Review on Energy Resilience

Applied Energy 166 (2016) 19–26

Contents lists available at ScienceDirect

Applied Energy

journal homepage: www.elsevier.com/locate/apenergy

Techno-economic analysis of solar integrated hydrothermal liquefaction of microalgae

http://dx.doi.org/10.1016/j.apenergy.2016.01.005 0306-2619/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. E-mail address: [email protected] (C. Sansom).

Matthew Pearce a, Mobolaji Shemfe a, Christopher Sansom b,⇑ a Bioenergy & Resource Management Centre, Cranfield University, Bedford, Bedfordshire MK43 0AL, UK b Precision Engineering Institute, Cranfield University, Bedford, Bedfordshire MK43 0AL, UK

h i g h l i g h t s

� Hydrothermal liquefaction and concentrated solar power provide integrated biofuel technology. � Heat kinetics and energy efficiency Aspen plus modelling of CSP and HTL. � Microalgae biofuel minimum fuel sales price of $1.23/kg.

a r t i c l e i n f o

Article history: Received 10 October 2015 Received in revised form 21 December 2015 Accepted 4 January 2016 Available online 20 January 2016

Keywords: Hydrothermal liquefaction Microalgae bio-oil Concentrated solar power

a b s t r a c t

Integration of Hydrothermal Liquefaction (HTL) of microalgae biomass with concentrated solar power thermal processing (CSP) for bio-oil production is a potential processing pathway for energy efficient gen- eration of renewable biofuels. Solar HTL infrastructure avoids additional bolt-on components of conven- tional solar parabolic trough systems used for electricity production including heat transfer fluids, counter current heat exchangers, fluid transfer interconnectivity and electrical power control systems. The absence of such capital intensive additional equipment considerably reduces the production costs of solar HTL biofuels compared to electricity generation from conventional CSP power systems. An eco- nomic and market appraisal of variance and system economic resilience is presented. It is hypothesised that the combination of nutrient recycling with HTL/CSP unification has the potential for economically sustainable microalgae bio-oil production. A microalgae biofuel minimum fuel sales price of $1.23/kg has been modelled. Further experimental work would be able to validate this integrated model.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

As the demand for energy intensifies amid growing concerns for drastic climate change, biofuels are needed more than ever as an alternative to fossil fuels. Third generation renewable liquid biofu- els derived from microalgae could potentially supplement incre- mental global energy demand. Microalgae grow rapidly, produce energy dense lipids, are able to utilise marine, freshwater and wastewater, grow on non-agricultural land and remediate waste or atmospheric carbon dioxide. Microalgae biomass feedstock for HTL bio-oil production benefits from reduced energy requirements for complete dewatering [1–20].

This paper begins with an introduction and review of current literature, the HTL process and heat integration using CSP are dis- cussed, then we calculate costs of microalgae derived bio-crude

production from a 1-ha site using a 100 m long parabolic CSP trough. Working methodology considers established CSP thermo- dynamics, heat transfer, present day market prices and the mass of engineering equipment and associated capital expenditure (CAPEX). Finally, this theoretical forecast of a commercial opera- tion is compared to industrially functioning global electricity CSP and evaluates how this new techno-economic analysis (TEA) can make strides from being present-day theory to the development of a new future scenario of commercially implemented technology.

The energetics of the HTL process are dominated by the energy required to heat the reactor, 6.51 MJ (kg microalgae)�1 [21]. Care- ful consideration of the EROEI (Energy Returned on Energy Invested) of HTL as a function of reaction temperature is required [22]. A sensitivity analysis of base case parameters indicated that modelled systems were particularly sensitive to the extent of heat integration from HTL suggesting that optimisation of heat integra- tion is necessary for minimisation of lifecycle greenhouse gas emissions [23]. HTL oil yields reported for higher temperatures (>200 �C) exceed the lipid content of the biomass, which

20 M. Pearce et al. / Applied Energy 166 (2016) 19–26

indicates conversion of other cellular constituents (e.g., protein, carbohydrate, algaenan) by HTL reaction processes [24,25].

Reaction times are a crucial factor for economical operation of HTL, short reaction times will reduce bio-oil yield whereas long reaction times may lead to higher gas and compromised bio-oil yield [26]. 30 min is the most appropriate HTL reaction time for Enteromorpha prolifera [27] and Dunaliella [20]. A hydrothermal liq- uefaction techno-economic analysis of a modelled 2000 dry tonne per day processing facility using defatted microalgae indicates that 66% external electricity should be supplied with an overall energy efficiency of 56% [28]. The well to pump lifecycle comparison of fossil energy use and greenhouse gas emissions were higher for HTL bio-oil than lipid extracted renewable diesel production on account of net heat energy input to establish the operational pro- cess conditions, however HTL used 1.8-fold less biomass than the lipid extracted renewable diesel production pathway [29]. The amount of energy required to dry algal biomass to levels typical of terrestrial crops for solvent based oil extraction would exceed the energy content in the algal oil [30]. The energy required for microalgae and biogas production from Nannochloropsis has been calculated to be as much as 8–11 times more than the bio-gas energy yield [31]. A trade-off between high algal oil yields and high energy recovery via catalytic hydrothermal gasification of the aqueous HTL solubles is amplified by using Escherichia coli grown on aqueous HTL solubles for secondary HTL, boosting the oil yield per unit of microalgae biomass and suggesting that recovery and recycling of aqueous phase product from HTL is instrumental to overall lifecycle economics [32].

An integrated modelling framework has been developed to predict biological cultivation and chemical HTL process pathways as a predictive tool for microalgae to fuel [33]. Whilst many authors’ have endorsed HTL nutrient recycling and the energetic transformation and optimisation of biomass conversion processes using HTL, to date there has been no known research on efficient heat delivery for achievement of the operational HTL process con- ditions, irrespective of the reaction constituents. With the volu- metric scale-up of this technology which will be required for industrial quantities of biofuel production, integration of efficient heat delivery is a pre-requisite. Key issues for future R&D of microalgae biofuels include both the utilisation of co-products and development of energy efficient thermo-conversion processes [34].

Various parameters affect product yield of HTL derived bio-oil including microalgae species, feed ratio of solids to liquid, reaction temperature, holding time, heating rate, cooling rate, presence of catalysts and effective product separation [35]. In recent years HTL process development from batch to continuous feed has occurred [36]. Continuous feed systems have advantages of higher feedstock flows and lower process and retention times, lacking uncertainties in heating and cooling rates common in batch run experiments [37]. Development of a continuous feedstock process requires thermal quenching to reduce temperature differentials, ensure preservation of reactant products and optimise the viable and scalable commercial integration into a CSP/HTL production system. Thermal retention, multi-phase flow fluid mechanics and feedback control optimisation within the core of the reaction pres- sure vessel should be identified to define reaction process bound- aries. Microalgae biomass of concentration 10–20% (w/v) is optimal for HTL boundaries of solids loading [38]. A 20% solid con- tent is estimated to be a reasonable trade-off between the capital costs for the HTL system and the costs for dewatering. Higher bio- mass solid input concentrations to HTL reduce the capital cost and make product separation easier but also incur greater dewatering costs [3]. Solar heat integration as the vector for biomass to biofuel transformation does not jeopardise holistic energetic transforma- tion pathways resulting in a more favourable energy return in

the LCA (Lifecycle Analysis) than energy input from fossil fuel gen- erated heat.

Engineering the integration of solar thermal energy for HTL bio- oil rather than electricity generation has not been widely reported by other authors’. The objectives of this study were to investigate a techno-economic analysis (TEA) of factors influencing the unifica- tion of HTL and CSP parabolic troughs for the processing of microalgae biomass into bio-oil. CSP parabolic troughs yield a tem- perature of up to 400 �C with oil as the heat transfer liquid (HTF); the use of molten salts as a HTF can attain a temperature much higher [39] whilst beneficial operational temperature require- ments for HTL occur within the range of 250–350 �C [36,40]. A thermodynamic assessment of parabolic troughs [41] with an eco- nomic analysis using experimental field trials of microalgae pro- ductivity justifies the potential viability of this technology unification. Aspen plus� and custom sizing equations have been used to determine the economic viability of the process. Finally, the influence of estimated parameters on the economic results was assessed via sensitivity analysis.

2. Materials and methodology

2.1. Process overview

This sized CSP plant could process 200 kg of daily microalgae biomass in 3 cycles. The schematic diagram of the solar-assisted HTL plant is depicted in Fig. 1, which describes the integration of solar CSP with the tubular HTL reactor aligned along the focal line of the parabolic trough.

A land surface area of a 1 ha site could produce in the region of 180–200 kg biomass per day from a high rate microalgae pond, additional waste biomass and recycled nutrients for secondary bio- mass growth further supplements HTL microalgae feedstock. Con- sidering a 30% biomass to bio-oil conversion with a ratio of 20% solids to water ratio in HTL, this would provide 1000 l feedstock per day. The HTL reactant volume space replaces the heat transfer fluid (HTF) as used in conventional electricity generating CSP plants. Likewise, HTF molten salt is replaced by microalgae bio- mass and water as the reactant components of HTL. Reactant inputs and discharge on alternative ends of each linear row of para- bolic troughs function as semi-continuous batch processing. A pro- posed diurnal thermal HTL capacity for 3 h either side of midday permits 3 batch runs per day (Table 4). Estimation of CSP plant size for processing of HTL feedstock is based on the 1000 l daily produc- tion of HTL feedstock at 20% (w/v) microalgae – 160 l HTL reaction core volume from 100 m of solar CSP parabolic troughs, with 226 m2 total solar aperture.

2.2. Process modelling and economics

Fig. 2 depicts the overall methodology employed in this study. The elements of the methodology used in this study are elabo-

rated in the succeeding tables and equations discussed in this sec- tion. Table 1 presents the typical dimensions and thermodynamic outputs of the commercially available CSP parabolic troughs for electricity production, and the methodology used to derive the val- ues in Table 1 is now discussed.

The value of DNI (Direct Normal Irradiance, ie direct sunlight) of 750 W/m2 is a conservative number, representative of locations with high concentrations of direct sunlight on an average basis (e.g. desert regions of North Africa, the Middle East, and the Americas) or peak levels of direct insolation in more temperate latitudes. For reference, DNI at the top of the earth’s atmosphere (the so-called ‘‘Solar Constant”) is approximately 1380 W/m2).

Table 1 also includes optical and thermal loss values from a parabolic trough solar concentrating mirror and absorber tube of

Fig. 1. CSP–HTL integrated plant. Microalgae biomass feedstock is semi-continually pumped into the reaction vessel core of the CSP parabolic trough, followed by storage of post-HTL products.

Fig. 2. Methodology summary.

M. Pearce et al. / Applied Energy 166 (2016) 19–26 21

the type used currently in CSP power plants. Since our design uses the same standard components these values are considered to be valid. Using these values enables us to calculate the total solar energy transferred into the reactor tube.

Total Optical efficiency (after losses), ROPT is given by:

ROPT ¼ Rs � Rt � Rg � RREF � RSoil � RHCE � RX � ð1 � AHCEÞ ¼ 0:975 � 0:995 � 0:985 � 0:945 � 0:975 � 0:985 � 0:975 � ð1 � 0:015Þ

¼ 0:833

where RX is a factor to account for other optical losses owing to stray light, atmospheric absorption, plus unaccounted optical misalignments.

Total Thermal efficiency (after losses), RTH is given by:

RTH ¼ EHCE � THCE � ð1 � ACoatÞ � ð1 � ECoatÞ ¼ 0:905 � 0:975 � ð1 � 0:018Þ � ð1 � 0:145Þ ¼ 0:741

These losses are primarily in the infra-red, attributable to the energy re-radiated from the reactor tube plus conductive and con- vection losses through the glass tube and its coatings and into the atmosphere.

Therefore the total heat absorbed by the reacting fluid, H is given by

H ¼ DNI � ROPT � RTH ¼ 750 � 0:833 � 0:741 W=m2 ¼ 462 W=m2

Table 1 CSP parabolic trough dimensions and thermodynamic outputs.

Description Symbol Experimental system

Units

Direct Normal Irradiance DNI 750 W/m2

Heat collector element shadowing Rs 0.975 – Tracking error Rt 0.995 – Geometry error Rg 0.985 – Clean mirror reflectance RREF 0.945 – Dirt on mirrors RSoil 0.975 – Dirt on heat collector element RHCE 0.985 – Unaccounted error RX 0.975 – Glass absorbance AHCE 0.015 – Glass emissivity EHCE 0.905 – Envelope transmittance THCE 0.975 – Coating absorbance ACoat 0.018 – Coating emittance ECoat 0.145 – Solar curved width 2.5 m Solar aperture width Wa 2.26 m Focal distance 0.68 m Ambient temperature 28.4 �C Wind speed 3.8 m/s Inner absorbance reactor diameter d 0.066 m CSP parabolic trough length 100 m Water specific heat capacity 4200 J/kg K Biomass specific heat capacity 1400 J/kg K Biomass:water ratio 20:80 – Reactant specific heat capacity 3640 J/kg K CSP concentration ratio CR 40 – Heat transferred to heat transfer liquid H 1044 W/m Total heat 104.4 kW

Table 2 CSP solar collector operational parameters.

Description Experimental system Units

Solar collector area 226 m2

Power input 169,500 W Heat required to attain 320 �C 7.804 � 107 J Time required to attain 320 �C 7.5 min Time required to attain 320 �C

(with thermal losses) 15 min

Cost of solar collectors 270 $USD/m2

Solar collector total cost 61,000 $USD

22 M. Pearce et al. / Applied Energy 166 (2016) 19–26

Given a 100 m line of parabolic trough collectors with an aper- ture width of 2.26 m this yields a collector area of 226 m2 and H = 462 � 226 W, or 104.4 kW.

Table 2 shows the heating energy kinetics and heating rate to attain the required operating temperature of 320 �C in the HTL reactor and also the solar collector purchase equipment costs.

With mass m = 67 kg of feedstock within the reactor tube, the heat required to heat the mixture up by 320 �C would be given by

Table 3 HTL reaction biomass conversion kinetics [42].

Reaction pathway Reaction conditions Reaction temp k (m

Reaction 300 1p Protein ? AP 0.2 1l Lipid ? AP 0.35 1c Carbohydrate ? AP 0.35 2p Protein ? Biocrude 0.13 2l Lipid ? Biocrude 0.11 2c Carbohydrate ? Biocrude 0.0001 3 Biocrude ? AP 0.14 4 AP ? Biocrude 0.12 5 AP ? Gas 0.0004 6 Biocrude ? Gas 0.0002

H ¼ m � s � H; where s is specific heat of the mixture: Hence H ¼ 67 � 3640 � 320 J ¼ 7:804 � 107 J This assumes a specific heat of 3640 J/kg K (based on values of

4200 J/kg K for water and 1400 J/kg K for biomass in water, and a mixture containing 20% biomass in water).

Hence it would take 7.804 � 107/104.4 � 103 s, or 12.5 min to provide the heat necessary to raise the temperature by 320 �C. For prudence, this is rounded up to 15 min in Table 2. This would be followed by 30 min at temperature (when it might be necessary to defocus some of the solar collectors in order to keep the reaction temperature constant), then pump out the mixture to cool it down.

Table 3 illustrates the reaction kinetic parameters for the con- version of microalgae into biofuel within the HTL reactor at 300 �C and 350 �C. The size of the HTL reactor is derived from rate equations using kinetic parameters reported by Valdez and Savage [42] illustrated in Eqs. (1)–(6). Table 4 presents design parameters for the HTL reactor, which are subsequently used to calculate the purchase equipment cost and in turn the CAPEX (Capital expendi- ture) of the vessel. The CAPEX of the feed pump and storage tank were derived from sizing and cost models provided in Aspen plus�.

The HTL reactor is sized based on the mass balance below: For A ? products

�rA ¼ � d NAV � � dt

ð1Þ

For first-order reactions at constant volume:

� dCA dt

¼ kCA ð2Þ

� ln CA CAo

� � ¼ kt ð3Þ

dXA dt

¼ kð1 � XAÞ ð4Þ

t ¼ � 1 k lnð1 � XAÞ ð5Þ

where t = the time for the entire batch operation to occur, XA is the conversion of A to products and k is the rate constant. The volume of the HTL reactor is determined from Eq. (6).

VR ¼ _mt q

ð6Þ

Based on a reactor and CSP trough length of 100 m, and corre- sponding equipment mass thereof, purchase equipment cost is cal- culated as a function of reactor mass (Table 4), and current market prices of electrical CSP solar receivers (Table 2). HTL reaction kinet- ics are calculated from the decomposition and re-polymerisation of biological material (protein, lipid and carbohydrate) into hot aque- ous phase material and subsequent re-constitution into bio-crude,

in�1) tr (min) Reaction temp k (min�1) tr (min)

300 350 350 3 0.28 2 2 0.35 2 2 0.35 2 5 0.28 2 6 0.33 2

6931 0.001 217 5 0.3 2 6 0.26 3

1733 0.0014 495 3466 0.0009 131

Table 4 HTL design parameters.

Description Experimental system Units

Reactor length 100 m Feed volume 0.12 m3

Volume allowance 180 % Reactor volume 0.34 m3

Inner reaction diameter 0.066 m Time to fill 20 min Heat or cool to required temperature 15 min Reaction time 30 min Empty 20 min Entire batch operation 2 h Reaction temperature 320 �C Mass flow rate 67 kg/h Density of feed 1100 kg/m3

Reaction pressure 15,000 kPa Wall thickness 0.009 m Outside diameter 0.07 m Density of reactor 7850 kg/m3

Reactor mass 2678 kg Purchase equipment cost 25,379 $USD

Fig. 3. Distribution of CAPEX of equipment.

M. Pearce et al. / Applied Energy 166 (2016) 19–26 23

bio-gas and carbonaceous aqueous liquid. Modelled reaction path- ways for these chemical conversion kinetics are considered from previous literature in Table 3.

Since the focus of this paper is on the downstream processing of microalgae into bio-oil, it does not address continued research into

Fig. 4. Distribution

algae culturing, harvesting and de-watering. However, operational expenditure estimated investment costs were based on microalgae productivity for a high rate pond at 20 g dry mass/m2/day�1 [43]. Microalgae production costs range from €0.43 to 4.95/kg�1

[44,45]. CAPEX costs are derived from commercial costs of solar CSP collectors.

For the operating cost, raw material cost of $300/ton is used [3]. Other contributory operating cost, including maintenance cost, operating labour, overhead costs are estimated from the total CAPEX based on fundamental engineering costing principles. The minimum fuel selling price (MFSP) were derived from net present value (NPV) analysis, when NPV equalled to zero at discount factor of 10%, for a product yield of 70 tonnes/year with a 40% income tax applied.

3. Results and discussion

3.1. Economic results

The economic results are presented in Figs. 3–5. The TEA (Techno-Economic Analysis) assumes an equipment lifecycle of 20 years. The TEA model estimates that with a CAPEX of $242K, OPEX (Operating Expenses) of $43K/year and bio-oil product of 70 tonnes/year from a 1 ha site a MFSP (Minimum Fuel Sales Price) of $1.23/kg bio-oil Table 5. This TEA is based on a processing sys- tem which simplifies feedstock processing capacity by integrating the solar heat directly with the solar collector. The system requires the implementation of pressure release valves before and after the reactor core to allow for incremental heating and passive cooling during the 2 h semi-continuous batch reaction. The HTL reactor and the solar collectors constitute 69% of the total capital invest- ment. Main CAPEX costs of the pond construction is made up of containment geomembrane liners. The economic assessment does not include further downstream processing such as liquid, gas and solid separation or hydro-process upgrading of biofuel via catalytic hydrogenation. Operating labour costs account for 46% of OPEX, followed by overheads 23%, maintenance 16% and feedstock raw materials 12%.

Direct comparison between CSP DSG (Direct Steam Generation) and CSP HTL is valid as the only difference between the solar field components of the HTF is the replacement of water with microal- gae and water at 20% inclusion (w/v). Replacement of HTF using DSG is not yet widely applied commercially for CSP electricity gen- eration. The major bottleneck constraint to the use of DSG instead

of total OPEX.

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16 18 20

$U SD

T ho

us an

ds

Years

Cashflow Income OPEX

Fig. 5. Cashflow, income tax and investment repayment.

24 M. Pearce et al. / Applied Energy 166 (2016) 19–26

of molten salt or synthetic oil is energy storage [47,48]. In contrast, CSP HTL is not prone to the constraint of energy storage, as HTL bio-oil is the penultimate final liquid energy product (prior to hydrogenation), rather than extra requirement for downstream energy conversion from heat to electricity. Table 6 presents 3 CSP plant dimension scenarios and economic investment for both con- ventional plants and envisaged CSP/HTL [48,49]. Solar aperture and site size are considerably reduced for a CSP/HTL plant.

The energy density of the final bio-oil can be estimated as 35 MJ/kg � 200 kg feedstock with 35% yield oil yield = 2450 MJ/ day � 30 = 73,500 MJ or 20.4 MW h/month. The net capacity factor of a solar CSP plant is about 30%, so a 50 MW solar CSP plant (stan- dard size) would produce (50 � 30 � 24 = 36,000 MW h) �

Table 5 Capital cost estimation.

Equipment name Total capital investment (USD)

Fixed capital cost (USD)

I (

Pump 15,800 15,048 HTL reactor 105,525 100,500 5 Storage tank, ponds & construction 59,875 57,024 2 Solar collectors 61,000 57,950

Total 242,200 230,522 8

Table 6 Electricity CSP and HTL CSP site comparison.

Pitz-Paal (2007) CSP parabolic trough Mo

Total power plant size 47 MW 75 HTF substance Direct steam generation Dir

cyc Solar aperture (m2) 337,076 35 Site size (ha) 144 15 Solar field (€/m2) 190 20 Power generation block (€/kW) 700 98 Land (€/m2) 2 2

Table 7 Microalgae biomass production cost estimates.

Author Norsker [45]

HTF substance Water + algae b Microalgae dry weight production from 1 ha (kg/year) 64,000 Microalgae growth system Open pond Latitude 51.4�N Microalgae production cost (€/kg) 4.95 Microalgae production cost/year 316,800

0.3 = 10,800 MW h/month. By contrast, solar bio-oil CSP is only 0.19%, the energy productivity of a 50 MW solar CSP plant. How- ever, solar bio-oil only requires 2 ha of land, whereas a 50 MW solar CSP electricity plant requires 150 ha. 2 ha of solar electricity CSP would produce 144 MW h and therefore solar bio-oil CSP is 20.4/144 = 14% the efficiency of electricity solar CSP. Further con- siderations of the energetic comparisons are the energy versatility of the liquid fuel for transportation requirements, recycling of nutrient water solubles back into the microalgae culture ponds to improve productivity and capital investment economics. There is additional scope for an improvement of the energy return on energy invested of algae HTL with recycled heat from the combus- tion of bio-oil and gas by-products.

Table 7 shows 2 trials and 4 microalgae production costs [44,45]. The Dutch economic productivity investigation forecast with an inflation rate of 5% used microalgae costs of €0.7/kg based on growth in the Dutch Antilles in the tropics [45]. By contrast, another economic evaluation of cost of microalgae production in Spain indicated a production cost of €69/kg. This is due to higher operational and processing costs including carbon dioxide, syn- thetic nutrients, centrifugation, freeze-drying and labour costs. It was concluded by the Spanish trial that process automation, low- labour costs, waste flue-gases and waste nutrients could reduce production costs further [46].

CSP is still a relatively young and actively researched technol- ogy. Table 8 shows the incremental commercial production inter- est and amplified scale in commercial systems within the last 5 years. CSP/HTL research could complement this field.

There exist various parameters’ affecting the conversion of bio- mass into bio-oil. Coordination of biomass input and bio-oil output within the scope of this TEA requires reliable definition of biomass

ndirect cost USD)

Total direct cost (USD)

Equipment cost (USD)

Equipment weight (kg)

760 11,780 3800 77 076 78,684 25,379 2678 880 44,600 14,400 1134

716 135,094 43,579 3889

ntes (2011) CSP parabolic trough Pearce et al. (2015) CSP parabolic + HTL

MW 64,000 l bio-oil/year ect steam generation/combined le gas turbine

Water + algae biomass

8,592 226 3 <0.2 0 241 4 0

2

Norsker [45] Norsker [45] Chisti [44]

iomass

Open pond PBR Open pond 12.2�N 12.2�N Not stated 1.28 0.7 0.43 81,920 44,800 27,520

Table 8 Major commercial worldwide CSP electricity plants [50].

Name Date Location Power (MW) Land area (ha) Cost ($USD)

Solar Energy Generating Systems 1987 California, USA 150 648 Integrated Solar Combined Cycle (ISCC) 2005 Hassi R’Mel, Algeria 150 150 M Nevada Solar One 2007 Las Vegas, USA 64 162 266 M Andasol 2008 Granada, Spain 150 200 Solana (Abengoa) 2010 Arizona, USA 250 775 1450 M Integrated Solar Combined Cycle (ISCC) 2011 Kuraymat, Egypt 146 220 M Genesis (NextEra Energy Resources LLC) 2013 California, USA 250 1950 Godawari 2014 Jaisalmer, India 50 150 145 M Mohave (Abengoa) 2015 280 714 1600 M Integrated Solar Combined Cycle (ISCC) Current Ain Beni Mathar, Morocco 470 160 620 M Shams-1 Current Abu Dhabi 100 1012 570 M Ouarzazate (ACWA Power) Current Morocco 500 3000 1180 M Bokpoort (ACWA Power) Current South Africa 50 150 340 M Kaxu Solar One (Abengoa) Current Pofadder, South Africa 100 1100 860 M

Fig. 6. MFSP sensitivity to ±20% change in fuel yield, capital cost, operating cost, fuel yield and income tax.

M. Pearce et al. / Applied Energy 166 (2016) 19–26 25

productivity at scaled production. Commercial microalgae biomass production is dominated by extremophile species tolerant of wide ranges of pH, temperature or salinity [51]. This TEA study inte- grates biomass input (productivity) with bio-oil output (HTL con- version), hence estimates land area, equipment costs and conversion efficiency into a dynamic economic model which pro- vides a predictive forecast range for further applied experimental work. One of the only feasible energetically efficient conversion pathways for the production of fuel from microalgae is whole microalgae biomass using thermochemical conversion, by contrast energy efficient conversion of biodiesel transesterification is com- promised by water and petroleum derived solvent evaporation [52]. Bio-oil from HTL does not use embedded energy from petro- leum derived substances, and input heat energy could be supplied via CSP. With optimised heating to reaction temperature, cooling to ambient temperature, efficient filling and emptying of the reac- tor it is feasible that more than 3 semi-continuous batch processes could be run per day. Furthermore, optimal site location, CSP geometry, and solar concentrator tracking could enhance produc- tivity and conversion efficiency beyond that reported in this study. Biomass input into this CSP/HTL design does not have to be solely of microalgae origin, as HTL has found to have been effective with other feedstocks including swine manure [17], macroalgae [27] and E. coli [32]. The unification of CSP and HTL described herewith and not previously reported by other authors’ provides scope for further work to evaluate the practical implementation of this research.

3.2. Sensitivity analysis

The sensitivity of the final MFSP to specific parameters are examined here. Fig. 6 shows the shows the results of the analysis for the input-based parameters, including fuel yield, CAPEX, OPEX and income tax.

An increment of 20% in fuel yield resulted in a decrease of 16.7% in MFSP. A decrease of 20% on the other hand, resulted in an increase of 25% in MFSP. An increase of 20% in OPEX and CAPEX resulted in an increase of 12.7% and 7.3%, respectively and vice versa. MFSP increased by 15.4% when income tax was increased by 20%. On the other hand, MFSP decreased by 11.8% when tax was decreased by 20%. These results imply that the profitability of the proposed process is mostly sensitivity, by order of degree of influence, by fuel yield, OPEX, income tax and CAPEX.

4. Conclusions

This study has identified how nutrient recycling and HTL/CSP unification could viably develop sustainable microalgae bio-oil production. A MFSP of $1.23/kg and a positive cash flow has been demonstrated. However, further questions remain over the scaled productivity, HTL conversion efficacy and corresponding economic viability. This can only be demonstrated with an experimental application to validate this proposed integrated technology.

However the practical details presented in this work, particu- larly those regarding the solar field components, were selected with considerable care. The calculations were performed on the basis that standard concentrating solar power plant components would form the basis of any experimental prototype. As such, the parabolic trough concentrator is assumed to be identical to those manufactured in Spain, Germany, Italy, or the US by suppliers such as Rioglass, Flabeg, Schott, DLR, Archimedes (ASE), or SkyFuel. Sim- ilar comments could also be made about tracking and control sys- tems, optional heat storage media, pumps and pipelines. However the receiver or absorber tube, which provides the novelty in the design by virtue of acting as the reaction vessel for the process, would necessarily be a modified design of the standard receiver tubes currently used to carry heat transfer fluids at high tempera- tures to night storage tanks or direct to steam turbines. In fact, the lower temperature of our microalgae reaction would allow a relax- ation of the absorber tube specification, since the radiative losses would be lower.

Future work would require building either a full-scale or a 1/10th scale prototype unit to demonstrate field based operational viability and scalability of this proposed technology. Consideration of fluid pumping to fill and empty the reactor, liquid and gaseous

26 M. Pearce et al. / Applied Energy 166 (2016) 19–26

pressure release and longevity of material structural tolerance to physical, chemical and heating cycles would also be paramount research.

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