since it is not cost-efficient. The leads to three operational models for our system: solar only, solar and propane combined, and propane only. For simplicity of our model, we assume a fixed demand of 8kW as hourly usage. In reality, an approximate demand for daytime would require 1 MW, yet we choose a less amount of demand since our work is accepted as a prototype, and a demand of 8kW would be sufficient for a building (i.e. administrative building). 3. SYSTEM MODELING 3.1. Organic Rankine Cycle (ORC): Following analysis and some of the equations are inspired from [12]. For this analysis, a nominal thermal efficiency of the ORC (ηth,o) of 10% is assumed corresponding to hot (QH at TH) and cold (Qc at Tc) heat transfers at TH=373 oK and TC=298 oK. TO is the reference temperature and is equal to TC. As the Carnot efficiency suggests, the actual thermal efficiency of the ORC will vary with the values of TH and TC. The actual ORC at METU NCC is designed to operate with TH = 373 oK with an ignorable hit rate, and therefore in this analysis, TH is assumed as constant at 373 oK. However, TC is related to and therefore will vary with the environmental conditions. Since a wet cooling tower is used, TC will vary with both the dry bulb temperature (Tdb) and relative humidity (φ) of the environment, both of which are given in typical meteorological year (TMY2) formatted data sets. However, modeling the impact of both dry bulb and relative humidity on TC and the resultant variation in thermal efficiency with TC requires relatively complex modeling which is beyond the scope of our analysis. Therefore, actual thermal efficiency (ηth ) is assumed to only vary with Tdb according to the following model in (1), based on Carnot efficiency’s variation with TC. ƞ ƞ = , ƞ = ƞ (1) , Solving (1) for ηth yields as in (2): ƞ o = ƞ (2) , As mentioned earlier, To = 298 o K and TH = 373 K. Based on the definition of Carnot efficiency. Qorc = Worc / ηth (3) Equations (2) and (3) are combined to yield as in (4). ⎡ ⎢ ⎢ ⎣ = ƞ ̇ ̇ ̇ , ⎤ ⎥(4) ⎥ ⎦ , The rate of heat transfer to the ORC is described in (5). = (5) 3.2. Parabolic trough collector Model: The model results in the heat transfer from the collector to the heat transfer fluid per unit aperture area, (QPTC/Aa), for each hour of the year. For this analysis, a total collector field aperture area (Acoll) of 216 m2 is assumed with 1-axis tracking about an EastWest horizontal axis and a mean collector operating temperature (Tm,op) of 120 °C. = ( ) ( ) − ( − ) (6) 3.3. Fresnel Collector Model: In comparing Fresnel Lenses with PTC collectors, collector model based on aperture area was used from the SRCC sheets [11]. The approximated formula for Fresnel is explained in (7), where diffuse radiation is also accounted, different than that of PTC. In the analysis, the price used is approximated from deposited commercial cylinder of 10 kg used in home cooking. The propane is assumed to be in its gaseous state and the density is taken as 0.51lt/kg at 15 degrees Celsius [8]. 3.4. Boiler Model: The boiler for METU NCC system uses propane gas. As noted in the Introduction section, propane is used only to meet with the minimum electrical energy demand. The rate of heat transfer of the boiler is described in (8). ̇ = ̇ − ̇ ≥0 (8) The heat transfer per unit mass of propane to the heat transfer fluid (HTF) in the collector loop is assumed equal to propane’s heating value (HV). The value of HV is 45 982 kJ/kg for liquid propane combustion at 25 °C with water vapor in the products [6]. The rate of heat transfer to the HTF in the boiler becomes as in (9). ̇ (9) = ̇ With (9), the mass flow rate of propane gas is calculated for every hour of the year. Assuming 100% theoretical air and complete combustion and modeling air as 21% O2 and 79% N2, the chemical reaction for the combustion of propane described in (10). To quantify the impact on climate change for this system, we also note that three moles of CO2 are produced for every mole of propane burned in our work. + 5( + 3.76 )→3 + 4 + 18.8 (10) 3.5. Cooling Tower Model: The interest in modeling the cooling tower is to perform an order-of-magnitude analysis for the water consumed during system operation. For this purpose, a simple model of the cooling tower is sufficient in which all of the low temperature heat transfer from the ORC (Qc) is used to evaporate water at 25 °C; e.g., only isothermal latent heating and not sensible heating effects are modeled. ̇ℎ ̇ = (11) 550 450 350 250 150 Jan In (11), ρH2O represents the density of liquid water, VH2O stands for the volumetric rate of water consumed by the cooling tower and hfg is the enthalpy of evaporation for water. For the base case analysis, we assume ρH2O as 1.0 kg/liter and hfg is evaluated at 25 °C as 2442 kJ/kg [6]. greater for Fresnel. When compared to the PTC systems, Fresnel collectors are more effective in terms of solar fraction values. Fresnel systems are also economically feasible since they use evacuated glass tubes, and mirrors which are available in the market at lower prices than the costly parabolic shaped collectors of the PTC. They also occupy less land use than of PTC’s. It is observed that the minimum CO2 emissions are in January and the maximum is in August which can be reasoned by the effect of ambient temperature. Ambient temperature is the factor that reduces the main energy production due to Carnot efficiency. Carnot efficiency depends on the hot and cold temperature values. In the model, collector temperature value is a constant term but the value of Tcold relies on the ambient temperature which varies with the season. For Northern Cyprus, one would expect to get higher solar fractions within winter months due to ambient temperatures. However, the optimal temperature and available solar radiation are in September. To note some of the important aspects of this analysis, it is indicated that available Qfresnel from the collectors are greatly increased, mainly due to the diffuse irradiation. The solar fraction, (fs) is higher for Fresnel case, due to effect of diffuse radiation on Qptc values. The water consumption is same for both systems since the same ORC system is used. Although the water consumption is similar, the CO2 emissions are less and solar fraction values are more for Fresnel collectors. Once economic aspects are investigated, it is also obvious that the PTC systems are highly expensive due to the production of the parabolic shape compared with Fresnel systems. Including the demand profile, comparisons are made with the PTC collectors and FS collectors, of which the results are summarized in Table 1 and Table 2, respectively. In Figure 3, solar irradiation absorption of both systems is compared with each other. In Figure 4, the comparison of CO2 emissions are made and we evaluate the operating cost of both systems in Figure 5. 4. RESULTS AND EVALUATION Consistent with the theory, the amount of carbon dioxide emissions and water consumption increases with ambient temperature, both peaks in summer months. The average yearly value of solar fraction is Qpt c kWh/ day Dec (7) Annual ) Nov − Oct ( Sep − Jul ( ) Aug ) Jun ( Apr + M ay ( ) M ar ) Feb ( Insolat ion kW h/ day = Qf resnel kW h/ day Figure 3. Total amount of irradiation comparison absorbed by PTC and Fresnel panels. 11,08 0,17 2,25 1,83 11,14 0,10 2,44 M ar 205,01 1,83 11,34 0,12 2,44 Apr 255,08 1,88 11,83 0,15 2,50 M ay 239,80 1,99 12,46 0,13 2,65 Jun 275,72 2,05 13,06 0,15 2,74 Jul 275,09 2,15 13,63 0,14 2,86 Aug 254,94 2,16 13,62 0,13 2,88 Sep 426,40 1,87 13,16 0,22 2,49 Pr ice (PTC) TL/ kw h Oct 353,43 1,86 12,62 0,19 2,48 Nov 342,54 1,74 11,83 0,20 2,31 Dec 228,52 1,78 11,32 0,14 2,37 Annual 275,29 1,90 12,26 0,15 2,53 Pr ice (FS) TL/ kwh Figure 5. Operating cost comparison of PTC and Fresnel systems for each month of the year. 5. CONCLUSION Table 2. STE Analysis of Fresnel Systems M ont h Annual 1,69 171,46 Dec 275,55 Oct Jan Feb Nov TL/ kWh Sep - Jul lt / kWh Aug kg/ kWh Jun kWh/ day 3.5 3 2.5 2 1.5 1 0.5 0 M ay Cost Apr fs M ar VH2O Jan m CO2 Feb Qp tc Price (TL / kW h) Table 1. STE Analysis of PTC Systems M ont h Q fr esnel m CO2 VH2O fs Cost kWh/ day kg/ kWh lt / kWh - TL/ kWh Jan 351,73 1,52 11,17 0,24 2,03 Feb 243,94 1,68 11,24 0,17 2,24 M ar 286,83 1,65 11,45 0,20 2,20 Apr 367,34 1,61 11,95 0,25 2,15 M ay 356,86 1,73 12,58 0,23 2,30 Jun 416,85 1,73 13,19 0,26 2,31 Jul 405,83 1,84 13,78 0,24 2,46 Aug 362,71 1,90 13,76 0,22 2,54 Sep 539,83 1,60 13,31 0,32 2,14 Oct 453,49 1,63 12,75 0,28 2,17 Nov 429,58 1,54 11,94 0,28 2,05 Dec 291,79 1,64 11,41 0,20 2,19 Annual 375,56 1,67 12,38 0,24 2,23 In this work, we analyzed, modeled, evaluated and compared two solar thermal energy systems, namely parabolic trough collector and Fresnel systems over Northern Cyprus topography. We pointed out the importance of the topography and weather conditions of a region while deciding on which type of STE systems to use to get better output and thus efficiency. As a result of our work, Fresnel systems have an advantage over PTC systems in terms of higher solar irradiation absorption, lower CO2 emissions, and lower operating cost for Northern Cyprus. 6. FUTURE WORK The work we conduct in this paper consists of two types of solar panels in general: Fresnel systems and PTC systems. Further work may be done to carry this over to a more general and detailed range by including photovoltaic solar panels (PV) or two-axis tracking systems. ACKNOWLEDGEMENT We would like to thank Ipek Alemdar, graduate student SEES program for taking part in the discussion for constructing the model and interpretations of the data. We would also like to thank Dr. Derek Baker for his valuable discussions of identifying the concept of the concentrating and nonconcentrating solar systems. His lecture notes were also used in formula development in this paper. 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