JSDEWES: Techno-Economic Model for a Quick Preliminary Feasibility Evaluation of Organic Rankine Cycle Applications

The enormous amount of waste heat which is released from industry often has a high potential to be transformed into mechanical work using one of the conventional or unconventional technologies. Some of the promising new technologies which are still under development and are usually considered for waste heat recovery are thermoelectric generators, phase change materials, Organic Rankine Cycles (ORC), Kalina cycles and trilateral flash cycles [1].

Thermoelectric generators make use of the Seebeck effect to generate electric energy [2]. Waste heat is applied to generate temperature gradient over the conductor, which further generates voltage difference. Although the efficiency does not exceed 10% [3], these generators distinguish their compactness and reliability, since they have no moving parts, as well as their operation has almost no environmental impact [4]. The maximal operation temperature of commercially available thermoelectric generators is 250 °C [5].

Paraffin mixtures are applied as phase change material, and mechanical work is utilised in a generator during the phase change from liquid to solid, and vice versa. The efficiency of this kind of electricity generation, which is still under laboratory testing, is well below 10% [1].

Organic working fluid is applied instead of water in the ORC. This way, the evaporation conditions of working fluid matches the temperature of the waste heat source better, which guarantees high flexibility and thermal efficiency of the ORC, thus, its applications have increased significantly in recent years [6].

Even better waste heat source temperature adaptation is possible when a mixture of two working fluids having different boiling temperatures is applied, such as in the Kalina cycle [7], which extracts source heat efficiently and is favourable at low temperature conditions [8].

Trilateral flash cycles use single working fluid. When boiling working fluid at high pressure, a flash expands, a vapour mixture is formed, which enters a separator. Here, the saturated vapour is produced, which can perform mechanical work when expanding in a turbine [9]. Flash expansion reduces thermal efficiency, which is, therefore, lower than in the case of an ORC working within the same temperature interval [10]. According to Yari et al. [9] trilateral flash cycles are still under technical development.

A short overview of possible technologies for mechanical work or direct electrical energy recovery from waste heat, shows that ORC and the Kalina cycle have the highest potential at the moment. When both are compared under the same operating conditions, it may be stated that ORC is more compact and exhibits higher thermal efficiency [11], while the Kalina cycle generates up to 3% higher power output [12], however, it is much more complex and demands higher investment costs [13]. Moreover, the applied heat exchangers have to be larger and corrosion resistant [14].

At the moment, ORC is the most promising and applied power plant technology for waste heat utilisation. Its commercial application exists in the electric power range from 20 kW _eto 20 MW_e. One of the most common applications is utilisation of geothermal energy. Here, the ORC may replace a standard flash cycle, or may even be applied as its bottom cycle to utilise the exhaust condensate from the separator of the flash unit [15]. Besides, ORC may be found widespread in other energy sectors too. Villarini et al. [16] investigated ORC application within a solar driven triregenerative system. Solar energy stored in a heat storage tank drove an ORC, while heat rejected from the ORC was applied within heat supply system, or to produce chill in an absorption chiller. Application of ORC within a wood drying plant fuelled with sawmill biomass is presented in Borsukiewicz-Gozdur et al. [17]. The authors have shown that the application of organic working fluids instead of water increased the system flexibility, and resulted higher electric power output and higher drying air temperature. ORC based waste heat utilisation in Combined Heat and Power (CHP) plants was shown by Qiu [18], who studied the application of different working fluids. He presents the comparison and optimisation of 8 mostly applied working fluids nowadays, and gives a preferable ranking. Furthermore, ORC can be efficient as a bottoming cycle in combined cycle power plants, as reported in Katulić et al. [19]. The authors carried out an exergoeconomic optimisation of a bottoming cycle where, instead of water, selected organic fluids were used in the lower pressure levels of the heat recovery steam generator. Their study proved that it is economically justified to use selected organic fluids instead of water in the lower pressure level of a combined-cycle power plants bottoming cycle. ORC application for waste heat recovery from cars and trucks is considered in the Automotive industry [20], while it also has high potential for waste heat recovery in heat intensive industries. Ustaoglu et al. [21] showed the efficiency increase of the wet rotary kiln process in the Cement industry by the utilisation of waste heat with ORC. Similarly, Campana et al. [22] found application of ORC effective for waste heat recovery in the Glass, Steel and Crude Oil industries. Walsh and Thornley [23] focused on coke ovens used in the Steel industry, and proposed an ORC based utilisation of low temperature heat from coke oven exhaust. They showed that the potential savings due to on-site electricity generation suggested a payback period of less than 4 years. Varga and Csaba [24] suggested ORC for low temperature waste heat recovery in crude oil processing as an effective technology for additional energy efficiency increasing and improvement of refinery competitiveness. Therefore, ORC may be of interest for possible investors in the Energy sector who, however, would need a quick answer as to whether their investment is going to be economically efficient or not. A simple decision model was, therefore, developed, which can be used to check the technical feasibility and economic viability of an ORC system, and to select the appropriate working fluid, based only on basic information on the waste heat source, i.e., source temperature and mass (heat) flow rate. No complex thermodynamic ORC model was integrated into the decision model, although, correlation was applied instead, in order to keep the model simple and highly applicable.

An ORC thermodynamic model was built in order to develop design correlations for simple and quick estimation of ORC operational parameters and component design. The ORC system layout is presented in Figure 1. Exhaust gases carrying waste heat flow through the set of heat exchangers (SH, EV and PH), and reject their heat to the working fluid. Exhaust gases are released to the atmosphere at approximately 120 °C, which is set as the lower limit, in order to avoid any water condensation within the exhaust. The produced live vapour expands in the turbine and then enters the regenerator, where the exhausted vapour rejects heat to the vapour cooler integrated within the water cooled condenser, where it finally condenses. The condensate is then pumped to working pressure and directed through the set of heat exchangers to produce live vapour. The condenser is cooled by cooling water. The latter is supplied by the cooling tower, where the final heat transfer to the surroundings takes place.

ORC operational points are calculated using a simple ORC model written in Excel, with the REFPROP database [25] applied as an Excel Add-in. The live vapour pressure and temperature (p₁ and T₁) and condensation temperature T₃ = T₄ = T_C are set as input data. Any other thermodynamic state is predicted by the model, as well as calculating turbine and pump specific work. The specific heat transferred to the working fluid in a super-heater, evaporator and preheater, and the specific rejected heat in the vapour cooler and condenser are also estimated by the ORC thermodynamic model.

Once all the temperatures and mass flow rates are known, it is possible to estimate the basic design of ORC components like heat exchangers, pump and turbine. The plate type heat exchangers are applied due to their compactness and high effectiveness [32], which result in less heat transfer area than that when using shell and tube heat exchangers. The heat transfer area is of main importance, since it dictates the cost of the heat exchanger which is used in the economic evaluation of the system. The effective heat transfer area of any heat exchanger may be estimated by:

$A_{\text{HE,}i}=\frac{{\dot{Q}}_i}{U_i\Delta T_{LM,i}}$ (2)

where ${\dot{Q}}_i$ is the heat flow rate, U_i is the overall heat transfer coefficient, and ∆T_LM,i is the logarithmic mean temperature difference.

Heat flow rate is a product of working fluid mass flow rate and specific heat transferred by the individual heat exchanger specified in Table 4. Overall heat transfer coefficient (U ) may be calculated by considering two convective heat transfer resistances in series (working fluid and secondary fluid sides):

$U=\frac{1}{h_{\text{wf}}}+\frac{1}{h_{\text{sf}}}$ (3)

When a plate heat exchanger is designed an optimisation is performed to find an optimal combination of number and dimensions (length and width) of plates, which results high heat transfer at both sides of the plate with as low a pressure drop as possible. This is possible only when an actual heat exchanger with known heat flow rate and characteristic temperatures is designed. A general approach which may estimate the optimal heat exchanger design within a broad interval of design conditions as applies in the case of the present decision-making tool is, therefore, almost impossible. A simplified approach was, therefore, used, and an achievable constant overall heat transfer coefficient was applied. A similar approach may be found in van de Bor and Infante Ferreira [33], where the overall heat transfer coefficient within heat pumps operating with R134a, R11, propane and butane was set to 1,200 W/m²/K. In the present study, U = 1,200 W/m²/K was used in the evaporator and condenser where the phase transformation from liquid to vapour and vice versa takes place, U = 400 W/m²/K was used to design all other heat exchangers. Logarithmic mean temperature difference is defined by:

$\Delta T_{LM}=\frac{\left(T_{\text{h,in}}-T_{\text{c,out}}\right)-\left(T_{\text{h,out}}-T_{\text{c,in}}\right)}{\ln \frac{T_{\text{h,in}}-T_{\text{c,out}}}{T_{\text{h,out}}-T_{\text{c,in}}}}$ (4)

where suffixes h and c are used for hot and cold media, respectively. Thus, any logarithmic mean temperature can be calculated using the characteristic system temperatures specified in Table 3.

The cooling tower is a forced convection type and consists of a plate type water to air heat exchanger and an axial fan forcing the ambient air through the heat exchanger. A cheaper brazed plate heat exchanger was applied due to the noncorrosive operational environment. The air flow rate depends on its temperature variation, and can be predicted from:

${\dot{V}}_{\text{CT,air}}=\frac{{\dot{Q}}_{\text{rej}}}{c_{p,\text{air}}{\rho }_{\text{air}}\Delta T_{\text{air}}}$ (5)

It was assumed that ∆T_air = 10 °C. Eq. (2) was applied to estimate the effective heat transfer area of the cooling tower heat exchanger. U = 400 W/m²/K was used and T_ambient = 25 °C was applied for the logarithmic mean temperature prediction.

Different profitability criteria may be applied in the economic analysis. PBP and the Net Present Value (NPV) were used in the present work. The standard procedure for determination of both factors was used [34], thus, it is not presented here, while the investment evaluation is presented in more detail. The characteristic economic parameters used in NPV and PBP calculation are summarised in Table 6. The values correspond to renewable energy source application projects in Slovenia, and were set as the default values, which, however, may be changed by the decision model user at any time. The electricity price (C_el) considers the average production electricity price in Slovenia, increased by the subsidies from the governmental renewable energy production support scheme.

A simple decision model was developed, which can be used to check the technical feasibility and economic viability of an ORC system, based only on basic information on the waste heat source, i.e., source temperature and mass (heat) flow rate. Two profitability criteria, NPV and PBP, were introduced for economic evaluation, while an ORC design correlations based model was developed and applied for prediction of ORC technical parameters and components design estimation. The model is applicable for three commonly adopted working fluids for technical applications, R134a, R11 and benzene, and predicts basic technical and economic parameters of the ORC system.

Model validation was performed with the previously published data for all three working fluids. Although simplified, the model predicted the basic ORC thermodynamic characteristics correctly. It gives quick answers, and can be incorporated easily into a decision algorithm, which, when supported by a set of ORC component design and cost functions, can serve as an effective tool for preliminary feasibility evaluation of any proposed ORC based waste heat recovery system.

An example of model application was made to show its basic capabilities. An ORC based bottoming system was applied to recover heat from the exhaust gases of an ICE. It was found that the benzene resulted in the highest electric power gain, although it demanded very high investment, and its application was limited to high waste heat source temperatures. From the economic point of view, R11 surpassed the other two working fluids at moderate to high source temperature, while R134a guaranteed the system operation at low source temperature. The results show clearly that R11 or similar working fluids may be preferable than the thermodynamically more efficient benzene or other overhanging fluids for the smaller turbine size (lower specific volume ratio v₂/v₁) which guarantees lower investment cost and higher system economy.

The research described in this paper was a part of the INTERREG Central Europe CE-Heat project which was supported financially by the European Regional Development Fund. Its support is greatly appreciated.