Numerical Simulation Of A Hybrid Concentrated Solar Power .

14th International Conference on Sustainable Energy Technologies – SET 201525th - 27th of August 2015, Nottingham, UKThe indirect storage component calculates the transient heat exchange between the water/steam and either thecollector s absorber tube or piping. The Fourier heat transfer differential equation is discretised in a twodimensional domain using a finite volume method, and in time by an iterative Crank-Nicholson method (Pawelleket al., 2012). As inputs, the IS block requires the estimated values for the water and steel mass, as well as theinner heat transfer coefficient at design conditions.Hybridisation was modelled trough a heat injection component, representative of the boiler. This component actsas an ideal heat exchanger, promoting the interface between the boiler output and the mass flow rate ofwater/steam. In order to control the boiler output, a code was created using EbScript to impose operating limitsaccording to the manufacturer data. The boiler output is controlled by the water/steam mass flow rate, acting asan auxiliary heater of the solar field. For simulation purposes, the Viessmann VITOMAX 200-HS model witheconomiser was used. Two different boiler sizes were the object of analysis, with nominal heat outputs of 380kWth (0.5 ton/hr of saturated steam) and 530 kWth (0.7 ton/hr of saturated steam), and both with a minimumthermal output of 100 kWth. The transient behaviour of the boiler was considered using the IS component, takinginto account that the boiler takes half an hour from cold start to design conditions.For estimating biogas consumption, a computer model was created using Ebsilon, consisting in a combustionchamber where the mix of air and biogas is burned, retrieving as output the flue gas. The combustion wasmodelled considering an excess of oxygen in the flue gas of 3%. The control of biogas and air mass flow rate isachieved considering manufacturer’s data regarding efficiencies and flue gas temperatures, either for nominal orpart load conditions. The flue gas exchanges heat with a steam evaporator and economiser (see Figure 2).Figure 2: Boiler simulation layout3Biogas consumption was estimated considering a yearly constant low heating value (19.27 MJ/m ). A study wasconducted at ENIT, for estimating biogas production with the local canteen residues. The estimated value is about360 m /day.The ORC sub-system was simulated using EES, since properties data of the organic fluid (SES36) were notavailable in the Ebsilon database. It was assumed that the ORC operates at steady-state and thermal inertia ofthe power block was neglected. The control was achieved through the organic fluid mass flow rate.Manufacturer s technical information for the turbine, pump and generator efficiencies were considered. As inputthe code requires the saturated steam mass flow rate from EBSILON. Neither pressure drops nor efficiencieswere considered for the heat exchangers. As main output results the code provides the gross and net electricalpower, organic fluid mass flow rate, as well as the parasitic consumption and the condenser thermalrequirements.Solar-only operation modeThe system steam production will be controlled by the mass flow balance at the steam drum. The same conceptwas used in the simulation model. During system operation the recirculating mass flow rate is kept constant atabout 0.5 kg/s. In real conditions the feed water pump will operate when the water level in the steam drum dropsbelow a predefined level. Since the simulations were carried out in an hourly basis, it was assumed that the massflow rate of the feed water should balance the water removed from the recirculating pump, and so is intrinsicallyrelated to the steam quality after the distributing header.If the saturated steam mass flow rate that leaves the solar field exceeds the maximum requirements of the ORC,it is then separated. In real operation, the solar collectors should change their state to partial defocused in order toSOARES 1664

14th International Conference on Sustainable Energy Technologies – SET 201525th - 27th of August 2015, Nottingham, UKcontrol the steam production. In the simulation model, the excess of energy is accounted as dumped, providing aninput for a PCM storage tank design. The last separation occurs before the ORC, through a water-steam drainer.This component is mostly used in the warm-up and cool-down profiles, to establish a more realisticthermodynamic balance.To obtain a more accurate approach in the simulation model, distinct operating profiles were created. A code wasdeveloped using EbScript to allow the automatic interchange between the profiles using dynamic variables, e.g.direct normal irradiation, hour of the day.The solar only operating mode is constituted by four profiles: warm-up, operating, cool-down and stop. At the2beginning of the day if DNI exceeds 200 W/m the collectors change their state to focus and the recirculatingpump is activated. This represents the warm-up profile. The operation profile starts with solar field steamproduction, and operates as described before.2At the end of the day, if DNI is less than 200 W/m , the collectors change their state to defocus and watercirculates until the system cools-down. At night, the system is off, with solar field collectors defocused and thepumps shut-down. Even during the night, the thermal inertia of the system as well as the heat losses to theambient were considered.Hybrid operation modeThe analysis was carried out based on the assumption of the system running 12 or 24 hours daily, at ORCminimum and nominal power. Concerning the control, the hybrid mode comprehends four and two operatingprofiles for the 12 and 24 hours regimes, respectively. The 12-hour operating regime differs from the solar only,on the warm-up and operating profile.The warm-up begins at 7:00 with the start of the boiler in order to warm-up the ORC and the feed water line of the2solar field. If DNI exceeds 200 W/m the recirculating pump is activated and the solar collectors focused. Duringthe operation regime (08:00 to 20:00) the hybrid mode is activated, where the boiler compensates therequirements (nominal or minimum) to drive the ORC turbine. This control is achieved by saturated steam flowrate balance. In preliminary simulation results it was noticed that during summer the system could start earlier (at7:00), and the 12 hour operation regime acted as constraint to the solar field. To overcome this issue, in the2beginning of the day if DNI is above 200 W/m the collectors are focused earlier.The 24-hour regime just encompasses two operation modes, hybrid and boiler-only. During the daylight period thesystem operates in hybrid mode, and at night the collectors are defocused and the solar field is cooled down.After that the system relies solely on the boiler.For both cases the boiler minimum power of 100 kWth was considered, in order to assure electricity generationstability, during the predefined time operating range. Otherwise, the boiler would be submitted to consecutivestartups and shutdowns, and shortages in the electrical generation would be expected, due to solar radiationtransients.4. RESULTSIf the system relies solely on solar energy the annual heat generated is about 663 MWhth. Hybridisation improvedthe solar field output by 3% (Table 1). This outcome is related with the system start-up, since the SF feed water isalready warmer, and consequently less solar energy is required to achieve steam generation. Furthermore, thisimprovement is extended to the solar field annual efficiency.The second improvement of hybridisation was the extinction of dumping rates associated with scarcity of energy.Almost one quarter of the heat is dumped in the solar-only operation mode, mostly related with energy dearth.The dumping rate results are divided in two items (excess and scarcity), representing the heat dumped due to theexcess of energy or due to insufficient energy to drive the ORC turbine, respectively.The scarcity of energy was surpassed with hybridisation, with the fulfilment of the ORC minimum thermal powerrequirements. On the other hand, the excess of energy increased. This fact is related with minimum operatingconditions of the boiler (100 kW th), leading to energy waste predominantly in the summer months, when solarradiation is highly available.The excess of energy can be reduced by implementing a storage tank in the system. The benefits extend beyondthe ability to store the excess of solar field thermal energy. If storage capacity can provide more than 30 minutesof thermal energy requirements to drive the ORC turbine, the need of having the boiler in permanent operation isSOARES 1665

14th International Conference on Sustainable Energy Technologies – SET 201525th - 27th of August 2015, Nottingham, UKeliminated. In other words, it can act as system buffer in order to compensate thermal output fluctuations from thesolar field and boiler, reducing the amount of wasted biogas.Despite the discontinuous operation of the boiler, due to the solar irradiance transients, the average biogas boilerefficiency is still high (about 93%) for all cases.Table 1: Simulation Annual ResultsHybrid - 24 hours operationSolar OnlyHybrid - 12 hours operation530 kWth380 kWthORC minimum530 kWth380 1799.41799.41799.41799.41799.4Annual Heat Generated S.F.663683683683683683683MWhthSpecific Thermal Field 8.6%38.6%38.6%38.6%%Annual Heat Generated - Boiler033592546178814251155865MWhthAnnual Combustion Heat - Boiler036192746191815341244928MWhthMean Annual Boiler Efficiency092.8%92.7%93.2%92.9%92.8%93.2%%Annual Biogas Consumption0655497347278225168dam3Average Biogas Consumption017951362951761617460m3/dayDirect Normal Irradiance - DNIMean Annual SF EfficiencyORC minimum2kWh/(m .a)2Solar Share100%17%21%28%32%37%44

Numerical Simulation of a Hybrid Concentrated Solar Power/Biomass Mini Power Plant João SOARES 1, Armando OLIVEIRA 1,2 1 Dept. Mechanical Eng. University of Porto, Rua Dr Roberto Frias 4200-465 Porto-Portugal, joaosoares@fe.up.pt 2 CIENER-INEGI, Rua Dr Roberto Frias 4200-465 Porto-Portugal, acoliv@fe.up.pt Renewable electricity generation systems are increasingly used as a means of reducing .