The transition to a renewable-based power and fuels system is challenging. Creating a production system that makes the most of solar and wind energy, as well as biomass and waste, and being responsible in the use of water, requires organized a systematic evaluation of the technologies, their alternatives, and their limitations.
Over the last 30 year, a number of technologies have been developed for the production of biofuels and renewable power. Among the biomass processing technologies, first-generation biofuels are being avoided due to the food vs fuel competition. Thus, the most common processes use lignocellulosic biomass to produce bioethanol via biochemical, or else thermal routes. Furthermore, synthetic fuels and intermediate chemicals such as methanol or hydrogen can also be produced following thermal routes from biomass.
On the other hand, solar and wind have increased their share in the energy mix. For solar, concentrated solar power technologies (CSP) or photovoltaics (PV) are the most commonly used to transform sunlight into power. Wind is used in wind turbines. However, the mismatch between supply of renewable electricity and power demand gives rise to challenges for deployment of novel technologies.
The possibility of storing that energy in the form of chemicals or fuels such as methane and methanol is an interesting alternative due to the current limitations in the capacity of batteries. In these processes, hydrogen is produced from renewable power via electrolysis and CO2 is captured and reused for synthesis. Furthermore, solar energy can be used to grow algae for biodiesel production. However, for it to be renewable, methanol, the alcohol needed, must also be obtained from sustainable resources. Both, electrolytic or biomass-based methanol can be used. Finally, hydropower is another technology capable of storing the excess of power by pumping up water to a dam for further use. The operation of such a system of technologies also requires power and/or thermal energy that must be produced within the system.
So far, each of the above processes has been studied separately. In particular, biofuels and power technologies have been studied by different communities. While electrical engineers are familiar with power production, biofuels have been in the domain of chemical and biological engineers. As a result, the development of power and chemicals supply networks have been evaluated as two different entities, and the challenges in handling power have not been properly addressed. The possibility of storing power in the form of chemicals, just as nature does in the form of biomass, can be of great help. Alternatively, an excess of power can be used to pump water. Based on these principles and process integration, different resources have been integrated lately in hybrid processes so as to mitigate the variability of solar or wind energy as a first stage to use a second resource to help satisfy demand.
The idea behind this work is to take a holistic view. We aim not at integrating one or two technologies, but rather to determine the best integration of renewable-based technologies to meet power and fuels demand in a region or an entire country. Mathematical optimization techniques allow for researchers to systematically select the technologies that transform the natural resources of a particular allocation into power and fuels. This approach can be applied at different scales: from a residential area, an industrial complex, a county, a state or to an entire country.
Economic, environmental, and social issues can be used as decision-making criteria for the selection of the resources to be used and the technologies to transform them. Although we have used a series of examples for provinces, regions, and the peninsular area of Spain, the proposed modeling framework is flexible and general so that it can be applied to other cases. Additional technologies can also be evaluated, as well as the effect of future development of the technologies in terms of their costs
Interesting results that were obtained are related to the efficient use of the resources. If we need to meet the demand for power and fuels of a particular region, the selection of the transformation technologies depends on the local availability and cost. However, the efficiency of the use of the resources may not be the best. For instance, if the wind velocity is not high enough but there is no other renewable source, this resource will still be used. However, power production cost will be high. The selection of the technologies to use the natural resources changes when a wider area is included in the analysis.
A more efficient solution is achieved when larger regions are considered so that resources of each region are exploited more effectively and exchanged among them. Thus, an effort towards integration is positive for the environment. Another interesting fact is the additional cost related to the uncertainty in the availability of solar and wind. To ensure that the demand is met, a larger investment is required since we need to have backup technologies in case the availability of resources is below what is expected. Finally, another advantage of the study is its capability to evaluate policies, providing a more realistic result where social issues are taken into account, including those related to developing more effectively the different regions.
These findings are described in the article entitled Optimal integration of renewable based processes for fuels and power production: Spain case study, recently published in the journal Applied Energy. This work was conducted by Mariano Martín from the University of Salamanca and Ignacio E. Grossmann from Carnegie Mellon University.