Microalgae and Photolysis as New Energy Technologies: Comparison
Please note this is a comparison between Version 1 by Patrick Moriarty and Version 2 by Peter Tang.

Because of the near-term risk of extreme weather events and other adverse consequences from climate change and, at least in the longer term, global fossil fuel depletion, there is worldwide interest in shifting to noncarbon energy sources, especially renewable energy (RE). Because of possible limitations on conventional renewable energy sources, researchers have looked for ways of overcoming these shortcomings by introducing radically new energy technologies. The largest RE source is bioenergy, while solar energy and wind energy are regarded as having by far the largest technical potential.

  • climate change
  • EROI
  • microalgae
  • photolysis
  • renewable energy

1. Introduction

Because of the near-term risk of extreme weather events and other adverse consequences from climate change and, in the longer term at least, global fossil fuel depletion, there is worldwide interest in shifting to noncarbon energy sources, especially renewable energy (RE). Although the ability of RE to provide for expected future global energy levels is hotly debated, some researchers have argued that RE cannot take over from fossil fuels (FF) [1][2][3][4][5][6]. They have variously argued that land constraints will limit output and that several existing forms of RE, especially bioenergy and hydro, have their own serious environmental problems. Further, the intermittent nature of the output of the two most promising sources, wind and solar, will necessitate energy conversion and storage [7][8]. Not only will costs rise because of the extra equipment needed, but energy losses at each stage (for example, converting intermittent electricity from wind turbines into hydrogen via electrolysis for storage, and perhaps later followed by reconversion into electricity) will significantly lower the system energy return on energy invested (EROI).
Researching new alternatives to conventional RE is, therefore, worthwhile, and this entry examines several cutting-edge RE technologies. In any case, as an eponymous 2019 article in New Scientist [9] documents, “The renewables revolution is stalling”. Furthermore, CO2 emissions from fossil fuel (FF) combustion, far from falling, are still growing [10]. Partly because of this disappointing growth in RE, a large number of even more ambitious schemes to counter global warming have been proposed. These include various carbon dioxide (CO2) removal schemes, either biological (reforestation, etc.) or mechanical (CO2 removal from fossil fuel stacks, or even directly from the air, followed by sequestration). Geoengineering proposals would aim to directly increase the planetary or regional albedo—the proportion of insolation directly reflected back into space—with solar radiation management the most popular proposal [8]

2. Microalgae for Fuels

Although, similar to fossil fuels, conventional bioenergy can be readily stored and so is available on a continuous basis, some researchers doubt that conventional bioenergy can be a major RE source in the future because of competition for fertile land and water from agricultural production needed for food (e.g., [11]). At present, all liquid biofuels are derived from food crops, even though global basic nutritional needs are not being met even today. Some researchers have argued that microalgal energy production can overcome this problem, since microalgae can utilize land not suited for agriculture and can use brackish water or even wastewater in need of purification [12][13]. Davis et al. [14] have even discussed Saudi Arabia as a suitable location for large-scale microalgae cultivation. Further, microalgae can be selected to produce high percentages of oil, which can be directly used in vehicles in a manner similar to biodiesel. Microalgae, along with bacteria, can also be selected or genetically engineered to produce hydrogen [15][16][17][18], possibly a key future fuel. Microalgae are very simple organisms, and 50,000 species have so far been described from the hundreds of thousands that are estimated to exist. These single-celled organisms, together with bacteria, form the base of the food web. Unlike higher plants such as trees, shrubs or grasses, microalgae do not need any roots, supporting stems or leaves. The whole of the plant can thus be harvested, unlike land-based bioenergy crops, where roots and often stover from cereal crops must be left in place to maintain soil fertility and prevent soil erosion. Ketzer et al. [18] recently reviewed the EROI for energy production from microalgae in both shallow open ponds and closed reactors. The energy inputs were mainly for cultivation and downstream processing, with a lesser input for harvesting. The studies reviewed showed very low EROI for production from closed reactors—all six studies had EROI values much less than 1.0, with several below 0.1. For open ponds, the values were higher, but only in three of the 17 studies reviewed did the EROI exceed 1.0. However, the high EROI value of Campbell et al. [19] was mainly the result of a very high estimate of annual microalgal yield of 109.6 ton per hectare. Larkum [20] and Walker [21] are both very skeptical of such high yield values, with Larkum pointing out that the natural primary production of microalgae (phytoplankton) in lakes and oceans is “very low”. Reasons for such low natural production include a low level of natural stirring (from wind or ocean currents) and a low level of available nutrients. The leaves of higher plants are very efficient for CO2 diffusion from air, but such is not the case for CO2 diffusion in a water medium. Energy intensive mechanical stirring is thus needed for both CO2 and nutrients to be available to microalgae. However, if ponds were located next to fossil fuel plants, flue gas CO2 could be used for CO2 fertilization. The unit costs for microalgal biomass are presently very high, mainly because the production is chiefly for specialty products produced in bioreactors. Fernandez et al. [22] have reported costs “ranging from minimum values of 5 €/kg for raceway reactors to 50 €/kg for tubular photobioreactors”. (The equivalent US costs are about USD 5.65 and USD 56.5 per kg, respectively.) Roles et al. [23], in a detailed cost analysis of biodiesel in an Australian context, found that estimated prices would be several times current prices for diesel but could be reduced to current levels given (optimistic) improvements. Although shallow ponds have much lower costs [24] and higher EROI than closed reactors, they have two significant disadvantages. Shallow ponds inevitably suffer from evaporation losses, a problem if water is scarce [25]. Deeper ponds such as used in raceways (typically around 0.3 m deep) would suffer smaller annual losses as a proportion of total water. Regardless of configuration, the need of sunlight for photosynthesis limits depth. While showing much larger production rates than ponds, bioreactors are similarly limited but in reactor solar tube diameter, with 0.1 m being a typical value [26]. Because of these restrictions, microalgal cultivation on a grand scale would still have high land requirements. Relative to land-based crops, energy production per unit land area from microalgae is greater, but it remains lower relative to other land use options such as PV. On an energy equivalent basis, depending on algae type and whether production occurs in a pond or bioreactor [26], in areas with moderate to high average insolation (20 MJ/m2/day), 2–6% of the available solar energy could be expected to be returned as energy in the liquid fuel produced. If used as a transport fuel, the low efficiency of combustion engines would result in an even lower return on the insolation via the liquid fuel relative to use of the insolation via PVs to power electric vehicles. Many of the papers published on microalgae for fuels stress its potential role in directly providing biodiesel for transport, without the conversion needed for bioethanol from terrestrial plants. However, many cities and countries have plans to banish internal combustion engine vehicles, some as early as 2030 [27]. In this case, liquid algae-based fuels, which cannot be expected to be produced in quantity for at least a decade, would have no future, even as a stopgap fuel. Nevertheless, algae-based hydrogen and air transport fuels would remain possibilities. There is a further problem: similar to all plant matter, microalgae need nutrients for growth. Reijnders [28] has pointed to the key fertilizer, phosphorus, being a limiting factor in future. A 2010 New Scientist article [24] acknowledged the much higher cost of bioreactor-produced fuels, but for shallow pond cultivation presented a graph comparing the cost of conventional diesel and microalgal biodiesel. In 2009, biodiesel was estimated as 10 times more expensive, but by 2017, microalgal biodiesel was projected to be cheaper than conventional diesel. Similarly, Wesoff [29] has detailed the rush to microalgae by venture capitalists between 2005 and 2012. He discussed some of the optimistic projections from that time as follows: “Jim Lane of Biofuels Digest authored what was possibly history’s least accurate market forecast, projecting that algal biofuel capacity would reach 1 billion gallons by 2014. In 2009, Solazyme promised competitively priced fuel from algae by 2012. Algenol planned to make 100 million gallons of ethanol annually in Mexico’s Sonoran Desert by the end of 2009 and 1 billion gallons by the end of 2012 at a production rate of 10,000 gallons per acre.” As with the low-cost forecast in New Scientist, these forecasts had no basis in reality, and the companies still involved are (again) concentrating on high-cost pharmaceuticals and food additives. Although inevitable disillusionment has set in, the hype cycle may yet repeat itself, as has happened with other technologies such as electric vehicles and hydrogen as a fuel. Notwithstanding the exit of (most) venture capital, research has continued seemingly unabated, as evidenced by the thousands of articles published since 2012.

3. Photolysis

The potential for solar energy appears enormous, since the quantity of annual solar radiation intercepted at the top of Earth’s atmosphere is four orders of magnitude larger than present global primary energy consumption [30]. (Nevertheless, de Castro et al. [2][3] argued that the technical potential for solar electricity is far less than usually estimated.) As is also a problem for wind power, solar energy is intermittent, though more predictable than wind. Particularly in regions with pronounced winters, solar energy output will be very low for several months each year and zero at night for all regions [5]. All the fossil fuels we use, as well as bioenergy, are ultimately products of photosynthesis. Nicola Jones [31] defined this process as follows: “Photosynthesis is all about using the sun’s energy to split water into its constituents, hydrogen and oxygen, and rearranging them into chemically more energetic molecules—in the case of plants, carbohydrates made with the help of atmospheric carbon dioxide”. With bioenergy, intermittency is not a problem, as the energy can be naturally stored as standing biomass or harvested and stored such as coal. Therefore, although plant photosynthesis for land-based crops averages only about one percent efficiency compared with 15–20% for commercially available PV cells, plants store their energy in the form of chemical bonds—fuel—rather than electrons [32]. The problem with natural plant photosynthesis is not only its low efficiency, but also that the products contain carbon as well as hydrogen; when combusted, they produce climate-changing CO2 as well as water. Although the CO2 produced will be recycled during the next growth, net reduction in CO2 from the atmosphere requires the carbon to be permanently stored in the biomass. The aim now is to employ artificial photosynthesis to produce a carbon-free fuel—hydrogen—leaving biomass to act as a carbon store, rather than a recycler of carbon. One possible solution to this intermittency problem is to use excess solar electricity (or other intermittent RE electricity surplus to grid needs) to power water hydrolysis to produce hydrogen. This hydrogen can then be compressed and stored or used directly in hydrogen fuel cells for stationary or vehicular power, or even mixed at low concentrations and fed into existing reticulated natural gas systems [27]. Although coupling such excess electricity to electrolysers for hydrogen production, then using the hydrogen directly as a fuel has greater overall efficiency than re-conversion of the stored hydrogen back to electricity (50–60% compared with 27–38% for the full cycle) [33], the overall efficiency could still be improved. (Of course, if overall efficiency is measured as the ratio of H2 output to solar energy input, the resulting figure is much lower.) A variant of this is to have the PV array dedicated to the electrolyser, which enables both optimization of the PV array for electrolysis and possible use of direct current, but in this case, hydrogen transport is needed. In either case, the arrangement is known as photovoltaic-driven electrolysis, and, in terms of components, at least, is a mature technology. Liquid electrolyte alkaline electrolysers have now been available commercially for over a century [34]. Photolysis (also called photoelectrolysis or photo-electrochemistry) is seen by many researchers (e.g., [31][34][35]) as a means of improving this efficiency, by integrating the electricity production and electrolysis steps. Nikolaidis and Poullikkas [36] have offered a simple description: “Photolysis, in general, is effected when the energy of visible light is absorbed with the help of some photo-catalysts and is then utilized to decompose water into H2 and O2 [...]. In photo-electrolysis, the sunlight is absorbed through some semiconducting materials and the process of water splitting is similar to electrolysis.” Dincer and Acar [37] reported a recent production cost estimate for photolysis hydrogen of 10.4 USD/kg, far higher than for any other H2 production method. (Nevertheless, Singh et al. [17] in the same year presented H2 production costs much higher than USD10.4/kg for nearly all production methods, including the use of conventional PV arrays and electrolysers. It appears that no H2 costs can be taken as given). An important question is whether integrating PV cells and electrolysis is worth doing. Its obvious competitors have already been mentioned: PV arrays dedicated to physically separate electrolysers, or electrolysers run off mixed RE electricity sources. Can PV-electrolyser integration both increase large-scale system EROI and lower costs compared with these alternatives? At the present early stage of development, these questions are difficult to answer. Land needs should be similar to those for conventional PV farms. The problem is in choosing the right semiconductor materials, and large numbers have been tried. For direct water dissociation to occur, the semiconductor material has to simultaneously meet a number of exacting chemical criteria [35]. This may explain, why, even after four decades of research, water photolysis can still be regarded as being in the early stages, particularly with regard to the ultimate aim of large scale H2 production. For large-scale commercial application, of course, the materials must be readily available at low cost, and the optimal tradeoff between efficiency and cost determined. A further factor acting to limit large-scale development is that while traditional electrolysis systems can operate under low light conditions and even overnight if coupled to a RE supplied grid or to a standalone RE system such as concentrating solar power with heat storage, photoelectrolysis systems can only operate when there is sufficient light. As mentioned, an important advantage of being able to physically separate the production of electricity from the electrolysis of water is that the hydrogen can be produced on site (for example, at a truck freight depot). The electricity transmission and distribution networks are vast, but the existing pipeline network for H2 is limited. Further, there is then no limit on the size of the electrolyser plants; they are presently commercially available in sizes ranging from a fraction of an MW size up to multi-MW size. If grid electricity is used, the electrolyser plants can also be run continuously.

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