Life cycle assessment of an alkaline fuel cell CHP system

Abstract

A life cycle assessment (LCA) of an alkaline fuel cell based domestic combined heat and power (CHP) system is presented. Literature on non-noble, monopolar cell design and stack construction was reviewed, and used to produce a life cycle inventory for the construction of a 1 kW stack. Inventories for the ancillary components of other commercial fuel cell products were consulted, and combined with information on the fuel processing requirements of alkaline cells to suggest a hypothetical balance of plant that would be required to produce AC electricity and domestic grade heat from natural gas and air.

The emissions from manufacturing and disposing of this fuel cell CHP system were estimated to be equivalent to 510–1000 kg of CO₂ and 1.0–2.0 kg of particulate matter. As with platinum based polymer electrolyte fuel cells (PEMFC), emissions of sulphur dioxide were the most significant impact, resulting in degraded human health in the regions where catalyst metals are mined. Improving the operating lifetime and reducing catalyst loadings were identified as the most effective routes to reducing this environmental impact, as they are with other fuel cell technologies.

These impacts were compared to the results of existing LCAs for other fuel cell technologies. It was found that an alkaline fuel cell stack produces less environmental impact than an equivalent solid oxide or phosphoric acid (SOFC or PAFC) stack, while no conclusive comparison with PEMFC could be made. The inclusion of energy consumption during stack manufacture and data on the more exotic material inputs were highlighted as a problem in these studies.

Introduction

Fuel cells have the potential to lower the environmental burden of meeting domestic energy needs, particularly greenhouse gas emissions and primary energy consumption [1]. However, the manufacture and disposal of the fuel cell equipment creates additional environmental impacts which are not accounted for, and must be recouped during operation to avoid merely substituting one problem with another. Simplification of the fuel cell stack and ancillary equipment would lower these environmental impacts; as would a shift away from energy intensive production stages and catalyst metals won in heavily polluting mine operations.

In these respects, alkaline fuel cell (AFC) stacks address one of the major problems of more popular stack technologies by offering the prospects of cheaper and simpler construction. AFC stacks can be produced from relatively standard materials and do not require precious metals or energy intensive sintering, while their balance of plant is less complicated than that of other fuel cell systems [2], [3]. Even the perceived intolerance to carbon dioxide does not pose a significant problem, as cost effective removal can be achieved with chemical-, thermal- or electrical-swing adsorbers [4], [5], [6], [7].

Decentralised energy generation with domestic micro-CHP is one of the first markets to have been entered by fuel cells. Four prominent fuel cell technologies are suitable for this market, and are in different stages of commercial development. In Japan, fuel cell products based on polymer electrolyte membrane (PEMFC) stacks have moved from advanced field trials into the start of assembly-line production and full commercialisation. Solid oxide fuel cells (SOFC) are expected to reach the same stage by 2010–2012, with Japanese and European energy companies forming strategic distribution deals with leading manufacturers. Phosphoric acid fuel cells (PAFC) have been proven in industrial CHP systems over the last decade, and have been demonstrated as 1 kW-class CHP systems [8].

AFC is not widely regarded as a technology for domestic CHP, having been forgotten by many in the fuel cells community since the surge of interest in PEMFC and SOFC [3]. Alkaline was the first fuel cell technology to be demonstrated in practical applications, being used to power the 1960s space missions [9]. R&D applications branched out, but never extended beyond niche markets such as golf carts, forklifts and marine power generation due to technical issues with lifetime and degradation, and the widely held misconception that pure oxygen is required due to the trace amounts of carbon dioxide in air [2]. Several companies however remain active in developing AFC technology for stationary power generation, including Apollo Energy Systems, Gaskatel, Hydrocell and Independent Power Technologies.

Life Cycle Assessment (LCA) is a methodology for estimating the environmental impacts associated with a product or action, and is commonly associated with calculating the ‘carbon footprint’ of consumer products or the embodied energy in renewable technologies.[10], [11] The strength of this technique lies in its ability to consider the entire life cycle from cradle to grave, rather than just those actions directly related to the process in question. For example, the mining, refining and transport of fuel to a power station can be considered in addition to just the on-site emissions from combustion. LCA is therefore the ideal tool for comparing the environmental impacts of competing products, and identifying key areas where improvements could be made.

LCAs are typically produced using a software package which combines the building blocks needed to describe the construction, usage and disposal of a product with the means to assemble these into a complete life cycle and then analyse its environmental impacts. An example of this process (not including the definition of goal and scope) would be as follows:

• The individual stages required to produce a fuel cell system are identified, either by observing and interviewing manufacturers or from reviewing literature on cell design and construction;

• Each stage is broken down into sequentially smaller processes, giving a hierarchy that extends from extracting raw materials from the earth up to the final delivery of the system;

• An inventory is produced for each of these processes, giving the material and energy inputs and the waste or by-product outputs. Data is acquired from peer-reviewed inventory databases provided with many LCA software package, or from further research for the more uncommon processes;

• The hierarchy of processes and their inventories are entered into an LCA software package, which ties every raw material and emission to an environmental impact, and can therefore calculate the total impact of producing the fuel cell.

Much of the interest in AFCs faded out in the 1980s before the field of LCA gained recognition, and so there appear to be no previous LCAs of alkaline fuel cells, or the related alkaline electrolysis technology [2], [12].