Electrolyser Efficiency

The efficiency of an electrolyser is calculated in almost the same way as for a fuel cell. If Vc is the operating voltage for one cell of a fuel cell stack, then it was shown in Chapter 2, 

In the case of an electrolyser the formula is just the inverse of this 

The losses in electrolysers follow exactly the same pattern as for fuel cells as described in Chapter 3. Real values of Vc are around 1.6 to 2.0 V, depending on the current density. Because the problems of cooling and water management are so much more easily solved, the performance of electrolysers is routinely as good as for the best of fuel cells, with current densities of around 1.0 Acm being normal. 

An electrolyser can be operated very efficiently, with K about 1.6 V, if the current density is kept low. Low current density means a slow rate of production of hydrogen or a large electrolyser - hence, higher costs. There is always a balance to be struck between efficiency of production, and high rate of production per dollars worth of electrolyser. Typical operating efficiencies claimed by commercial makers of units, including the energy needed to compress the product gas, are around 60 to 70%. 

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Sensitivity analyses are performed to evaluate the effect of changes in cost factor values on H2 production and PV electricity prices. The cost factors for H2 production are PV electricity electrolysers electrolyser operating capacity factor electrolyser efficiency (in terms of converting electricity energy input into H2 energy output) electrolyser O M expense and the discount rate. The cost factors for PV electricity... 

The sensitivity results reported in Table 5 for FF production price are as follows. A 0.01/kWh increase in electricity cost causes FF production price to increase by 0.55/kg. A 25 increase in electrolyser cost ( /kWdc-m) causes FF production price to increase by 0.04/kg. A 1% increase in electrolyser capacity factor causes FF production price to decrease by 0.02/kg. A 1% increase in electrolyser efficiency (LHV) causes FF production price to decrease by 0.04/kg. A 1% increase in electrolysis plant O M expenses, which includes water system and compressors, causes FF production price to increase by 0.09/kg. A 1% increase in the discount rate causes FF production price to increase by 0.09/kg. To evaluate the effect of a decrease in cost factor values simply reverse the sign, positive or negative, for the change in FF production price. 

Hydrogen production by electrolysis is a mature technique which has been used for over a hundred years. Most commercial electrolysers are based on the principle of alkaline electrolysis, but solid polymers are also used as an electrolyte. Electrolyser efficiencies range between 65 and 75 %.Units with a hydrogen production of liters to several 10.000 cubic meters per hour are available. 

In working Example 18.8, we have in effect assumed that the electrolyses were 100% efficient in converting electrical energy into chemical energy. In practice, this is almost never the case. Some electrical energy is wasted in side reactions at the electrodes and in the form of heat This means that the actual yield of products is less than the theoretical yield. 

Two distinctly different coulometric techniques are available (1) coulometric analysis with controlled potential of the working electrode, and (2) coulometric analysis with constant current. In the former method the substance being determined reacts with 100 per cent current efficiency at a working electrode, the potential of which is controlled. The completion of the reaction is indicated by the current decreasing to practically zero, and the quantity of the substance reacted is obtained from the reading of a coulometer in series with the cell or by means of a current-time integrating device. In method (2) a solution of the substance to be determined is electrolysed with constant current until the reaction is completed (as detected by a visual indicator in the solution or by amperometric, potentiometric, or spectrophotometric methods) and the circuit is then opened. The total quantity of electricity passed is derived from the product current (amperes) x time (seconds) the present practice is to include an electronic integrator in the circuit. 

Despite their high cost, they are used in industrial electrolyses, fuel cells, and many electrochemical devices. The large investments associated with platinum electrocatalysts usually are paid back by appreciably higher efficiencies. 

On this basis a demonstration plant having a capacity of 10 000 tonnes a-1 of chlorine was built in the Bayer production plant at Leverkusen. The plant was successfully commissioned on 4 January 2000. Figure 4.8 illustrates the electrolyser section of the plant, with the peripheral apparatus arranged mostly outside this building. The 76-element electrolyser was found to behave very smoothly and could be immediately operated up to 5 kA m-2 without any problems. Permanent operation is performed at 4 kA m-2. The power consumption was found to be about 1080 kWh tonne-1 CI2 with a typical current efficiency of nearly 100%. Chlorine purity is found to be 99.9%, which obviates the need for a chlorine liquefaction purification and therefore simplifies the plant drastically. 

The main objective for an intelligent system in electrolyser operations is to gather and process valuable information for a greater control and efficiency. To achieve the aforementioned objective, two key functions have to be performed properly. Firstly, accurate and precise real-time data need to be obtained and secondly, the system should be able to process and interpret these data based on fundamental and acquired industrial electrochemical knowledge. In the case of R2 s EMOS , this second key element refers directly to its capability to use embedded human expertise to find optimal operating solutions and to detect and correctly identify equipment degradation or other anomalies. 

All these modifications lead to the fulfilment of all aspects for an improvement of the Rol by KU single element technology and high efficiency of the latest cell design, such as low energy consumption with a high on-stream factor for the plant, simple and rapid maintenance of the electrolysers, plant load flexibility and high current densities. 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

The quest for higher efficiency and lower production cost is now leading inevitably to the development and adoption of larger electrolysers and higher current densities. As indicated from the calculation of optimum current densities at given electric power costs illustrated in Fig. 17.1, the optimum current density at a unit power cost of... 

As indicated in Fig. 17.2, the membrane process has long been characterised by substantial reductions in electric power consumption, through constant advances in membrane, electrolyser and electrode technologies. In the early years of its commercial establishment, some 25 years ago, it yielded a caustic soda concentration of 20% or lower, with less than 90% current efficiency. Today, the caustic soda concentration is 33%, the current efficiency is 97%, and the ohmic drop of the membrane has been lowered by approximately 1.0 V. During the same period, advances in electrolyser design have improved the uniformity of intracell electrolyte concentra-... 

For high current efficiency at high current densities, it is essential that the electrolyser provides efficient mass transfer to the membrane surface. The most important... 

The ML32NCH electrolyser equipped with the Aciplex F-4401 membrane has been in commercial operation at 6 kA m-2 for approximately one year at Asahi Chemical s chlor-alkali plant. As shown in Figs 17.16 and 17.17, the electrolyser has achieved a cell voltage of 3.17 V and a current efficiency of 96%, while operating at 6kA m-2. This operation is continuing the present plan is to investigate the performance of the ML32NCH at a current density of 8 kA m-2. 

The recent general outlook of AZEC B-1, the efficient bipolar electrolyser... 

Furthermore, liquefaction efficiency will always be less than 100%. Some of the chlorine produced must remain with the non-condensable tail gas. The relevant factors were addressed in a paper presented at the 1997 SCI London International Chlorine Symposium [3]. In the processing of the tail gas, up to about 4% of the chlorine produced in the electrolysers is diverted to lower value products such as bleach or hydrochloric acid. Small quantities of secondary products such as these materials can also present a marketing problem. A further loss of chlorine product can occur in the storage system, particularly in systems where padding air is employed. 

Production of hypochlorite takes place in a two-step absorption unit in which 23% caustic solution is fed counter-currently to the chlorine feed-stream. In the first step -the liquid jet-loop reactor - about 90% of the chlorine is converted to hypochlorite. In step two - a packed column - a very efficient absorption [1-3] is carried out in which the chlorine concentration in the off-gas is reduced to <1 ppm. The operating window of this apparatus with respect to chlorine load is quite large and varies from 100 to 6000 kg h-1 of chlorine. This high capacity is necessary for the consumption of peak loads from the electrolysis plant during short time periods. During start-up or shutdown of one electrolyser the total chlorine peak load can vary from 100 to 300 kg in just a few minutes. 

Pressurised alkaline water electrolysis technology has been state-of-the-art for many years. The system efficiency of real electrolysers ranges from 62% to 70%, including all auxiliaries (AC/DC converter, pumps, blowers, controls, etc.) based on the lower heating value of hydrogen. 

Alkaline electrolysers are at an industrial stage, especially commercialized for on-site production of ultrapure hydrogen for industrial applications. In general, this hydrogen is needed at low to moderate pressure, and the cost demand is set in comparison to the alternative, which is in general the supply by tube trailers. High purity water is fed to the electrolyzer. State-of-the-art commercial alkaline electrolysers typically operate at HHV systems efficiency of 60-75% [44], Current... 

SPE electrolysers are at present available at a scale of 0.5-10 Nm3 hydrogen per hour at an output pressure up to 200 bar. The available systems operate at a rather poor overall efficiency, 50-70% [44, 48], The projected price level of these systems, at industrial production level, is at least a factor of 3-5 too high. 

For both low temperature electrolysers, the biggest gain in efficiency is to be expected from an improvement in Balance of Plant components, taking into account the big gap between cell efficiency (80-90%) and system efficiency (50-60%). In the case of SPE electrolysers, catalytic research should therefore be directed to making the catalysts more tolerant to contaminants. For alkaline electrolysers, in addition to this, more active electrodes could lower capital costs. 

Solid Oxide Electrolysers (SOE) are in development for steam electrolysis. As electrolysis is an endothermic process, a supply of waste heat can be used beneficially to reduce the electrolyzer voltage, and thus increase its electrical efficiency. Combination with nuclear power generation and geothermal heat sources is often encountered in development programs for SOE. 

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