Liquid desiccant systems are of potential interest as a means of cooling greenhouses to temperatures below those achieved by conventional means. However, only very little work has been done on this technology with previous workers focussing on the cooling of human dwellings using expensive desiccants such as lithium salts. In this study we are designing a system for greenhouse cooling based on magnesium chloride desiccant which is an abundant and non-toxic substance. Magnesium chloride is found in seawater, for example, and is a by-product from solar salt works. We have carried out a detailed experimental study of the relevant properties of magnesium rich solutions. In addition we have constructed a test rig that includes the main components of the cooling system, namely a dehumidifier and solar regenerator. The dehumidifier is a cross-flow device that consists of a structured packing made of corrugated cellulose paper sheets with different flute angles and embedded cooling tubes. The regenerator is of the open type with insulated backing and fabric covering to spread the flow of desiccant solution. Alongside these experiments we are developing a mathematical model in gPROMS® that combines and simulates the heat and mass transfer processes in these components. The model can be applied to various geographical locations. Here we report predictions for Havana (Cuba) and Manila (Philippines), where we find that average wet-bulb temperatures can be lowered by 2.2 and 3°C respectively during the month of May.
Greenhouse cultivation is an energy intensive process therefore it is worthwhile to introduce energy saving measures and alternative energy sources. Here we show that there is scope for energy saving in fan ventilated greenhouses. Measurements of electricity usage as a function of fan speed have been performed for two models of 1.25m diameter greenhouse fans and compared to theoretical values. Reducing the speed can cut the energy usage per volume of air moved by more than 70%. To minimize the capital cost of low-speed operation, a cooled greenhouse has been built in which the fan speed responds to sunlight such that full speed is reached only around noon. The energy saving is about 40% compared to constant speed operation. Direct operation of fans from solar-photovoltaic modules is also viable as shown from experiments with a fan driven by a brushless DC motor. On comparing the Net Present Value costs of the different systems over a 10 year amortization period (with and without a carbon tax to represent environmental costs) we find that sunlight-controlled system saves money under all assumptions about taxation and discount rates. The solar-powered system, however, is only profitable for very low discount rates, due to the high initial capital costs. Nonetheless this system could be of interest for its reliability in developing countries where mains electricity is intermittent. We recommend that greenhouse fan manufacturers improve the availability of energy-saving designs such as those described here. INTRODUCTIONNo issue is more relevant today than the sustainability of processes designed or managed by humans. Among such processes, those concerned with the supply of food and clean water are among the most critical. Cultivation in greenhouses has much potential here, as it encourages the efficient use of land and water and can provide food all year round. However, greenhouses are an energy-intensive form of cultivation especially when artificial heating, cooling or lighting is used (Campiglia et al., 2007). Therefore energy conservation in greenhouses and the use of energy from non-fossil sources is of great interest. Recent reviews of heating and cooling technologies for greenhouses, based on solar and other forms of energy, have been provided by Sharma (2008 and.The focus here is on energy saving through the improved design of fan ventilation systems. Our objective is to explore and quantify, through mathematical models and practical experiments, the savings achievable. We will than evaluate the proposed designs from economic and environmental perspectives.
The urgent need to decrease greenhouse gases (GHG) has prompted countries such as the UK and Norway to commit to net zero emissions by 2050 and 2030, respectively. One of the sectors contributing to GHG emissions is agriculture, by approximately 10% in the EU in 2017. GHG reductions in the production side should involve avoidance at source, reduction of emissions and/or removal of those emissions, with the potential for negative emissions by carbon capture. This paper focuses on the utilisation of agricultural waste that can be converted into biogas, such as livestock and crops residues which represent around 37% of GHG emissions by agriculture in the EU. The biogas can be used to produce electricity and heat in a micro gas turbine (MGT). Then, the exhaust gases can be sent to a carbon capture plant. This offers the potential for integration of waste into energy for in-house use in farms and fosters a circular-bioeconomy, where the captured CO2 could be used in greenhouses to grow vegetables. This could even allow the integration of other renewable technologies, since the MGT offers flexible operation for rapid start-up and shut down or intermittency of other technologies such as solar or wind. Current carbon capture processes are very costly at the smaller scales typical of remote communities. The alternative A3C (advanced cryogenic carbon capture) process is much more economical at smaller scales. The A3C separates CO2 from process gas that flows counter-currently with a cold moving bed, where the CO2 desublimes on the surface of bed material as a thin layer of frost. This allows enhanced heat transfer and avoids heavy build-up of frost that reduces severely the heat transfer. The phase change separation process employed by A3C and the large thermal inertia of the separation medium gives good flexibility of capture for load changes and on-off despatch. This study integrates a combined heat and power MGT, Turbec T100, of 100 kWe output. This include developed models for the MGT using characteristics maps for the compressor and turbine and for the cryogenic carbon capture plant, using two software tools, IPSEpro and Aspen Plus, respectively.
A novel Advanced Cryogenic Carbon Capture (A3C) process is being developed using low cost but high intensity heat transfer to achieve high CO2 capture efficiencies with a much reduced energy consumption and process equipment size. These characteristics, along with the purity of CO2 product and absence of process chemicals, offer the potential for application across a range of sectors. This work presents a techno-economic evaluation for applications ranging from 3% to 35%vol. CO2 content. The A3C process is evaluated against an amine-based CO2 capture process for three applications; an oil-fired boiler, a combined cycle gas turbine (CCGT) and a biogas upgrading plant. The A3C process has shown a modest life cost advantage over the mature MEA technology for the larger selected applications, and substantially lower costs in the smaller biogas application. Enhanced energy recovery and optimization offer significant opportunities for further reductions in cost.
Selection and peer-review under responsibility of the scientific committee of the 11th Int. Conf. on Applied Energy (ICAE2019).
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