Energy efficiency analysis of outdoor standalone photovoltaic-powered photobioreactors coproducing lipid-rich algal biomass and electricity
Introduction
Microalgae represent an efficient solar-driven biotechnology resource for the environmentally sustainable production of biofuels, food, feed, cosmetics, fine chemicals, fertilizers, and biopharmaceuticals. The basis of the promise of microalgae as a biorenewable resource, include: (a) high lipid yields (up to 60 m3 ha−1 vs. 2 m3 ha−1 for Jatropha, or 0.2 m3 ha−1 for corn [1]); (b) high conversion of solar energy to product (theoretical maximum values of 10% for microalgae vs. 6% for C4 plants [2]); (c) rapid reproduction cycles allowing for semi-continuous or continuous harvesting; (d) potential for CO2 sequestration (1 kg biomass is equivalent to 1.8 kg CO2, [3]); (e) no requirement for high-value agricultural land, reducing competition with food-based crops; and (f) flexible inputs for culture systems including sea-/industrial, domestic and agricultural waste-water, flue gas, thus avoiding freshwater dependence [4], [5] and valorize waste streams [6], [7], [8], [9].
This combination of features has led many to view microalgal production as a panacea to the environmentally sustainable production of many bio-based products, rather than current efforts using agricultural plant-based systems. However, microalgal culture requires far greater energy input than the production of traditional terrestrial crops. For example, in their study of the environmental impact of oil production in Italy, Jez et al. [10] reported that oil production from microalgae still has greater negative environmental impacts compared to traditional crops (e.g., sunflower and rapeseed) due to excessive energy demand and input material consumption. The cultivation of microalgae, in addition to harvesting of biomass, was by far, the biggest contributor (60.9%) to the electrical energy needs and environmental impact [10]. The authors did note that the environmental impact of algal production could be reduced considerably by the use of renewable, specifically solar energy to provide the electricity to drive cultivation. Recently, Morales et al. [11] have indicated that there is a balance to be achieved between environmental impacts and energy when integrating photovoltaic panels with microalgal cultivation. Thus, to effectively exploit microalgae as a renewable bioresource, the energy efficiency of cultivation systems need to be considerably improved for commercial-scale production.
The two conventional systems for the commercial production of microalgae are open pond systems and closed photobioreactors [12]. Open ponds (e.g., classical raceways) are attractive commercially due to their low capital investment costs and are considered the cheapest technology for mass microalgal production [13], [14]. However, low biomass productivity and the inability to sustain year-round production due to a high rate of culture contamination are significant limitations associated with open ponds [1], [13], [15]. Closed photobioreactors can offer optimal biophysiological conditions that lead to higher biomass productivity with a lower tendency for contamination. Unfortunately, photobioreactors are prone to overheating under outdoor conditions, which results in lower productivity and high cell mortality. As such, temperature and thermoregulation of cultures in photobioreactors is a well-recognized problem in solar microalgal farming [16]. This can also contribute to high environmental and energy costs in those temperate areas of the world where the solar resource is ideal for microalgal culture, e.g., western USA, Israel, north-western Australia.
Under outdoor conditions, >50% of the solar radiation hitting the photobioreactor surface is within the infrared region (i.e., wavelengths above 700 nm) and directly contributes to overheating the culture [17]. Consequently, up to 95% of collected solar spectral energy is transformed to heat by the culture [18]. Microalgae have optimal temperature windows, in which maximum bioproductivity is achieved. In summer (especially in the tropics), supra-optimal (high) temperatures that are lethal to microalgae are easily reached in closed photobioreactors necessitating the use of cooling systems. In contrast, sub-optimal (low) temperatures occur in temperate regions, especially during winter, and these can lead to deterioration in growth and loss of productivity, making it necessary to heat cultures [19]. Year-round productivity in photobioreactors can then really only be achieved by cooling and heating photobioreactors and this is enough to lead to a negative energy balance of the system, even before considering other inputs such as materials, mechanical operations, and required nutrients. Therefore, effective temperature control of algal solar photobioreactors is a serious challenge to the overarching goal of cost-effective, environmentally sustainable, low-energy consuming microalgal production.
The problem of photobioreactor temperature control lends itself to novel approaches for the design of photobioreactors that are self-cooling (and require no heating in winter) and integrate photovoltaic electricity generation. These could then be optimized for maximal biomass productivity over the year to significantly decrease energy demand and address the negative net energy balance of algal photobioreactors. To this end, Moheimani and Parlevliet [20] proposed a microalgae production plant utilizing semi-transparent, spectrally-selective photovoltaic (PV) filters positioned above the culture facilities. This system could transmit a specific light spectral range to the culture while capturing and redirecting the remaining wavelengths to the PV cells for electrical energy generation. This idea paved the way for the design and development of an energy-harvesting spectrally-selective insulated glazed photovoltaic (IGP) photobioreactor [21], [22]. The IGP photobioreactor has a transparent (thin-film, CdTe) PV panel (40% transmission) and a low-emissivity (low-e) film [21]. The PV panel is glued to the upper part of the reactor to generate electrical power for production operations, removing the requirement for grid electricity. The low-e film is embedded in the illumination surface and selectively allows >70% of photosynthetically-beneficial wavelengths from sunlight to reach the microalgae culture, while simultaneously reflecting >90% of ultraviolet and infrared radiation [21]. Filtering out the non-photosynthetic wavelengths (e.g., above 700 nm), should keep the temperature in the photobioreactor below the upper critical limit without the need for freshwater-related cooling during the day [22]. In the same vein, the large temperature drops at night typical of conventional photobioreactors can also be mitigated by the insulated panels, ensuring a culture temperature above the lower critical limit for most microalgae species. Although on the surface this solution sounds attractive, the actual energy balance of the technology needs to be rigorously assessed.
The net energy ratio (NER) is a standardized parameter used to evaluate the energetic productivity of a system [23] and represents a quantitative and scientific evaluation of the ratio between total energy production and primary non-renewable (fossil) energy requirements in the production process during a technology’s life cycle [24], [25]. An NER ≥ 1 corresponds to the energy output exceeding the energy input, and such a system is obviously desirable [25], [26], [27]. Assessment of process sustainability for algal production systems (especially for biofuels) has been carried out mainly on systems based upon open ponds [23], [28], [29], [30], [31], [32]. The energy balance of closed photobioreactors, and particularly flat panel reactors, have been subjected to far less scrutiny. The reported range of the calculated NER for these types of systems varies widely. For example, Jorquera et al. [23] used a GaBi program to produce values of 4.5 and 1.7 for production of biomass and oil, respectively, from Nannochloropsis sp grown in a flat panel photobioreactor, while another research focussed on Scenedesmus obliquus reported values between 0.39 and 7.81 when cultured at mid-temperate latitudes [33]. In the most comprehensive treatment of photobioreactor energy efficiency, Tredici et al. [34] recently reported an NER of 0.6 for biomass production in an industrial-scale Green Wall Panel photobioreactor system culturing Tetraselmis suecica in Italy and 1.7 for a similar silicon-based PV-integrated system located in Africa. The data from the limited number of studies on photobioreactors to date seem to indicate that overall NER is due to both photobioreactor design, the species being cultured and the location of the facility. In fact, Morales et al. [11] have indicated that there is a compromise that needs to be made between optimizing energy efficiency and environmental impact when assessing commercial microalgal production using photobioreactors.
Only a few studies have investigated the supply of energy to the system using PV, but none of those have explored the actual integration of PV panels into photobioreactors themselves. Combining spectral filtering technology and PV electricity generation into an individual photobioreactor module should mean that more modules can be placed per hectare as well as reducing heating and cooling costs to maintain the microalgal cultures at temperatures for maximum biomass productivity.
This study aims to evaluate the NER of a pilot-scale flat panel photobioreactor that incorporates self-cooling and integrated photovoltaic energy generation for cultivation of Nannochloropsis sp.; a microalga often touted as a potential biofuel feedstock. The result of the energy analysis of this novel photobioreactor is compared to a photobioreactor utilizing a passive evaporative cooling (PEC) using the same system boundaries. The strength of this analysis is the use of experimental biomass productivity and power efficiency data obtained from the operation of both types of photobioreactor. However, the authors emphasize that the validity of the conclusions is only applicable within the defined boundary limits and use of the IGP photobioreactor system.
Section snippets
Functional unit, system boundaries, and source of data
For clarity and easy comparability, this energy balance analysis is carried out following the methodology of Tredici et al. [34], and utilizes similar system boundaries. The functional unit chosen for the current analysis is a 1-ha IGP photobioreactor plant. The choice of a 1-ha plant is not a reflection of the appropriate scale of an algae facility but a manageable size for industrial food or fuel-based applications of algae. It could be argued that a larger plant size would be needed to
Energy output
The ground areal productivity achieved using the IGP photobioreactor at the Murdoch University Algae R&D Centre in Perth during the austral spring (October–November 2018) was 16–23 g m−2 d−1, with no CO2 addition. Productivity could be increased by 70–80% with CO2 addition to the culture [52]. Notwithstanding this potential increase, a conservative figure of 20 g m−2 d−1 was chosen for the biomass productivity over the course of a year for this analysis. This level of productivity can be
Conclusions
The integration of semi-transparent photovoltaic panels to spectrally-selective insulated glazed photobioreactors offers a trinity of benefits: (a) sourcing local electricity for the plant operation; (b) eliminating freshwater-based cooling of photobioreactors, and (c) a strong reduction in diel temperature fluctuation. These could neutralize the strong external cooling water and electrical energy requirements of microalgal photobioreactors. In this study, the primary energy inputs and outputs
CRediT authorship contribution statement
Emeka G. Nwoba: Conceptualization, Methodology, Investigation, Formal analysis, Validation, Writing - original draft, Writing - review & editing. David A. Parlevliet: Conceptualization, Formal analysis, Funding acquisition, Resources, Methodology, Validation, Supervision, Writing - review & editing. Damian W. Laird: Conceptualization, Funding acquisition, Resources, Supervision, Writing - review & editing. Kamal Alameh: Conceptualization, Funding acquisition, Resources, Methodology, Validation,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by Murdoch University. The authors would like to thank Emeritus Prof. Michael Borowitzka for his intellectual advice.
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