4.2
Mixing in algal raceways is normally achieved by paddlewheels (Section 2.2.3), but how much energy of the potential calorific yield of micro-algae is used to maintain fluid flow?
Energy return on operational energy investment EROOI
4.2.1
Energy return on energy investment (EROEI or EROI) is the ratio of the energy produced compared to the amount of energy invested in its production.
However as previously discussed (Section 1.2) problems arise in deciding which inputs and outputs count.
It is often unclear in the literature what has been included in a particular calculation. Reported EROIs often exclude the embodied energy in process equipment, but the embodied energy in the equipment does not appear to have been reported as major factor in the production of micro-algal biofuels. The embodied energy within process equipment is not considered in the
energy balances of this work. The major energy inputs in the production of micro-algal biofuel are operational energy and the embodied energy of nutrients (Aquafuels, 2011). The use of low embedded energy sources of
water, nutrients and CO2, such as wastewater and flue gas have been assumed
through-out. The embodied energy of materials has been excluded from the energy balances calculated subsequently unless specifically mentioned in any results such as that for flocculation harvesting (Section 6.9).
The term energy return on operational energy invested (EROOI) is ratio of the energy output to the operational energy input. The output will be the HHV of the biomass or where biogas is the end product the HHV of the methane in the estimated biogas production. The input will be the operational energy
requirement, heat and electricity, of the process equipment. EROOI is used throughout this section and subsequent sections to measure energetic viability and net energy return.
Basis of calculation
4.2.2
Section 4.1 indicated that a pragmatic yield is 25 g m-2 day-1 for 20 % lipid micro-algal biomass having a calorific value of 6.0 kWh kg-1. The total potential energy produced per day in micro-algal biomass is therefore
0.15 kWh m-2 day-1.
The total hydraulic power required to move a micro-algal suspension around a raceway can be calculated from the head losses as previously discussed in Section 2.2.2.3 together with Sections 2.2.2.6 & 2.2.4
Results and discussion
4.2.3
Table 9 shows the estimated head losses in lined and unlined raceways of different dimensions based on Equation 3, 7 & 10 and assuming a friction factor of 0.01 for lined and 0.02 for unlined.
Table 9 Head losses in lined and unlined raceways a. Head loss m Velocity m s-1 0.15 0.3 Friction Factor 0.01 0.02 0.01 0.02
Raceway Dimension Area m2
Width / Straight Length w*2L + πw2
1m / 50 m 103 0.010 0.016 0.040 0.065
10 / 100 m 2314 0.010 0.018 0.041 0.070
20 m / 200 m 9267 0.013 0.026 0.050 0.106
a Calculated from Equations 3, 7 & 10
The head loss in lined raceways is approximately half that of unlined, halving the power and energy required for maintaining fluid flow within a raceway. Liners, therefore, are valuable in reducing raceway operational energy in addition to reducing seepage, improving cleanability and reduced water clouding problems due to the suspension of soil particles (Section 2.2.1).
For a pilot-scale raceway with a smooth liner surface, of the type used in Spain by the University of Southampton (Manning friction factor 0.01, 1 m wide,
0.3 m deep, with straights 50 m long and a mean fluid velocity of 0.3 ms-1) a
hydraulic power requirement of 35 W can be calculated (Milledge, 2011b). If a
raceway is mixed for 24 hours the hydraulic energy required is 0.84 kWh day-1.
The hydraulic power required to mix the raceway is thus over 5 % of the total energy available in the biomass production of 25 g m-2 day-1. Typically, quoted paddlewheel efficiencies are 10 to 20 %, and therefore, from one quarter to over half of the total potential higher heating value of the algal biomass could be used in mixing such a raceway. Optimised paddlewheels can have higher efficiencies, but a 40 % efficient paddlewheel would still use the equivalent 13 % of the energy in algal biomass to mix a small raceway.
Table 10 shows the calculated paddlewheel energy (assuming 40% overall efficiency), expressed as a percentage of micro-algal biomass energy content (based on a yield of 25 g m-2 day-1 and a calorific value of 6.0 kWh kg-1 for various size lined raceways at fluid velocities of 0.1 m s-1 and 0.3 m s-1.
Table 10 Paddlewheel energy consumption as percentage of micro-algal biomass calorific value
Paddlewheel Energy as % of Algal Calorific Value
Velocity
m s-1 0.15 0.3
Raceway Dimensions m Area m2
Width / Straight Length
1 m / 50 m 103 1.70% 13.50%
10 m / 100 m 2314 0.70% 6.20%
20 m / 200 m 9267 0.50% 3.80%
Larger racewaysgive a better energy return on mixing investment and reducing
the fluid velocity also significantly improves energy return. The lower mixing energy requirement as a percentage of the potential micro-algal biomass energy yield in larger ponds compared to smaller ones is a result of the higher proportion of straight sections with a relatively low head loss. The eight-fold reduction in power requirement from halving of fluid velocity is due to hydraulic power being a function of the cube of fluid velocity.
The results in Table 10 are in agreement with the recalculated experimental data from Green et al. (1995), for an 0.1 hectare raceway in California, with a mixing energy requirement of 0.12 to 0.25 kWh to produce 1 kg of algae in an
open pond operating at fluid velocity of 0.15 m s-1, equivalent to an energy
requirement for mixing of between 2 - 4 % of the calorific value of the micro- algae produced. LCA s have used figures between 0.05 and 3.3 % (Collet et al., 2011).
The power required to give a flow velocity of 0.15 m s-1 in a 0.3 m deep unlined
raceway of one hectare using a paddlewheel with 40 % efficiency has been
estimated at 18 kWh day-1 ha-1 (1 horsepower) (Benemann and Oswald,
1996).This is equivalent to an energy requirement for mixing of 1.2 % of the calorific value of micro-algae and is again in good agreement with the results in Table 10.
Conclusions
4.2.4
The spreadsheet developed to calculate the mixing energy requirement for raceways produced results that are in agreement with other published data. A simple EROOI has been produced, using this calculation and a pragmatic yield. This will be further developed and used in calculations of operation energy returns for entire micro-algal bioenergy production systems.
The required energy inputs relative to the biomass calorific value decreases with the addition of a liner, reduction in fluid velocity and increase in raceway size. For maximise EROOI ponds should, therefore, be lined and as large as practicable. The fluid velocity should be the minimum to provide mixing and regular exposure of the cells to light. It is possible that small gains in micro- algal yield due to improved mixing and exposure to light through increased fluid velocity may not be energetically or economically efficient, and sub-
optimal micro-algal yields at lower flow-rates could offer a better energy return on energy investment. The effect of fluid velocity on EROOI is investigated further in Sections 5.1.2.1, 6.2.1 & 6.3.2.4, and the results of the work in this chapter were used in the development of a more extensive energy balance model, in the next chapter