THERMODYNAMIC DEVELOPMENT AND MODELLING OF A NOVEL UNIFIED BIOMASS-BASED TRI-GENERATION ENERGY SYSTEM

Population growth and economic development in many parts of the world has brought serious environmental concern, because most energy generation processes emit pollutants, many of which are dangerous to the ecosystem. These primary energy generation methods require the burning of fossil fuels which results in the release of large amounts of greenhouse gases, particularly carbon dioxide. Current energy research shows that renewable energy systems are a global warming mitigation option, especially for residential purposes, as they can help reduce greenhouse gas emissions

System

4.5        Results from Parametric Simulations

4.5.1    Effect of Pinch Point Temperature on Exergy Efficiency

4.5.2   Effect of Pinch Point Temperature on Heating and Cooling

4.5.3   Effect of Turbine Inlet Pressure on Exergy Efficiency and Turbine Output

4.5.4   Effect of Pinch Point Temperature on Hot Water Mass Flow Rate

4.6   Effect of Turbine Inlet Temperature on Exergetic Efficiency

References

Population growth and economic development in many parts of the world has brought serious environmental concern, because most energy generation processes emit pollutants, many of which are dangerous to the ecosystem. These primary energy generation methods require the burning of fossil fuels which results in the release of large amounts of greenhouse gases, particularly carbon dioxide. Current energy research shows that renewable energy systems are a global warming mitigation option, especially for residential purposes, as they can help reduce greenhouse gas emissions (Abam et al. 2018). Renewable energy is a source of energy which comes from natural resources like rain, wind, tides, sunlight, waves, biomass and geothermal heat. These energy sources are naturally replenished when used and are inexhaustible. Biomass is a renewable energy source from organic biological materials. As an energy source, biomass can be used directly, or changed into other energy products like biofuels (Cohce et al. 2011). 

At present, biomass resources are used principally in the production of heating, cooling, electricity generation and hydrogen production in energy conversion systems (Ahmadi et al., 2013; Filho and Badr, 2004). Additionally, the most common method of biomass energy conversion is direct combustion with coal and provides the ultimate potential for large-scale utilization of biomass energy in the future (Hughes and Tillman, 1996). Other thermochemical conversion technologies like gasification and pyrolysis are technically feasible and marginally efficient, compared to combustion, for power generation. However, most of these biomass technologies, although potentially feasible, are not yet cost competitive and efficient (Filho and Badr, 2004). Consequently, biomass is not the most favourable options at present. However, increasing political and environmental concerns have impelled the development and utilization of renewable energy. This has resulted to an increase in both the demand and cost of biomass resources.  Surely, the paradigm has shifted towards more effective conversion and efficient utilization of biomass resources (Van den Broek and Faaij, 1996). With the incorporation of biomass based ORC applications, effective energy conversion can be achieved.

The ORC technology is novel and characterised by high energy efficiency. Its flexibility allows the production of electric energy from renewable sources like geothermal wells, biomass (which is used for this research work), solar irradiance and waste heat energy from industrial processes. It is based on the Rankine cycle with high-molecular-mass organic fluid which evaporates on significantly lower temperature than water in classic Rankine cycle (Abam et al., 2019, Julije et al., 2013). Interestingly, since many industrial processes and applications are characterized by the availability of low enthalpy thermal sources with temperatures lower than 400°C, like the ones derived from industrial processes (e.g. combustion products from gas turbines and internal combustion engines, technological processes and cooling systems) or renewable sources (e.g. solar and geothermal energy) the ORC is proving to be a promising process for conversion of heat at low and medium temperature to electricity. However, a certain challenge is the choice of the organic working fluid and of the particular cycle architectural design for its implementation. For these reasons, vast researches have been done on the choice of refrigerants for its implementation, especially as it relates to the depletion of the ecosystem. 

Recent implementation of the ORC technology has been on the generic cycle which traditionally comprises a pump, an evaporator (which replaces the boiler), a turbine and a condenser. With this simple arrangement, theoretical and experimental studies have been conducted with an avalanche of refrigerants, different heat sources and condensing methods, cost analysis, optimisation and several condensing and evaporating pressures (Velez et al. 2012; Quoilin et al. 2011; Manolakos et al. 2009; Canada et al. 2004; Chen et al. 2010).  However, in addition to the foregoing, attention is now paid to the modification of the basic ORC structure with other components to effect augmentation in turbine work, system efficiency and a reduction in the running cost of the system. Of note is the presentation of a framework for the energy and exergy evaluation of the basic and three modified ORCs where obtained results showed substantive improvement in both overall energy and exergy efficiencies and specific turbine work (Sahar and Fereshteh, 2015). All the proposed ORCs have a measure of efficiency and power production based on the given architecture and the quantity of waste heat powering the evaporator. Unlike the other bottoming cycles such as the Kalina cycle, the ORC is always operated to produce a single output. 

Recent studies have included the configuration of the Kalina cycle for simultaneous power and cooling (Wang et al., 2016). Models were mathematically established to simulate the combined system at steady-state conditions. Exergy destruction analysis was conducted to display the exergy destruction distribution in the system qualitatively. A study of the low waste heat operated ORCs suggests a novel configuration which can produce cooling, heating and power production simultaneously and powered entirely by biomass.  This need is met in this study by the development of a novel ORC-based configuration which can produce power, heating and cooling. The thermodynamic models which demonstrates the intended high energy and exergy efficiency of the proposed system will be conducted using a number of refrigerants. The results will add to existing body of literature in the application of biomass energy source in power generation, heating and cooling with an even simpler configuration.

The most common method of biomass energy conversion is direct combustion with coal and provides the ultimate potential for large-scale utilization of biomass energy in the future. Other thermochemical conversion technologies like gasification and pyrolysis are technically feasible and marginally efficient, compared to combustion, for power generation. However, most of these biomass technologies, although potentially feasible, are not yet cost competitive and efficient. Consequently, biomass is not the most favourable options at present. However, increasing political and environmental concerns have impelled the development and utilization of renewable energy. This has resulted to an increase in both the demand and cost of biomass resources.  Surely, the paradigm has shifted towards more effective conversion and efficient utilization of biomass resources. With the incorporation of biomass based ORC applications, effective energy conversion can be achieved. This need is met in this study by the development of a novel biomass-based ORC configuration which can produce power, heating and cooling. The thermodynamic models which demonstrates the intended high energy and exergy efficiency of the proposed system will be conducted using a number of refrigerants. The results will add to existing body of literature in the application of biomass energy source in power generation, heating and cooling with an even simpler configuration.

The aim of this research work is the thermodynamic development and modelling of a novel unified biomass-based tri-generation energy system. The specific objectives are as follows:

The research scope will cover the following areas: 

An avalanche of energy sources is available for powering energy conversion systems operating on the ORC. For all energy sources available, the general objective has always been the development of relatively simple energy system with reasonably high performance. In literature a number of such studies are handy. However, it is not always easy to obtain a robust configuration with good energy and exergy efficiency. Ahmadi et al. (2013) developed a novel multi-generation system based on a biomass combustor, an organic Rankine cycle (ORC), an absorption chiller and a proton exchange membrane electrolyzer to produce hydrogen, and a domestic water heater for hot water production. The energy condition of the heat source which powers the vapour absorption system is a comparatively weak refrigerant stream after expansion in the turbine and condensation in the condenser. Consequently, the amount of cooling is severely reduced, resulting in minimal exergy efficiency of the plant at 22 %. To remedy this drawback and augment the overall system performance, the proposed cycle incorporates turbine bleeding at a high energy level for cooling, in addition to whole power generation in the turbine as well as heating. Additionally, as contained in (Nonso, 2017), the details for powering a novel ORC configuration was only limited to specified energy states and quanta which can come from an avalanche of energy sources. Here, a specified and properly modelled energy source which powers such novel arrangement is proposed in biomass. This will not only enhance efficiency but will result in environmental sustainability consequent upon the utilization of biomass for its operation. This particular enhanced configuration is not in open literature and hence, justifies this research work.