With the ability to attain overall efficiencies above 90%, micro-Cogeneration units meet the demand for heating, space heating and/or hot water (and potentially cooling) in buildings, while providing electricity to replace or supplement the grid supply.
As per the micro-Cogeneration definition featured in the European Union’s Energy Efficiency Directive, micro-Cogeneration can be applied in private dwellings, public and commercial buildings to supply a range of heat usages.
The reduction in primary energy usage results in greenhouse gas (GHG) emission reductions and the mode of operation of the micro-Cogeneration unit can support the grid integration of variable renewables.
The large majority of commercially available micro-Cogeneration technologies are based on Stirling engine, Organic Rankine Cycle or internal combustion engine (ICE) technologies, characterised by high heat-to-power ratios. This makes them most suitable for installation in existing buildings. Newer technologies based on fuel cells were launched onto the market by the then largest field trial in Europe, ene.field project, successful predecessor of the PACE project.
The most highly-efficient fuel cell micro-CHP technologies can be operated according to electricity demand when installed in new low-energy buildings-but are also suitable for existing buildings.
Fuel Cell micro-Cogeneration
Fuel Cell micro-Cogeneration (also knowns as Stationary Fuel Cells, Fuel Cells micro-CHP, Fuel Cells Micro-Combined Heat and Power), is a technology that uses a single fuel (hydrogen, natural gas or LPG) to produce both heat and electricity for a building.
Compared to other micro-Cogeneration technologies the Fuel Cell micro-Cogeneration has a low ‘heat-to-power ratio’(meaning it produces a relatively low amount of heat and a relatively high amount of electricity compared to other micro-Cogeneration technologies) – so is well suited to the evolving trend in buildings towards higher electricity use and low space heating demand.
There is growing potential for the use of these micro-cogeneration systems in the residential sector since they have the ability to efficiently produce both useful thermal energy and electricity from a single source of fuel.
These products aim at meeting the electrical and thermal demands of a building for space, the domestic hot water heating, and potentially, cooling absorption.
The fuel cell works by combining hydrogen produced from the fuel and oxygen from the air to generate dc power, water, and heat. A system must be built around the fuel cells to supply air and clean fuel, convert the power to a more usable form such as grid quality ac power, and remove the depleted reactants and heat that are produced by the reactions in the cells.
Water is created in the electrochemical reaction, and then pushed out of the cell with excess flow of oxygen. The net result of these simultaneous reactions is current of electrons through an external circuit-direct electrical current. The hydrogen side is negative and it is called the anode, whereas the oxygen side of the fuel cell is positive and it is called the cathode. Each cell generates about 1 V, so more cells are needed in series to generate some practical voltages.
Two fuel cell technologies will be deployed in the trials: PEM and SOFC fuel cells.
At the heart of a PEM fuel cell, there is a polymer membrane that has some unique capabilities. It is impermeable to gases, but it conducts protons (hence the name, proton exchange membrane). At the interface between the porous electrode and the polymer membrane, there is a layer with catalyst particles, typically platinum supported on carbon electro-chemical reactions happen at the surface of the catalyst at the interface between the electrolyte and the membrane.
Hydrogen, which is fed on one side of the membrane, splits into its primary constituents – protons and electrons. Protons travel through the membrane, whereas the electrons travel through electrically conductive electrodes, through the outside circuit where they perform useful work and come back to the other side of the membrane. At the catalyst sites between the membrane and the other electrode they meet with the protons that went through the membrane and oxygen that is fed on that side of the membrane.
As with the other cell types, it is necessary to stack SOFCs to increase the voltage and produce power. Since there are no liquid components, the SOFC can be cast into flexible shapes: tubular or planar. Development work for cells operating at 1000°C is focused on increasing the mechanical resistance of the cell materials to alleviate the impact of thermal mismatch and to develop techniques that will decrease interfacial changes of the various material layers during thin film cell fabrication.
Fuel cell technology offers many potential benefits as a distributed generation system. They are small and modular and capital costs are relatively insensitive to scale. This makes them ideal candidates for a diverse amount of applications where they can be matched to meet specific load requirements. The systems are unobtrusive with very low noise levels and have negligible air emissions.
These qualities enable them to be placed close to the source of power demand. Fuel cells also offer higher efficiencies than conventional plants.