Chapter 1
Introduction

Chapter Overview

1. World Population Growth, Energy Demand and its Future
2. World Energy Future
3. Introduction to Fuel Cells and Associated Terms
4. Gas Turbines
5. Coupling of Microturbines with Fuel Cells to Obtain 'Hybrid Systems'
6. Conclusions

The current and future energy scenarios faced by the international community are discussed in this chapter, starting with a brief presentation of the energy landscape and related issues, including the increase in demand and environmental aspects. A list of possible solutions to existing and foreseen problems is presented and discussed, setting the framework to highlight the significant potential of fuel cells for future power generation. Following on from this, the performance characteristics of fuel cells are introduced, including an analysis of their different types and corresponding differential features. Additionally, attention is devoted to hybrid systems based on the coupling of high-temperature fuel cells and microturbines (mGTs).

1.1 World Population Growth, Energy Demand and its Future

A study carried out by the United States Census Bureau (USCB) [1] estimated that the world population exceeded 7 billion on 12 March 2012. Now, at the time of writing in August 2016 with the global population standing at about 7.4 billion [2], this figure is expected to continue rising over the coming decades [2]. As the world population grows, in many countries faster than the global average of 2%, the need for more and more energy is intensifying in a somewhat similar proportion, thus putting pressure on the natural resources available and existing infrastructures. This higher energy consumption is not only due to the growth in world population, but also to the improved lifestyles leading to a greater energy demand per capita (two features that inevitably come together). This is best exemplified by the fact that the wealthy industrialized economies comprise 25% of the world's population but consume 75% of the world's energy supply [3]. A recent study (from ref. [4]) shows that the total world consumption of marketed energy is expected to increase from 549 quadrillion British thermal units (Btu) in 2012 to 629 quadrillion Btu in 2020, and to 815 quadrillion Btu in 2040 - a 48% increase from 2012 to 2040 [4].

Indeed, the landscape of future energy demand and generation projected for the world seems rather bleak, as most nations, including the most developed ones, depend primarily on conventional energy sources such as oil, coal and gas to generate power not only for the domestic and industrial sectors but also for transportation. This dependency results in global warming, contributes to rises in fuel prices that constitute a burden on economies, and can lead to delays in energy production and supply [5, 6]. Furthermore, even if the global production of fossil fuels is currently sufficient to cover the world's needs, the exponential rise in the exploitation rate of this finite, fast-depleting resource would pose a risk to the future energy demand and generation balance [7-9]. In the long run this global dependence on conventional fuel sources for power production will prove problematic because the world will eventually fall short or run out of these resources. Renewable energy sources are often set forth as a feasible alternative to this fossil-fuel dominated world [10], although many of their inherent features, such as their low energy density, intermittency and geographical distribution, pose a number of challenges that remain to be solved today.

1.2 World Energy Future

Due to the heavy reliance of most nations worldwide on fossil fuels for power generation and transportation, the atmospheric concentrations of carbon dioxide and methane have increased by 36% and 148% respectively, compared with pre-industrial levels [11]. These levels are indeed much higher than at any time during the last 800,000 years, the period for which reliable data have been extracted from ice cores. This observation is further confirmed by less direct geological observations that also show that carbon dioxide concentrations higher than today were last seen about 20 million years ago. These findings suggest that the root cause for such high concentrations is anthropogenic, mainly hydrocarbon-based fuel burning (responsible for three-quarters of the increase in CO2 from human activity over the past 20 years) and deforestation [11]. Other environmental factors, including air pollution, acid precipitation, ozone depletion and emission of radioactive substances, are also of concern and raise awareness of the negative impact of human activity on the environment [3].

As a consequence of this massive production of anthropogenic carbon dioxide and other greenhouse gases (trace gases in particular [12]), global temperatures in 2016 were 0.87°C above the long-term 1880-2000 average (the 1880-2000 annually averaged combined land and ocean temperature is 13.9°C), which translates into a warming rate of around 0.61°C/century over the last few decades. In particular, the average temperature of the Atlantic, Pacific and Indian oceans (covering 72% of the Earth's surface) has risen by 0.06°C since 1995. The situation regarding global warming is far from being under control. As stated by the US Department of Energy's forecast, carbon emissions will increase by 54% above 1990 levels by 2015, making the Earth likely to warm by 1.7-4.9°C over the period 1990-2100 (see Figure 1.1). Such observations demonstrate the need for efforts towards alleviating energy-related environmental concerns in the near future [3].

Figure 1.1 Global mean temperature probability changes, for the years 1990-2100 and 1990-2030.

Source: Omer (2008) [3]. Reproduced with permission of Elsevier.

Achieving higher efficiencies and, if possible, the utilization of renewable energies in power generation technologies will be vital steps in mitigating or reducing these environmental problems, whilst meeting the expected rise in energy demand in the future. With increasing fuel prices and significant pressure to reduce emissions, increasing energy efficiency is considered amongst the most feasible and cost-competitive approaches for reducing CO2 emissions. For instance, Britain wastes 20% of its fossil fuel and electricity which, if used efficiently, would translate into a potential £10 billion annual reduction in the collective fuel bill and a reduction of some 120 million tonnes of CO2 emissions [3]. Unfortunately, even if energy is currently recognized globally as being at the centre of the sustainable development paradigm, the industrial and social development paths favour energy consumption rather than conservation [3].

The significant fuel consumption and CO2 emission issues have to link with the fact that conventional thermal power plants (regardless of the type of fuel used) cannot convert all of the thermal energy supply into useful (mechanical) work. In most cases, more than 50% of the heat added to the working cycle is rejected to the environment. Combined heat and power (CHP) installations are able to use a part of this heat, which would otherwise be wasted in a conventional power plant, to raise the overall first law efficiency to values higher than 80% for the best available technology [3]. This concept enables drastic reductions of the primary energy consumption and cost compared with the independent production of both forms of energy (electricity and thermal energy).

Complementary to energy conversion at high efficiency, substituting fossil fuels with renewable energy sources is envisaged as another means to tackle the aforecited social, economic and environmental problems. Renewable energies are broadly regarded as energy sources that are naturally replenished over a short timescale (i.e. in comparison to the lifetime of a human being), such as sunlight, wind, rain, tides, waves and geothermal heat. They have shown the potential to replace conventional fuels in various distinct areas, such as utility-scale electricity generation, hot water production/space heating, fuels for transportation, and rural (off-grid) energy services [13, 14]. Renewable energy sources have the potential to constitute the future energy sector's backbone, despite some evident shortcomings such as low density and inherent intermittency.

According to the REN21's 2014 report [15], renewables contributed 19% to the world's energy consumption in 2012, and 22% to electricity generation in 2013, using both traditional (biomass) and more innovative renewable energy technologies such as solar power, large wind farms and biofuels [10]. The importance of renewable energy sources has been disseminated widely, and several nations worldwide have decided to invest large sums of money in renewable technologies; such is the case in the US with a total investment of more than $214 billion in 2013, whereas other countries like China are following close behind [15].

Hybrid systems based on the coupling of a microturbine with a high-temperature fuel cell are highly regarded as a solution for future power generation due to their high efficiency, ultra-low emissions and their ability to run on fuels such as hydrogen produced from renewable sources. These systems can achieve very high efficiencies: more than 60% electrical efficiency using natural gas (depending on the low heating value). This efficiency is virtually independent of plant size due to...