تجزیه و تحلیل انرژی اقتصادی و زیست محیطی در سیستم های RET-هیدروژنی در ساختمان های مسکونی

The aim of this study was to analyze energy, economic and environmental performances of a set of scenarios dealing with the production and the use of hydrogen as energy carriers in residential applications in combination with renewable energy (RE). The authors also made an investigation into the required economic conditions necessary for making H2–RE residential systems competitive with conventional ones, which are based on the use of grid electricity and natural gas. A case study was enacted in a small residential district in Palermo (Italy) made by five multi-storey buildings. Many energy systems have been considered according to several fuel-device combinations (electric grid, fuel cell, PV panels, wind turbines, boiler etc.). The software HOMER (hybrid optimization model for electric renewables), developed by NREL and Midwest Research Institute (USA), was used, in order to study the energy balance of the system and its components. Moreover, it was possible to simulate the hourly operation of each system and to calculate technical, economic and environmental performance parameters. The net present cost and the cost of energy are the two main parameters used to compare economic performances of the systems with both actual and expected costs in the medium term. A sensitivity analysis was carried out in order to appreciate the most important parameters influencing the economic performances of the systems and to define possible future scenarios of competitiveness between technologies. Emissions of CO2 (the most important greenhouse gas) and other pollutants have been considered for an environmental benefits analysis.

Hydrogen production represents one of most promising solutions for solving the problem of intermittence in the power production by renewable sources by reducing local impacts of energy conversion and diverting it to several final uses. Hydrogen is a clean energy carrier. It can be produced by electrolysis of water using electricity generated by renewable energy (RE). Electrolytic hydrogen is made from water and is recycled as water. Its oxidation in fuel cells gives an output of electricity and water. Only when hydrogen is not recombined with pure oxygen, and air is used as the oxidant instead, such as occurs in engines or gas turbines, is nitrogen oxide produced [1] and [2]. Moreover, the reaction among the oxygen and the nitrogen in the air can produce much lower emissions of nitrogen oxides than the combustion of fossil fuels [3]. The market penetration of technologies based on hydrogen conversions requires large technological and infrastructural changes. Geopolitical implications could be enormous. Shifting from fossil fuel to the plentiful and more dispersed hydrogen could alter the power balances among energy-producing and energy-consuming nations, possibly turning today's importers into tomorrow's exporters [4]. Fulfillment of this target depends on social, environmental and economic concerns. It is well known that there is an increasing interest in environmental themes and the chance to obtain higher environmental performance with the same distribution of power, which could be directed toward a more wide employment of clean technologies. The use of RE systems in remote, off-grid areas is fairly well established around the world. Applications include telecommunication stations, single homes and small villages. Cotrell et al. [5] have concluded that fuel cell systems supplied with electrolytic hydrogen produced by wind turbines (WT) and photovoltaic (PV) systems appear competitive in a 148-kW village power system if fuel cells price is reduced to 40% of their capital cost [5]. Santarelli et al. [6] have established that the costs of a PV-hydrogen system feeding a residential building in an isolated valley of the Alps are not competitive in the actual energy market, and only considering the elimination of the costs of the distribution medium, voltage lines, and external cost due to pollution, the competitiveness of this system should be increased. Many experts foresee hydrogen produced by non-fossil, RE sources to be technically, economically and ecologically relevant in the next 20 years. However, hydrogen will not be a suitable or economically feasible energy carrier for all markets and countries, so that the use of methanol and synthetic liquid hydrocarbons in fuel cell vehicles or natural gas for stationary fuel cell applications, with reformed technology for central or on-site hydrogen generation, will have an important role in the energy supply. Lokurku et al. [7] compared electricity generation costs of a 200 kWe combined heat and power (CHP) natural gas–fuel cell system with a conventional CHP package having identical electrical power for stationary use. They found that natural gas–fuel cell systems for CHP will be competitive with an 80% investment and maintenance cost reduction and by improving fuel cell performance (increasing stack running time and efficiency). This paper analyses the use of hydrogen technologies in residential buildings connected to an existing electric grid. The aim of the study is to investigate the economic and environmental impacts of the use of hydrogen fuel cells as a substitute for electricity provided by grid and heat from a gas boiler. The building is thought of as a self-sufficient system, and the electric grid represents only an emergency device that operates when the fuel cell is not running (for breakdowns or maintenance). A small residential district has been studied. where a conventional energy supply system was used as a “base system” and some alternative systems operating with the H2 carrier were designed. HOMER [8] has been used to simulate the operation of the system and to calculate, for each configuration, several technical, economic and environmental parameters. The main economic indicator used to compare the system is the “net present cost (NPC),” which is the present value of the costs of investment and operation of a system over its lifetime. CO2, CO, NOx, SO2, unburned hydrocarbons and particulate matter are indicators used to analyze the environmental performances of each system. A comparison among conventional and hydrogen systems has been done.

Results of our simulations have provided some important indications. They might be summarized as follows: • Assuming current costs of devices, fuels and electric energy purchased from the grid, the system which uses the FC supplied with natural gas and the system which couples electrolytic hydrogen with WT are the most cost effective; in particular the latter is competitive with the reference system “Grid” in the case of electricity rate increases of 100% or more. • Economic competitiveness of “alternative” systems is obtainable only with a significant reduction of capital costs together with a strong increase in energy prices. Hydrogen production by PV needs the highest reduction of capital costs or, alternatively, a very high electricity cost increase (500%). • In the next 15–20 years, the use of FC supplied by hydrogen-pipeline will become competitive with traditional systems only if either electricity price increases (more than 200%) or hydrogen price decreases (more than 50%). • If FC is supplied with natural gas, a grid rate increase of 120% is sufficient to make the system competitive at the current natural gas cost. • Energy systems which use fuel cells have the best environmental performances, especially when hydrogen by renewable sources is used instead of natural gas. In fact, the hydrogen electrochemical reaction does not produce CO2, but only a very low quantity of NOx, while use of natural gas requires a pre-reforming phase that causes significant emissions of CO2. • However, the system which uses natural gas FC has emissions much lower than systems “Grid” and “Grid and boiler”. In fact, pre-reforming phase and the subsequent hydrogen galvanic burning produce moderate emissions of NOx, CO, unburned hydrocarbons, particulate matter and SO2. Moreover, CO2 emissions are 28% lower than the ones related to the base system. The good economic performances of NG based configurations, together with the opportunity to use existing NG distribution networks, can be a start-up factor for the diffusion of fuel cell technologies. • In the medium and long term, the priority should be to substitute natural gas with hydrogen produced by RE sources. • Systems based on “fuel cell–hydrogen renewable sources” show some important aspects: (1) the production of electrolytic hydrogen allows the preservation of surplus energy from PV or wind plants that otherwise would be lost; (2) the opportunity to use this energy in deferred ways (by a fuel cell) and in a different place (such as in the examined systems “3” and “5”) make the hydrogen energy carrier able to increase the value of RE sources; (3) total emissions imputable to a fuel cell do not depend only on the composition of fuel, but also on the manner employed to make it available; so that the important absence of carbon and other potentially harmful elements inside the hydrogen structure imparts the greatest benefits, but only when hydrogen production is linked with energy from renewable sources rather than reformation of natural gas.