Elsevier

Energy

Volume 108, 1 August 2016, Pages 155-161
Energy

Comparisons between energy carriers' productions for exploiting renewable energy sources

https://doi.org/10.1016/j.energy.2015.07.033Get rights and content

Highlights

  • Hydrogen from renewable energy can be used for the synthesis of chemicals.

  • We examined the productions of three energy carriers NH3, CH4, and methanol.

  • We assumed two different carbon sources CO2 and torrefied biomass.

  • Ammonia production outperforms all other scenarios in terms of economic efficiency.

Abstract

This paper provides comparisons between three different energy carriers that could be produced within areas having cheap electricity. The comparisons were made in terms of the technological efficiencies and economical viabilities for five different scenarios. Economical comparisons were based on IRR (internal rate of return) calculations and sensitivity analyses covering different independent variables such as carbon tax, electricity price and capital costs. It was discovered that SNG (synthetic natural gas) and LNG (liquid natural gas) production is economically uncompetitive compared to ammonia and methanol productions. Ammonia production would be the better choice if there were a carbon tax between 0 and 83EUR/t of CO2. At a carbon tax higher than 83EUR/t, methanol production would be the more economical option.

Introduction

Ever increasing energy demands and the commitments to reduce CO2 emissions have brought renewable energy sources to the fore [1]. The EU is committed to reducing its emissions by 20% during the Kyoto Protocol's second period. In order to achieve this goal the intermittent renewable energy sources (wind and solar energy) must be stored and transported as efficiently as possible. In addition, one of the more important issues is also transferring the energy from places having lower prices for electricity such as for example Iceland (geothermal and hydro energy). In the future it is expected that places with relatively high sun radiation (the Middle East, and North Africa) will also need technologies for converting the electricity to useful products.

Several projects with the intentions of exploiting solar energy using CPV systems (concentrated photovoltaic) or CSP systems (concentrated solar power) have already been planned or are under development [2] in areas with high solar irradiance. This includes Africa, Southern Europe, Asia, Australia, most states in the Southwest of the U.S., and the Middle Eastern countries. Another promising technology is solar updraft power technology [3], [4], [5]. SUPPs (solar updraft power plants) have the potential of generating highly sustainable energy in worldwide deserts [4] and several are at the predesign stage in countries throughout the world [4]. There are several advantages of SUPPs compared to CSP and CPV systems, such are [3]:

  • The technology is simpler and readily available.

  • The solar collector absorbs direct and diffused solar radiation.

  • Easier energy storage by placing a layer of thermal storage material such as a closed water-filled system on the ground under the collector's roof.

  • Low maintenance and operating costs and high reliability, as additional fossil fuels are unnecessary due to the reliable operation of SUPP day and night. In addition, cooling-water is not required during the operation.

  • The designed service lives of commercial SUPPs are intended to be between 80 and 120 years.

The disadvantages of this technology can be summarised as follows:

  • High investment costs.

  • Earthquakes could destroy high towers.

  • Environmental concerns may arise.

The cost of producing electricity using CSP or large PV (photovoltaic) plants lie close to 200EUR/MWh, and 180EUR/MWh, respectively, whereas 80EUR/MWh seems possible for SUPPs when considering the scaling effects of plant sizes [5]. Furthermore, after an amortisation period of around 30 years the cost of producing electricity using SUPPs would be expected to decrease to less than 20EUR/MWh [5].

Considering the above facts, there is vast potential for solar power generation in places with high solar insolation in northern Africa and the Middle East countries. Some of this energy could also be exported to Europe because of relatively higher sun radiation compared to Europe [2].

On the other hand, besides solar energy, geothermal energy is one of the more important renewable energy sources. In addition geothermal power generation is economically very favourable. The LCOE (levelled costs of electricity that represent the minimum costs that must be charged over time in order to pay for the total cost of power production) for geothermal power in Iceland amount to around 33.5EUR/MWh [6], which is very close to hydro power production 33.4EUR/MWh [6]. Iceland has extensive reserves of renewable energy potentials. The geothermal energy potential for electricity generation has been estimated to be 35 TWh/yr, and the hydro power potential in the country has been valued at 30 TWh/yr [7]. This potential is not fully utilised, as only 12,657 GWh/yr, and 5,234 GWh/yr of electricity is produced from hydro power plants, and geothermal plants, respectively.

Besides electric cars, the synthetic fuels such as methanol, DME (dimethyl ether), LNG (liquid natural gas) could be one of the solutions [8], [9] for storing renewable energy meant for the transport sector in the future, especially for aircraft, ships and trucks [8] where the need for liquid fuels is inevitable [10]. On the other hand there is also a vast potential for replacing fossil fuel based chemicals production [8] such as for example ammonia. In this paper the focus is placed on the production of synthetic fuels and chemicals using hydrogen produced from an electrolysis unit that employs renewable electricity. The source of carbon for the synthesis could be biomass or CO2 extracted from existing power plants.

There are several publications relating to this topic. Ridjan [10] et al. identified potential pathways for producing synthetic fuels with a focus on co-electrolysis of CO2 and H2O within a SOEC (solid oxide electrolyser cells). Pozzo et al. [11] proposed a novel process design for producing DME using a wood biomass gasifier and a high-temperature co-electrolysis unit. Varone and Ferrari [8] presented different scenarios for the production of gaseous and liquid synthetic fuels for a case of Germany. Stempien et al. [12] analysed the production of methane gas from water and carbon dioxide feed using renewable electricity. Nzihou et al. [13] presented a review of synthetic fuel production from biomass in combination with concentrated solar energy. Johansson [14] implemented a case study regarding synthetic natural gas production from biomass for steel industry. An experimental study on co-electrolysis performance and durability was performed in Ref. [15] suggesting that the durability would need to be improved if the electrolyser were to be powered using intermittent renewable energy sources. Davis and Martin [16], [17] presented the production of methane from water electrolysis and CO2 by comparing the use of solar PV systems and wind turbines on a monthly basis for the case of Spain. A renewable energy storage system via coal hydro-gasification with co-production of electricity and synthetic natural gas was presented in Ref. [18].

On the other hand ammonia is also one of the better choices for storing renewable electricity. Liquid ammonia has a high volumetric energy content, as it contains more hydrogen than liquid hydrogen per volume [19]. About 80% of the ammonia produced globally is used for the production of fertilisers and the remainder is used in explosives, pharmaceuticals, as working fluid in refrigeration units etc. [20]. Furthermore, ammonia could also be used as a liquid fuel in internal combustion engines as a carbon free fuel [19]. There are few publications regarding the production of ammonia from renewable electricity. Morgan et al. presented a case study of producing ammonia from a wind power plant in order to replace diesel fuel requirements on isolated islands [19], a combined scheduling and capacity planning model for a flexible electricity-to-hydrogen-to-ammonia plant is described in Ref. [21], and new electrochemical routes for producing ammonia were reviewed in Ref. [20]. It can be concluded that more research efforts are needed to develop efficient synthetic fuel production whilst ensuring reasonable standards of living throughout our world [22], [23], [24], [25].

The main purpose of this work was to compare the productions of three chemicals in terms of the technological efficiencies and economical viabilities. All three chemicals could be transported to Europe from places with lower prices for electricity. The synthetic fuel could be sold on the markets across the world. Five different scenarios are considered in this work: MC (production of methanol from CO2), MB (production of methanol from biomass), LNG (liquid natural gas production from CO2), SNG (synthetic natural gas production from biomass) and NH3 (ammonia production).

Section snippets

Materials and methods

In order to evaluate all three scenarios the production efficiencies of each chemical were determined in terms of the amounts of electrical energy used per kg of synthetic fuel. The efficiencies of converting the biomass were calculated for the scenarios where biomass was to be used as feedstock.

Results

Economic comparisons between all the processes were based on IRR (internal rate of return) over a 30 year period. The IRR values were calculated in MS Excel using the following equation:0=I0+i=1nCA,i(1+IRR)iwhere I0 is the total capital cost (EUR), n is the plant life time (years), and CA,i is the total annual profit (EUR). The value of IRR is in fact the discount rate at which the net present value is equal to zero.

It was discovered that the SNG scenario would not be economically feasible at

Conclusion

Renewable sources of energy in places where they are abundant have the potential to be utilised for producing several different chemicals. The productions of three different energy carriers were analysed in this work, ammonia, methanol, and synthetic natural gas. Two different carbon sources were used torrefied biomass and CO2 extracted from coal power plants, resulting in five different scenarios.

From the calculations it can be stated that the SNG and LNG scenarios are uncompetitive in terms

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