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Power Generation

Parabolic trough collector (PTC)-based CSP plants, using a conventional synthetic thermal oil as a heat- transfer fluid (HTF). are the most mature CSP technology. Solar power tower (SPT) technology and linear Fresnel reflector (LFR) technology with flat mirrors and simple structure are proposed as promising alternatives to the РТС-based CSP plants. The solar power tower technology is cost-effective for large- scale applications (> 50 MW). The linear Fresnel reflector technology has a lower optical efficiency (Nixon and Davies, 2012; Xie et al., 2012) and requires a much higher area of installation compared to that of a РТС-based CSP plant of the same capacity (Desai and Bandyopadhyay. 2015). The paraboloid dish system is the least applied concentrated solar power technology for power generation, relative to the other technologies.

The conventional steam Rankine cycle is widely used in commercial concentrated solar power plants. Depending on the capacity of the CSP plant and steam conditions at the inlet of the turbine, the thermal efficiency of the steam Rankine cycle is in the range of 20% to 40%. Modular CSP plants with a few kV to a few MW, capacity offer solutions in industrial as well as off-grid applications. For such plants, ORC power systems have been demonstrated to be an efficient solution for electricity production (Quoilin et ah, 2013). Existing concentrated solar energy-powered organic Rankine cycle-based commercial, medium-scale plants (> 500 kWe) for different applications are listed in Table 1 (NREL. 2019; Petrollese et ah, 2018; Turboden, 2019; Wendt et ah. 2015). In addition, there are a few micro and small-scale CSP- ORC plants, mainly built for research and development purposes, which are not commercially viable and are, therefore, not included in the list.

A simplified schematic of a typical concentrated solar thermal energy-driven organic Rankine cycle power system is given in Figure 2. The system can be equipped with a thermal energy storage for storing the excess energy. When the stored energy is available, the ORC power system runs at frill load. However, when the storage is at a minimum level and solar radiation is not sufficient, the heat transfer fluid mass flow rate is adjusted such that the solar field outlet temperature is controlled. The power system mass flow rate and turbine power output are also affected by the variations in the heat transfer fluid flow rate. Part-load efficiencies of the equipment are lower than the design condition efficiencies, and therefore, appropriate models need to be used for predicting the performance of the system. A summary of previous works on medium-scale (a few hundred kW to a few MWJ concentrated solar thermal energy-powered organic Rankine cycle power systems is given in Table 2. It can be observed that the parabolic trough collector and linear Fresnel reflector are typically used for medium-scale plants. Recently, a novel nanostmctured polymer foil-based concentrated solar power system, which avails the advantages of low capital cost, low operation and maintenance cost, and two-axis tracking, has been analyzed (Desai et al.. 2019a; Desai et al., 2019b). This system uses a nanostmctured focusing plastic film that is adhered to a glass plate.

It is important to select a proper working fluid for an organic Rankine cycle power system for cost- efficient utilization of any available heat source. For low and medium-grade heat sources, the dry and the isentropic fluids are the preferred organic working fluids, as the condition at the outlet of the turbine is always either saturated or super-heated vapor, avoiding expansion in the two-phase region (Hung, 2001; Lui et al., 2004). The promising organic working fluids for CSP-based plants are n-pentane, isopentane, hexamethyldisiloxane (MM), toluene and cyclohexane; see Table 2. In commercial, medium-scale actual plants (> 500 kV) n-pentane, MM or isobutene are used as working fluids in the ORC system; see Table 1. Apart from the techno-economic performance, environmental, safety, health, and legislative aspects need to be considered in the final selection of the working fluid for the ORC power system.

Table 1. List of concentrated solar energy-powered organic Rankine cycle-based commercial/medium-scale actual plants (> 500 kWe) for different applications (NREL, 2019; Petrollese et al., 2018; Turboden, 2019; Wendt et al., 2015).

Name (Location)

Start

year

Solar

field

Solar field area (in2)

Storage

Application (net capacity)

Saguaro Power Plant (Arizona, USA)

2006

PTC

10,340

-

Electricity' generation (1 MWJ (currently non-operational).

Rende-CSP Plant (Calabria, Italy)

2014

LFR

9,780

Electricity' generation (1 MV) The facility is combined with an already operating biomass-based plant (14 MW,).

Airlight Energy Ait-Baha Pilot Plant (Ait Baha, Morocco)

2014

PTC

6,159

Packed-bed rock (5 h)

Electricity' generation from CSP and waste heat from cement industry (hybrid plant)

(2 MWf

Stillwater GeoSolar Hybrid Plant (Fallon, USA)

2015

PTC

24,778

Electricity' generation. About 17 MW4 from CSP combined with geothermal energy producing 33 MWe. Additionally, 26.4 MW, of a solar photovoltaic plant.

Aalborg CSP-Bronderslev CSP with ORC project (Bronderslev, Denmark)

2016

PTC

26,929

Combined heat and electricity' production fi'om CSP (16.6 MWt J and biomass combustion (hybrid plant) (3.8 MW^),

Ottana Solar Facility (Sardinia, Italy)

2017

LFR

8,600

Two-tank dried

Power generation (0.6 MW^ additionally 0.4 MW, of solar PY.

Authors

Solar field

Storage

ORC capacitv/working fluids

Max. temp, of ORC/HTF

Remarks

Casartelli et al. (2015)

PTC, LFR

-

2.94 MW and 3.57 MW (Toluene)

295 °C (ORC)

F or cost parity, the cost of the LFR solar field should be about 50% of PTC solar field.

Cocco and Sena (2015)

LFR

Two-tank direct, thennocline

1 MW (Siliconic oil)

305 °C (ORC)

The cost of energy for a thermocline storage system is 420 €/MWh and for the direct two-tank system 430 €/MWh.

Cocco and Cau (2015)

PTC, LFR

Two-tank direct

1 MW (Siliconic oil)

305 °C (ORC)

Cost of energy (1 №V, 2 h storage): LFR-based plant: 380 €/MWli; РТС-based plant: 340 €,MWh

Rodriguez et al. (2016)

LFR

Two-tank, thennocline

1 MW, (Cyclopentane)

300 °C (ORC)

Specific cost for a thennocline storage system (€/kWli]h) is about 33% on average lower than that of the conventional two-tank storage system.

Desai and

Bandyopadhyay (2016)

PTC, LFR

1 MWi (R113, n-pentane, Cyclohexane, MDM, MM, Heptane, Toluene, R245fa, and other)

337 °C (ORC)

Cost of the LFR field to reach cost parity with a РТС-based plant: for SRC-based plants: 48% of PTC field cost; for ORC-based plants: 58% of the PTC field cost; the Steam Rankme cycle is a preferred option.

Gaig et al (2016)

PTC

Packed bed

500 kW_ (Isopentane, R152a, butane, isobutene, R245fa, and other)

275 °C (HTF)

Hybrid plants (5, 50 and 500 kW,) powered by waste heat and solar thermal energy Isopentane is the preferred working fluid

Tzivanidis et al. (2016)

PTC

Single tank direct

1 MW (Cyclohexane, toluene, water, MDM, and other)

270 °C (HTF)

Techno-economically, Eurotrough ET-150 is a better solution compared to other PTC technologies.

Cyclohexane is the prefened woiking fluid.

Russo et al. (2018)

LFR

Thermocline

1 MW( (Not given)

300 °C (HTF)

For thermocline storage, forced circulation of molten salts is better compared to the natural circulation.

Javanshiret al. (2018)

SPT

Butane, ethanol, isobutene, Rll, R141b

350 °C (ORC)

For a max cycle temperature lower than 300 °C, an ORC system (with R141b) is a better option. For high temperature, combined cycles are the better option.

Bellos and Tzivanidis (20 IS)

PTC

Single tank direct

238 kws to 845 kwe (Toluene, cyclohexane, MDM, n-pentane)

300 °C (HTF)

Hybrid solar-waste heat-powered system. Toluene is the prefened w'orkmg fluid.

Petrollese and Cocco (2019)

LFR

Two-tank direct

716 kW to 730 kW, (MM, n-heptane, toluene)

222 °C (ORC)

Multi-scenano approach for the plant design. MM is the prefened w'oikmg fluid.

Desai et al. (2019a)

Foil-based

Two-tank indirect

1 MW< (n-pentane, MM)

225 °C (ORC)

A foil-based CSP plant can reduce the LCOE by up to 40% compared to the РТС-based CSP plant.

In ORC power systems, the expander is the most important component as it has the most effect on the techno-economic performance of the system. Expanders for the ORC power system can be grouped into two types: (i) turbo expanders (axial and radial turbines), and (ii) volumetric expanders (scroll expanders, screw expanders, reciprocation piston expanders, and rotary vane expanders). Turbines with an organic working fluid can reach a very high iseutropic efficiency with only one or two stages. In systems with high flow rates and low pressure ratios, axial turbines (100 kW to a few MW) are the most widely used. In contrast, radial-inflow turbines are suitable for the systems with low flow rates and high pressure ratios. However, with decreasing power output and. hence, turbine size, the rotational speed increases proportionally. Therefore, for the low power range (mainly using radial-inflow turbines, <100 kWe), it is necessary to design an adequate bearing system and to employ a high-speed generator and power electronics. Radial outflow turbine design allows a high volume flow ratio with the constant peripheral speed along the blade span (Zanellato et al., 2018). Radial-outflow turbines can be used for small to medium-scale applications with an advantage of reduced rotational speed, allowing direct coupling to a generator (Maksiuta et al., 2017). In systems with a capacity less than 50 kV, the turbines cannot be used due to high rotational speed and high cost (Imran et al., 2016). Reciprocating piston expanders (Wronski et al., 2019) and screw expanders (Bao and Zhao. 2013) can be used for small capacity plants. Scroll expanders and rotaiy vane expanders can be used in small or micro-scale ORC power systems (Bao and Zhao, 2013).

Apart from the expander, the heat exchangers (evaporator, recuperator, and condenser) represent a significant share of the total ORC system cost. Temperature driving force (pinch point temperature difference) and pressure drops are key performance parameters regarding heat transfers, and each heat exchanger in the power system should be sized based on these parameters. The most commonly used heat exchangers for ORC power systems are shell and tube heat exchangers (for large-scale power systems) and plate heat exchangers (for small-scale power systems, due to compactness) (Quoilin et al.. 2013). Organic Raukine cycle feed pumps should meet the requirements of efficiency, controllability and low net pressure suction head. In addition, the ORC power system should be leak-proof, because the organic fluids are expensive (compared to water) and can be toxic, flammable, and have high values of global warming potential and/or ozone depletion potential. In a conventional steam Rankiue cycle system, the pump electricity consumption is very low compared to the power output (low back work ratio). On the other hand, in an ORC power system, the irreversibility in the pump can reduce the overall cycle efficiency significantly (Quoilin et al.. 2013).

As for the thermal energy storage technologies, the most widely-used systems for CSP-driven organic Rankiue cycle systems are the conventional indirect two-tank molten salt storage technology (for large capacity) and the direct thermal oil storage technology (for small capacity). Sensible thermal energy storage using a single tank packed-bed that consists of solids (such as rocks) as the heat storage medium and a heat transfer fluid in direct contact with the solids has also been analyzed in the literature (Cocco and Sena, 2015; Russo et al., 2018). The latent heat thennal energy storage is still at the proof of concept stage because of the low thennal conductivity, resulting in slow charge and discharge processes.

Thermodynamic analysis

2.1.1 Solar collector field

The solar collector field useful heat gam, 0CL, can be calculated as follows:

where >/0 CL is the optical efficiency of the solar collector field, U] j and U,, are the heat loss coefficients based on the aperture area of the solar collector field, Ap CL is the aperture area of the solar collector field, TmCL is the mean temperature of the solar collector field, J is the ambient temperature, and DNI is the direct nonnal inadiance. The incidence angle modifier (IAM) represents the reduction of the optical efficiency due to the incidence angle in parabolic trough collector fields and due to the incidence and the transversal angles in linear Fresnel reflector fields. The IAM for the system with two-axis tracking (paraboloid dish) is one. The cleanliness factor (f dtm) is the ratio of the optical efficiency in average dirty conditions to the optical efficiency with the same optical element in clean condition.

2.1.2 Organic Rankine cycle power system

The organic Rankine cycle feed pump increases the pressure of the working fluid (from state 11 to 12 in Figure 2). The power consumption of the pump, Wp, is computed as follows:

where m0RC is the mass flow rate of the organic working fluid, //r p is the isentropic efficiency of the feed pump, and Jr denotes specific enthalpy at i-tli state point. Hie index s refers to a state achieved after an isentropic compression expansion.

The organic working fluid in the liquid state at the maximum operating pressure (state 13) enters the heat exchanger. In the heat exchanger, heat is transferred from the high temperature heat transfer fluid, heated through the solar collector field, to the organic working fluid. Typically, this heat exchanger consists of three parts, a preheater, evaporator, and superheater. The heat transfer rate in the heat exchanger, Oe, is given as follows:

The power output of the turbine, Wr and the gross electric output. JPelgmss, are calculated as follows:

where //(j T is the isentropic efficiency of the turbine and //^ is the generator efficiency.

In the case of diy organic working fluids, the state point after the expansion in the turbine is superheated. The organic liquid at state 12 enters a recuperator (this component is optional) where the low-pressure organic fluid vapor from the turbine (state 9) supplies heat. Finally, the turbine exhaust is condensed in a condenser after part of its heat has been transferred in the recuperator. The heat transfer rate in the condenser, Qc, is calculated as follows:

 
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