Impact of storage in the operation of Hybrid Solar Systems

Transcription

1 Impact of storage in the operation of Hybrid Solar Systems Garcia, Miguel da Fonte Gomes Abstract There has been a growing concern with which energy sources are being used to generate the electricity. The renewable energy sources are being integrated into the energy systems more and more. In Portugal, due to the shortage of new locations to implement new hydro power plants, the focus is shifting to solar and wind energy. With this increase, in these two energy sources, there is a big problem that arises, the intermittency of the generation for these sources. Storage systems are being used to try to diminish the effects of the intermittency. Extra energy is stored for later use. In this study, the storage system studied was the use of batteries. This solution is also able to stabilize the system in addition to being able to store energy. It also has the advantage that it is an element that is able to act both as generator and as load in an energy system. To evaluate the reliability of the system two indicators were used, the LPSP Loss of Load Probability and the WEP Wasted Energy Probability. LPSP measures the probability of the energy generators not being able to supply the load, WEP measures the probability of the generators exceed the energy required by the load.. Index Terms Intermittent Generation, Batteries, LPSP and WEP δ Φ β ω γ ε 0 θ z θ kt NOMENCLATURE declination, in degrees; latitude, in degrees; module tilt, in degrees; hour angle, given in degrees; azimuth angle, in degrees; eccentricity factor; azimuth angle, given in degrees; incidence angle, given in degrees; clearness index; G, G0 Irradiance and extraterrestrial irradiance on the horizontal plane; Gd, Gr, Gt the tilted plane; diffuse, reflected and total irradiance on ground albedo; Rb ratio between direct and tilted irradiance; Gn normal irradiance; B, C adimensional variable used in ashrae model; Fss angle between the surface and the sky; Fsg angle between the surface and the earth; N number of PV modules; A m η r η pt β t Tc, Tr temperature; Ta NOCT area of a PV module; instantaneous efficiency; reference efficiency; MPP efficiency; is the temperature coefficient; cell temperature and cell reference ambient temperature; is the normal operation cell temperature; C bat (t) battery capacity in instant t; C bat (t-1) battery capacity in instant t-1; σ self-discharge rate; DOD Depth of discharge; ICM Internal Combustion Motors E total E load WE LPS Total energy generated; Load energy; wasted energy; Loss of power supply; inversor efficiency; battery efficiency.

2 I. INTRODUCTION Global consumption of fossil fuels is the main reason for global warming. [1] The high fuel price in conjunction with a greater public awareness is leading a change in energy generation. The main fuel of the energy industry is changing, from coal and fossil fuels to other renewable energies. Renewable energy sources are defined by being energy that comes from resources that renew themselves in a human lifetime. [2] This include tide, wind and solar amongst others. Wind Energy is one of the most promising renewable energy. [3] It has a few advantages: High efficiency; Fixed cost; Allows for decentralized production; Lowest C0 2 emissions. (see figure 2) In Terceira the energy system has a few more power generators, the geothermal and the waste energy. Next we must develop an algorithm that can detect the unit commitment and decide which group or groups of thermal generators should be available. Having done this, the last step is to calculate the LPSP and the WEP to be able to compare between configurations. [4] A. Photovoltaic model The power generated by the PV module doesn t depend on the solar radiation alone. It depends also on the temperature of the cell and the angle that the sun rays make with the module. To calculate the solar radiation that hits the module, at the incidence angle, two models were studied, ASHRAE method and the Erbst et al. model. Irradiance and Insolation The solar irradiance is the instantaneous power in a specified step in time; it is given in W/m 2. If we integrate these values over a period of one hour we get solar insolation, which is a unit of energy, the sum of power over a determined period of time; it is given in Wh/m 2. In this study, we take as granted that all values are constant in one hour, this means that numerically the irradiance and the insolation values are the same. Erbst et al. model [5] Figure 1 - Support by Energy Source In this model, the hourly data on a horizontal surface and site coordinates are required. The following equations from equations 1 to 4 represent the first steps in getting the Erbst model working. (1) (2) Figure 2 - C0 2 emissions Another energy source that has a high potential is the solar energy. There are two main technologies when generating solar energy, photovoltaic modules and solar collectors. Photovoltaic modules are made to generate electricity and solar collectors are made to heat water. In this study only the photovoltaic modules were studied. II. METHODOLOGY First, the models that describe the different energy generators must be explained. The system studied used wind energy, photovoltaic, hydro and thermoelectric power plants. (3) (4) Having the equations, the next step is to find the kt, the hourly clearness index. (5)

3 With the clearness index it s possible to find the direct, diffuse and reflected component of the insolation. Using equation 6 it is possible to find the diffuse component. 21 September October { The total insolation is then given by equation 7: (6) 21 November December ( ) ( ) Rb can be found by using equation 8. (7) (8) The total irradiance for any inclination is the sum of the beam, diffuse and reflected component. (10) The direct component of the irradiance is represented in equation 11. ASHRAE model [6] This model is simpler than the Erbst et al. As such it can be used as a first estimation for the total insolation. The normal irradiance is given by equation 9. Where Fss is, (11) (12) (9) Table 1 - B, C values for the Ashrae model The reflected component is, (13) Data B C 21 January February Where Fsg is, (14) 21 March April May June July August The power generated by the photovoltaic modules, in either method, can be estimated using equation 15 can be found using equation 16. (15) [ ] (16) Tc is the cell temperature and can be found using equation 17.

4 ( ) (17) (19) A m, η r, β t and NOCT are parameters that are available in the module datasheet. B. Wind Energy model The model used to estimate the energy generated by the wind turbines was the power curve. By using the power curve, represented in figure 3, it is possible to estimate the energy generated as long as the data for the wind speed is available. Carefully observing figure 3, it is seen that for wind speeds below 3 m/s and above 25 m/s the wind turbine is not working. The lower wind speed value is the cut-in speed, the speed at which the wind is strong enough to rotate the turbine. The higher wind speed, the cut-out wind speed is the maximum speed that the turbine can operate at without receiving damage. z and z 0 are the height of the rotor and the reference height respectively. and are the wind speed associated with those heights. C. Storage System model The storage system has three states, charging, discharging and doing nothing. When the total energy generated by the system is greater than the load required the system is charging. The state of charge when the batteries are charging is given by equation 20. ( ) (20) When the batteries are discharging the equation 21 describes the state of charge of the battery. ( ) (21) At any given moment in time, the battery state of charge should be between, Figure 3 - Power curve E44 Enercon The power curve can be approximated by a polynomial, in this case a 10 th degree polynomial. (18) Where a, b and C are constants that better approximate the curve. Prandtl Law: To change the wind speed from the reference level to the rotor level of the turbine we use equation 19, (22) The minimal value that the battery state of charge can take is given by the DOD, depth of discharge. D. Thermoelectric Power model Unit commitment (23) For a better allocation of the internal combustion motors (ICM) a publicly available algorithm was used. This algorithm receives the maximum and minimum of each generator, the spinning reserve that should be available and the ramp that describes the power variations. The costs of operating in the ICM are also an input. The unit commitment then, given a load prediction will be

5 able to determine the states of the ICM for the full year. Because of the costs associated with operating the ICM the unit commitment algorithm tends to operate on the same generators in a large sequential number of hours. The algorithm developed to study the reliability uses those grouped hours to reduce the ICM and try to use more renewable energies instead. Internal Combustion Motors It was stated above that the unit commitment algorithm clusters sequential hours with the same ICM. The developed algorithm receives these clusters of hours and attempts to lower the ICM generation by adding renewable sources to the energy system. If the opposite happens, the energy generation is not enough to satisfy the load then the algorithm attempts to increase the generation, first by trying to add more renewable sources and then, if there are not enough by adding another ICM. E. Modeling reliability Generation equal to consumption ( ) (25) In this case, none of the batteries suffer any change, only the self-discharge, inherent to all electrochemical processes. No reliability indicators are changed. F. Reliability Taking into account the various states the system can find itself in, the LPSP and WEP indicators are described by the equations 26 and 27 respectively. (26) In any given energy system, the reliability is defined by the system capacity to, at any given moment, equal the load consumption. Defining LPSP as the ration between the non-satisfied load and the total load in a year, it is easy to deduce that a LPSP of 0 means that the load was fully satisfied, all year that means that the system is 100% reliable. In the Opposite, if the LPSP is 1 then the generation was never capable of satisfying the load and the system is totally unreliable. Where T is a year s time, in hours. III. DATA AVAILABLE (27) Generation Exceeds the Consumption This happens when the system energy is greater than the energy requested by the load and the batteries are full. The energy loss when this happens can be quantified and it is given by equation 24. EDA provided the data on the wind, hydric and thermoelectric generation. Some of that data had some errors, in order to correct that two different methods were used. Interpolation [ ( )] Consumption is greater than the generation (24) When the error in the data was isolated, then the error was corrected with and interpolation of the two values next to the error. The equation to do that is 28. (28) This can happen when the generation and the batteries don t have enough energy to supply the load. It is important to note that the batteries are considered empty when they reach the minimum state of charge that doesn t damage the battery, the DOD. The energy not satisfied by the generation can be described in equation 25. Where y a is the value that is missing, x a is the hour of that missing data; x 1 and x 2 are the times next to the missing hour, y 1 and y 2 are the data from those hours Characteristic days To find each season characteristic day a week must be divided in three, working day, represented by Wednesday, Saturday and Sunday.

6 With these two methods the data provided by EDA is corrected and the algorithm can get it. Table 2- A year characteristic days B. Hydro Power plant The data for this type of power plant was just subjected to the correction that all data were. There is no algorithm that controls this unit. In figure 5, a diagram of the year 2012 is shown. Days of the Season Spring 21/06) Summer 23/09) Fall 22/12) Winter (20/3- (22/06- (24/09- (23/12-19/03) A. Load diagram Work Days Saturday Sunday 16/05 19/05 20/05 22/08 18/08 19/08 17/10 20/10 21/10 19/12 22/12 23/12 The estimate the load diagram of the year a sum of all the generation was made. As the system energetic balance must be null then the load diagram is equal to the generation but with opposite signs. In figure 4, it is possible to see this estimation. Figure 5 - Hydro Generation Diagram IV. RESULTS The methodology was applied to 4 different situations: No reserve and no batteries; Reserve with 50% of the wind power and no batteries; Reserve with 50% of the wind power and 25 units of 2MWh batteries; Reserve with 50% of the wind power and 50 units of 2MWh batteries. Has we are dealing with a stochastic system all the results are made with the mean of 300 different tests, this way the results get less of a deviation from the intended results. In all four situations depicted below the blue represents the value of LPSP or WEP and the red are its mean value over the 300 tests. Figure 4 - Load Diagram

7 Situation 1 Figure 6 - LPSP for the 300 tests. No Reserve. No batteries Figure 11 - WEP for 300 tests. With Reserve. With 25 batteries Situation 4 Figure 7- WEP for the 300 tests. No Reserve. No batteries Situation 2 Figure 12 - LPSP for the 300 tests. With Reserve. With 50 batteries Figure 8 - LPSP for 300 tests. With Reserve. No batteries Figure 13 - WEP for 300 tests. With Reserve. With 50 batteries Increasing number of batteries without reserve Figure 9 - WEP for 300 tests. With Reserve. No batteries Figure 14 - Increasing the number of bat without reserve Situation 3 Increasing number of batteries with reserve Figure 10 - LPSP for 300 tests. With Reserve. With 25 batteries Figure 15 - Increasing the number of bat with reserve

8 Table 3 - Number of times the ICM turn on and off in the different situations. Nº of ICM turned ON Nº of ICM turned OFF Situation 1 Situation 2 Situation 3 Situation VI. FUTURE WORK A future work could focus on the economic gains achieved by using the batteries as storage for the electric system of Terceira. It might also be advantageous to develop a more comprehensive model for the load estimation. A. Comments It can be seen, by comparing the different situations, that the greater the reserve and the number of batteries the closest to 0 we will get the LPSP indicator, this is to be expected, increasing the reserve will increase the available power and if there is an increase in the number of batteries in the periods of time that the generation exceeds the consumption the battery will charge and have energy available for when it is needed by the load. It is also interesting to note that the change from no batteries to any battery brings a big change to the value of LPSP, but as the number of batteries increases the value of LPSP does not decrease as fast as it did when there were no batteries. There appears to be a horizontal asymptote which means that after a certain number of battery increases adding more batteries won t do anything for the system in terms of the LPSP indicator. The WEP value doesn t appear to suffer from the same problem, increasing the number of batteries continuously decreases the number of wasted energy. The batteries also appear to diminish the variations between tests, which is why they are said to be stabilizers. In table 3 it is possible to see the number of state changes that occurred to the ICM s because of the developed algorithm. VII. REFERENCES [1] IPCC, [Online]. Available: [Accessed ]. [2] O. Ellaban, H. Abu-Rub and F. Blaabjerg, Renewable energy sources: Current status, future prospects and their enabling technology, Renewable and Sustainable Energy Reviews. [3] Eólicas de Portugal (ENEOP), [Online]. Available: =180&id_canal=110. [Accessed ]. [4] S.Diaf, D.Diaf, M.Belhamel, M.Haddadi and A.Louche, A methodology for optimal sizing of autonomous hybrid PV/wind system, Energy Policy, vol. 35, pp , [5] J. A.Duffie and W. A.Beckman, Solar Engineering of Thermal Processes, John Wiley & Sons, Inc., [6] ASHRAE handbook: HVAC applications, Atlanta, Georgia, V. CONCLUSION The main objective of this study was to evaluate the use of batteries in a stochastic energy system. Using the indicators LPSP and WEP we can conclude that the batteries improve the reliability of the system but in the case of the LPSP only up to a certain point. Adding more batteries indiscriminately will only help with the energy losses due to over generation, as it was seen in the section IV. It is also seen that the number of state changes to turn ON the ICM decreases a lot if we add batteries to the system. This might be the biggest gain economically because we can start reducing one of the highest expenses in the energy system of Terceira island and any other electric system that is based around the thermoelectric power plants.