|An electric car, symbol of the trend towards an electric economy
Electricity is increasingly coming to dominate the supply of energy, and the reasons may be obvious. Appliances such as induction equipment and microwaves are being used more and more for cooking worldwide, there is a rise in the use of electric pumps to heat and cool buildings, and electric vehicles are emerging rapidly.
For reasons to do with a fear of scarcity of fuel, and the threats of climate change and poor air quality, renewables are being promoted as a way to help provide this power. Yet renewables suffer from intermittency, to which, some say, the solution is hydrogen. However, this risks sending energy policy makers down the wrong track.
Peter Hardy of the British Institution of Gas Engineers & Managers wrote in the November 2012 issue of PEi that hydrogen will play a crucial role in the race to diversify energy sources1. He refers to a project by E.ON in Germany in which the intermittency of renewables will be addressed by converting the excess power that they produce into hydrogen, injecting it into pipelines and converting it back to electricity at a later stage. In reality this scheme may be a waste of precious energy and money.
GERG, the research co-operative run by the major European gas companies, has initiated the Power to Gas project to look at the injection of hydrogen into natural gas streams. The European Commission has already contributed substantial sums of money to the hydrogen injection idea via the so-called NaturalHy project2, and the former Dutch government had put aside millions of euros for research into the conversion of electricity into hydrogen gas.
But another disadvantage is that substantial and variable amounts of hydrogen in natural gas will seriously diminish the quality of the gas. Hardy’s statement that minimal work is required to supply customers with up to 50 per cent of hydrogen mixed with natural gas is not correct. Moreover, the extra output from PV systems and wind turbines that would create this hydrogen occurs so seldomly that investing in capturing it will never be economical.
A substantial market for hydrogen exists in the world. According to marketsandmarkets.com3, 53 million tonnes of hydrogen were produced globally in 2010. The energy chemically stored in that is 6.35 PJ (or 6.35 million GJ), or 151 million tonnes of oil equivalent (toe). This is roughly 1.2 per cent of the global energy supply in 2010, and represents a great deal of energy. About half of the hydrogen is used for making ammonia for fertilisers and the other half is used mainly in refineries to build lighter components from crude oils. Most hydrogen is derived from natural gas, oil and coal via steam reforming. The equations that govern the process are:
Electrolysis is barely used for bulk hydrogen production because of the poor energy efficiency and high costs. The theoretical maximum efficiency to be reached in producing hydrogen by using electrolysis is still only about 70 per cent, notwithstanding extensive research4. The fuel efficiency of the electricity supply system varies from 25-50 per cent, so the ultimate energy efficiency of converting fuel to hydrogen via electrolysis is 17-35 per cent. Steam reforming, however, can be as energy-efficient as 65-75 per cent.
If hydrogen were to be injected into high-pressure gas transmission pipes, often at pressures of around 60 MPa, the process of compression would require a substantial amount of energy. The reason is that hydrogen has only about a quarter of the volumetric energy density of natural gas. The high mass-based calorific value of hydrogen1 is therefore of no relevance. Compressing 1 kg of hydrogen from atmospheric pressure at ambient temperature to 6 MPa for pipeline injection takes about 7 MJ of compression energy in a multi-stage process with intercoolers. Taking into account the losses of the driving electric motor, compressing hydrogen to pipeline pressure consumes about 6 per cent of the energy of the compressed gas. This assumes that the electric motor driving the compressor is running on peak electricity from wind or PV.
Also, transporting hydrogen by pipeline would consume more energy than transporting natural gas. The reason is again the volumetric energy density of hydrogen, which is lower by a factor of four compared with natural gas. Clearly the transport capacity and storage capacity of pipeline systems is drastically reduced when hydrogen makes up a large fraction of the gas mixture. Ultimately the conversion of electrical energy into hydrogen for a gas supply system might reach a maximum energy efficiency of 60 per cent. A subsequent conversion of the hydrogen back into electrical energy, even with a high efficiency of, say, 50 per cent renders a maximum total cycle efficiency for converting electricity to hydrogen and then to electricity again of only 30 per cent. In practice the figure might actually be as low as 25 per cent. In contrast pumped-hydro power storage systems have a cycle efficiency that is about three times higher.
Renewables’ peak power
Many people are of the opinion that the output of wind turbines and PV systems should never be curtailed. However, an economically acceptable case cannot be made for harvesting excess electricity from renewables when the winds are high, the sun is shining and consumption is low. The reason is that peak wind and solar power only occur sporadically. This is illustrated in Figure 1 by the half-hour values of wind power fed into the grid of the German 50 Hz transmission system operator over 2012. The area the TSO covers is almost 110,000 km2, which accommodates 12.7 GW of installed wind power capacity. This equalled 41 per cent of all German wind power in 2012. The distribution curve of the 50 Hz TSO wind power output (Figure 2) shows that peaks over 7 GW occur just a fraction of the time. The full output of 12.7 GW is never reached. Close to identical curves are valid for Danish and UK wind power output.
|Figure 1: Wind power feeding into in Germany’s 50 Hz transmission system (17,250 data points )|
|Figure 2: Distribution curve for wind power in Germany’s 50 Hz transmission system|
Accepting occasional high peaks in output from wind turbines has to be based on a fair balancing of environmental interests and financial burdens. Table 1 gives the result of a number of simulations in which the output from the wind turbines as given in Figure 1 has been curtailed. It is interesting to note that if the output from wind turbines were reduced to only 7 GW instead of the absolute peak of 10.2 GW, the reduction in the annual amount of energy produced by wind would be only 2.7 per cent. Such curtailing would be needed during only 384 hours of the year.
A very low capacity factor of 4.4 per cent would result for any specially built system for transmission or storage of this peak energy, so investing in expensive transmission lines and low-efficiency hydrogen-based storage systems to cover these peaks can never make economic sense. It is much better to curtail such high peaks by switching off part of the renewable capacity. The money saved can be better spent on more renewable capacity and proper backup systems.
For the energy production from PV systems in the German 50 Hz transmission system area, accepting occasional peaks makes even less sense. Figure 3 and Table 2 show that only a very small amount of energy (2.7 per cent) is lost when the maximum in PV power output is reduced from 4.5 GW to 3 GW. This would mean a cap in output for 285 hours of the year. With PV the real issue is not the loss of this small amount of energy by curtailing the peaks in output but rather the large time spans over which there is no output, which is half of the time, as Figure 3 shows. This confirms once more that renewable energy sources need substantial flexible backup capacity.
|Figure 3: Distribution curve of solar PV power in the German 50 Hz transmission system|
Adding outputs from wind-based and solar-based generators (Figure 4) helps improve the median value of a system’s distribution curve. This value is the one in the middle of the x-axis. During the summer the wind is weaker than in winter, and solar radiation is poor in winter. But from the point of view of economics and for the combined system, some curtailing of the peaks is the best approach. It is a mistake to say that occasionally curtailing the output of renewables goes against good environmental policies, and it could be costly. Excessively expensive ways of producing and storing renewable energy are a burden on modern economies that rely on energy. Uneconomical investments will trigger the use of more energy to maintain levels of wealth. So the hydrogen route should not be used for storing sporadically occurring peak energy from renewables. In addition, special high-voltage transmission systems erected for transporting high peaks that occur only sporadically will never be economical.
|Figure 4: Distribution curve of the power output of wind turbines and PV combined|
Deterioration in gas quality
Natural gas is versatile and safe and offers excellent opportunities for efficient and clean energy use. It can be used in power stations, vehicles and home heating appliances. In the predicted electric economy of the future, which will largely run on renewables, natural gas can play a new role because of its excellent potential as a backup battery. Gas power is produced by fast-responding generators driven by combustion engines and gas turbines, and can guarantee a stable and reliable electricity supply until energy storage technologies other than that based on the hydrogen cycle become widely available.
Equipment that runs on natural gas requires the properties of the fuel to be virtually constant if optimum functioning is to be achieved with maximum efficiency and minimum emissions. The Wobbe index of a gas is an important indicator for maintaining a constant air-to-fuel ratio λ and power output P of gas-fuelled equipment. The Wobbe index is given by:
in which H is the volumetric calorific value and ρ is density.
Air-to-fuel ratios deviating from the design value create combustion instabilities, higher emissions of NOx and CO and a loss of efficiency Stable low-noise combustion can only be guaranteed when the flame positions in burners are virtually constant. Deviations in combustion velocity can cause pulsing, flame blow-off or flashback, possibly leading to destruction of the equipment.
Hydrogen has very different properties from gases such as methane, the main constituent of natural gas. Figure 5 shows how the volumetric fraction of hydrogen in a reference natural gas consisting of 91 per cent methane, 6 per cent ethane, 2 per cent propane and 1 per cent butane by volume affects the Wobbe index and the calorific value. Hs is the upper calorific value and Hi the lower calorific value.
|Figure 5: Changing gas properties caused by the addition of hydrogen|
Adding hydrogen to natural gas decreases the Wobbe index. The reference natural gas has a Wobbe index of 55.62 MJ/m3 at 273.15 K and 101.325 kPa. If the gas composition changes to 50 per cent hydrogen, the Wobbe index falls to only 48.12 MJ/m3, which is just outside the range proposed by the European gas industry body EASEE-gas of 48.96-56.92 MJ/m3,6. Without adjustments of the gas supply to the application, the air-to-fuel ratio λ will vary with the Wobbe index:
If the Wobbe Index changes from, say, 50 MJ/m3 to 56 MJ/m3 while an appliance is running at an air-to-fuel ratio of 1.05, the new λ will be 0.94, resulting in a poor fuel efficiency and high amounts of CO in the exhaust. For gas turbines, such large changes in Wobbe index can induce flame instability in the combustors and overheating of the turbine intake. Small changes to the amount of hydrogen – up to, say 3 per cent by volume – have no deteriorating effect on combustion engines, larger amounts drastically reduce the knock resistance of the fuel. If the makeup of the reference gas mentioned earlier is changed so that a mixture with 25 per cent hydrogen results, the so-called methane number7, decreases by some 10 points.
The methane number is a measure of the knock resistance. Pure methane has a methane number of 100. Hydrogen has a poor knock resistance and is therefore given a methane number of 0. The volumetric percentage of methane in a hydrogen-methane blend gives the methane number for that blend. Consequently, a mixture of 75 per cent methane and 25 per cent hydrogen has a methane number of 75.
Engines give the best performance when the methane number is 80-plus, so good-quality gas is needed for engines. In The Netherlands alone, some 10 per cent of the natural gas delivered is applied in engine-driven cogeneration installations. So the role of engines in the world will increase considerably because of the need for flexible and fast backup generators to compensate for the intermittency of renewables.
When it comes to the safety of combustion processes, many such systems require the purging with air of combustion chambers, boiler runs and exhausts before the actual combustion begins. With hydrogen in the gas, the lower explosion limit of mixtures of gas and air will widen, so much more purging is needed. The high combustion velocity of hydrogen compared with natural gas increases the risks of high-pressure explosions in confinements. On top of this, hydrogen is a small molecule and therefore leaks more easily through valves and gaskets.
The Wobbe Index range for Europe as proposed by gas suppliers via EASEE-gas is already too wide for most gas applications. In the USA, close co-operation between consumers, the equipment industry, gas suppliers and policy makers has resulted in a much more acceptable gas quality range8. In Europe, ENTSO-E, the umbrella organisation of electricity transmission system operators, is developing new rules called the Network Code on Requirements for Generators. This aims to keep the European electricity grid stable and reliable when intermittent renewables increase substantially as energy sources. Fluctuating gas compositions and low methane numbers make it very challenging for fast-reacting generators to meet the new dynamic output demands. Surprisingly, ENTSO-G, the sister organization of ENTSO-E for gas, seems not to be focused on good gas quality for electricity generators.
Figure 5 shows that adding hydrogen to natural gas has a much greater negative effect on the calorific value of gas than on the Wobbe index. Having just 10 per cent of hydrogen in the reference gas decreases the calorific value from 39.54 MJ/m3 to 36.65 MJ/m3. The customer receives some 7 per cent less energy for the same volume of gas as measured by a gas meter. The intermittent nature of hydrogen addition and the inertia in the pipeline system would mean the customer would never know what kind of gas they were receiving. Customers are supposed to be charged for the amount of energy consumed, not for volume.
There is no rational argument for special investment in systems that cater for the occasional peaks in the output from wind turbines and PV systems. Investments for harvesting such peaks will never be economical,and they will only waste precious capital. In fact, it is irrational to convert such peaks into hydrogen that is then injected into natural gas so that the mixture is later used as fuel and converted back into electricity. The efficiency of this power-hydrogen-power cycle is far too low to be economical. Injecting hydrogen into natural gas streams will deteriorate gas quality and result in the decreased performance of many gas applications, and increase the risk of damage to equipment. Moreover, a serious problem occurs with hydrogen when measuring the amount of energy delivered to customers. Instead of spending precious money on research and investment in the hydrogen-based cycle, it would be better to spend it by investing in renewable energy sources and on more promising energy storage systems and backup power systems.
1. Peter Hardy, Putting hydrogen on the low-carbon energy map, Power Engineering International, November 2012
3. marketsandmarkets.com, Hydrogen Generation Market – by Merchant & Captive Type, Distributed & Centralized Generation, Application & Technology – Trends & Global Forecasts (2011-2016), report code: EP 1708, December 2011
4. Kaveh Mazloomi, Nasri B. Sulaiman, Hossein Moayedi, Electrical Efficiency of Electrolytic Hydrogen Production, Int J Electrochem. Sci, 7 (2012), 3314-3326.
6. Geir Kvæl, The Harmonisation of Gas Quality in Europe, FLAME, Amsterdam, 23 February 2005.
7. Jacob Klimstra, Angel Benito Harnáez, Wim H Bouwman, Antoine Gerard, Bent Karll, Vittorio Quinto, Graham R Roberts and Hans-Jürgen Schollmeyer, Classification Methods for the Knock Resistance of Gaseous Fuels – An Attempt towards Unification, paper no. 99-ICE-214, ICE-Vol. 33-1, 1999, Fall Technical Conference, Ann Arbor, MI, USA, 17-20 October 1999.
8. NGC+ Interchangeability Work Group, White Paper on Natural Gas Interchangeability and Non-Combustion End Use, AGA/FERC, 28 February 2005, www.beg.utexas.edu/energyecon/lng/documents/NGC_Interchangeability_Paper.pdf
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