An application of pinch analysis in the design of a stand-alone thermochemical cycle, hydrogen hub sourced from renewables in rural South Africa

Abstract

Economic and social development are closely related to the accessibility of electricity. Meanwhile, non-grid connected rural areas shoulder the burden of health and environmental risks since extending the grid is considered uneconomical. Renewable hydrogen, hybrid energy systems are viewed as a promising solution in remote areas where grid extension is costly and fuel costs increase parallel to remoteness. Hence, this study applies heat and power pinch analysis in the conceptual design of an isolated, decentralized thermochemical cycle hydrogen & biogas energy hybrid, to satiate the electricity needs of a non-grid rural area in South Africa. This study highlights the value of using heat pinch and PoPA tools as a mid-term supplement to combat increasing energy costs, reduce negative environmental impacts, improve profits, and more importantly as a contribution to ensuring temperatures are kept well below 2℃ above pre-industrial levels. The hybrid plant was designed to supply electricity to 39 village households, housing 156 inhabitants with daily electrical usage of 273 kWh. A 4-step hybrid CuCl thermochemical process including an electrolysis step coupled with Anaerobic Digestion (AD) was employed in the hybrid design. The 4-step hybrid CuCl cycle process was chosen since it has greater thermal efficiency and practical viability relative to the other thermochemical cycles whilst AD was chosen since the chosen site is rich in biomass. A Parabolic Trough Concentrator (PTC) to capture solar radiation and a Proton Exchange Membrane Fuel Cell (PEMFC) was combined with the thermochemical plant to convert hydrogen to electricity. Considering the demand, the solar and biogas plant was sized at 30 & 35 kW respectively. Once sized, heat pinch analysis was applied to improve heat distribution coupled with Power Pinch Analysis (PoPA) for effective electricity distribution. In applying heat pinch analysis and constructing the Grid Diagram (GD), a cooling energy target of 43.86 kW was achieved whilst reducing the total number of heat exchangers from 19 to 12, leaving 5 unsatisfied streams. However, the 5 unsatisfied streams were accepted since reaching its target temperatures were considered insignificant. PoPA was applied whilst maintaining an Available Excess Electricity for the Next Day (AEEND)>Minimum Outsourced Electricity Supply (MOES), thereby ensuring no grid electricity requirement. The initial envisioned operating times and capacity resulted in an oversized system with a daily electricity surplus of 586.99 kWh. This was then revised by shifting operating times to provide an electrical surplus of 12 kWh per day. The original capacity was not changed but the plant power output can be ramped down to achieve continuous operation of the biogas plant, thereby providing an electrical surplus of 8.99 kWh. The process of ramping output up or down is known as cycling. The solar and biogas plant was initially oversized to 30 and 35 kW respectively, thus accounting for power losses, increased migration to the village considering the available power source and potential extension to neighbouring villages. Consequently, cycling was recommended to minimize electricity surplus. The minimum surplus of 8.99 kWh a day was achieved by reducing the power output (cycling) of the solar and biogas plant to 10 kW & 12 kW respectively. PoPA revealed that a biogas plant power output of 12 kW was ideal to ensure continuous power supply should weather conditions thwart effective operation of the solar plant. A cost estimate was performed revealing a unit cost of $3/kWh & $2/kWh. Consequently, the system was concluded as expensive. However, it has been recommended to scale-up production to ensure a more accurate estimate whilst reducing the unit cost with economy of scale. Furthermore, exploiting economy of scale could further highlight the benefits of using pinch tools to combat rising energy costs and negative environmental impacts associated with power production.