Biomass-based energy – both for the replacement of traditional liquid fuels and for the generation of renewable electricity – is a vital component of any plausible strategy to reduce and reverse the effects of global warming.
Despite its promise, the realistic potential of bioenergy is limited if it is derived from traditional terrestrial crops, such as corn or palm oil or switchgrass. Any crops that compete with food production for access to arable land and to freshwater for irrigation can never be responsibly deployed on the very large scales necessary for energy applications.
The Sahara Forest Project provides the opportunity to cultivate crops on desert lands that are naturally unproductive for food, using only seawater and its derivatives. Because desalinated water can be used within the Sahara Forest Project to grow valuable food and fodder, the most interesting sources of biomass for energy purposes are those that can be grown in or irrigated with saltwater, those that can grow in soils too saline for food crops, and those native desert species that are high in energy content that can thrive in the humidified intra-hedge growing spaces with little or no irrigation. These are the algae, the halophytes, and some particular desert species that are suspected of having a high energy density. The best species for bioenergy cultivation will be characterized in field trials at SFP facilities, and assessed by their biomass production efficiency, freshwater requirements, energy density, and ability to provide other ecosystem services.
In addition, the organic wastes of the horticultural and agricultural operations at the Sahara Forest Project will be available as additional bioenergy feedstock.
Plant materials contain not only carbon-based energy but also, as might be expected, many nutrients vital for plant nutrition. It is highly preferred to utilize biomass energy conversion processes that leave significant fractions of these nutrients behind in a form usable for soil enrichment and conditioning within the Sahara Forest Project’s growing areas. In this way the energy resources of plants are harvested while many of their nutrients are returned to the soil to feed the next generation. Without this closing of the loop, bioenergy generation can result in a net export of nutrients from the soil. Two technologies are available to extract energy from biomass while preserving some part of its nutritional value. One is anaerobic digestion (AD), in which methane-producing (methanogenic) bacteria digest biomass in an oxygen-poor environment, generating biogas and a nutrient-rich digestate sludge. This biogas typically requires cleaning before it can be burned in a turbine to make electricity, and needs further refinement to almost pure methane in order to be sold as natural gas. The second method is pyrolysis, in which biomass is burned in an oxygen-poor environment to generate bio-oil, syngas and carbon rich biochar. The syngas can be refined to hydrogen or converted to methane, and the biochar is a rich soil conditioner that also sequesters carbon in the soil.
AD systems are a relatively mature technology deployed many times over on municipal scales. The biological basis of AD systems – bacteria – makes them more flexible and robust to wet or alternative feedstock, such as food, livestock, or municipal waste. However, this biological basis can also create unpredictability in performance upon the addition of different feedstock mixes, and can occasionally lead to unwanted bacterial activity in the nutrient-rich digestate.
Pyrolysis is an ancient technology, practiced in simple forms for millennia to make charcoal. However, capturing the gas produced in the pyrolysis process is a more complex undertaking, and the industrial application of the technology for energy generation is not yet robust. While pyrolysis can accept a wide range of plant material feedstock, ranging from stems to wood to shells, it becomes energetically unfavorable if the water content of the feedstock exceeds 60%. Balancing these challenges is the fact that biochar is an extraordinary soil conditioner, particularly for desert soils. It improves soil structure, water retention, and nitrogen-use efficiency, while providing nutrients such as phosphorous, potassium, and calcium, and is much longer-lived than uncharred organic residue.
Biogas utilization – CSP Hybridization
CSP plants are now frequently built in a hybrid configuration with natural gas power plants. The synergy between the technologies is very good: the same power block can be shared by both systems, with the steam turbine driven by the heat of concentrated solar energy during daytime hours and by the combustion of natural gas during the night or cloudy periods. Because natural gas-fired plants can begin or increase electricity production at very short notice compared to other power plants, they serve as an excellent back-up and complement to intermittent solar resources, able to cover unexpected cloudy periods and to boost output at peak times in demand. However, in general, this gas + solar configuration does not provide the long-term environmental and resource sustainability of a purely renewable solution. If, however, the methane feeding the gas turbine is generated from biogas, the whole system becomes entirely renewable, while reaping the operational benefits of a hybrid system. These benefits – 24-hour availability and dispatchability – will become very important, and have significant commercial value, when renewables shift from their current fringe position to form a major part of the power infrastructure. In effect, such a system captures the sun’s energy in two ways – with mirrors and in plants – and utilizes the natural storage capabilities of plants to transfer some of that energy into sunless periods to drive 24-hour power generation. Moreover, the hybrid system eliminates transport costs and closes the resource loop, extracting the biomass’s energy on site, keeping the nutrients at the SFP facility to foster future plant generations, and exporting nothing but electrons. Moreover, combusting the gas on-site will provide additional CO2 for use in the greenhouses and the algae ponds, recycling the already carbon neutral emissions into additional food and biomass. However, because the energy density of biomass is quite low, this hybridization will only make sense at large scales for which biomass production is very high, or in situations when a significant additional bioenergy feedstock is available, such as municipal or hotel waste streams.
At smaller scales where hybridization does not make sense, the methane produced from biomass can be put to valuable use elsewhere in the SFP facility by providing heating to the greenhouses during cold or humid winter nights. For most of the year, the heating requirement is easily met using the waste heat from the CSP. However, there are some crops that cause significant seasonal spikes in heat demand, which can be better met using methane.