Before discussing the different measures in more detail, it is important to first recapitulate the definition and quantification of energy use. The key issue is that energy inputs and outputs come in different forms having differing utility, in particular chemical (for example, heating value of fuel), electricity, and heat. While one could simply add different energy types by unit conversion, different forms of energy are not interchangeable. For example, it takes more than one unit of heat to make one unit of electricity. One approach to put different energy forms on a comparable basis is the idea of primary (or source) energy. The precise definition of primary energy varies, but in general it includes the heat or fossil inputs needed to make electricity. In some cases, it also includes indirect energy use associated with delivering fossil fuels. For the U.S. energy system, one common conversion used is 3.4 megajoules of upstream primary energy per kilowatt hour of electricity. Analysts often use different definitions of energy and do not always explicitly state which definition is being used. Care is needed when comparing energy results from different studies.

Energy outputs generally include only those utilized; that is, waste heat is not included. Energy output is estimated by direct unit conversion, conversion to source energy, or in some cases, using the energy needed to make products that coproducts replace. NEV forms the difference.

NEV = Energy outputs of fuels and products – Energy inputs.

Fossil inputs measure quantities of fossil fuels used in processing. EROI and NEV refer generically to energy, which could be supplied by fossil or renewable forms. Fossil inputs are thus specific to how heat and electricity are supplied and thus require definition of the enveloping energy system.

4.4.6 Information and Data Gaps

Much uncertainty remains as to the current and future energy properties of algal cultivation systems, pointing to critical gaps. Scarcity of data on material flows at existing scales of algal biofuel production presents a challenge in assessing EROI. LCA studies of algae use process modeling to estimate energy consumption. Additional empirical data can help validate these models.

Although there are gaps in data, data collection by itself will not resolve the uncertainties of life-cycle energy implications of algal biofuels. The true energy behavior is a result of scale-up and learning processes that bring algae from the laboratory and pilot scale to industrial scale. While future energy behavior is challenging to forecast, given the substantial investment and path dependence associated with bringing an energy technology to scale, due diligence demands that a serious forecasting effort be made. While there are a variety of cost forecasting methods available, such as learning curves and scaling factors, methods to forecast energy and material flows of developing technologies are undeveloped. Efforts need to be made to develop such methods. Increased data availability from laboratory and pilot scales is critical to calibrate and validate the forecasting methods that emerge.

4.5 CONCLUSIONS

A review of published literature suggests that the scale-up of algal biofuel production to yield 37.8 billion liters of algal oil (10 billion gallons) would place an unsustainable demand on energy, water, and nutrients with current technology and knowledge. Estimated



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