12-06-2012, 04:45 PM
a low cost inverter for domestic fuel cell application
a low cost inverter for domestic fuel cell application.docx (Size: 476.73 KB / Downloads: 38)
INTRODUCTION
In the future, many local energy sources, such as photovoltaic units, fuel cells, small turbines, small hydroelectric plants, and other dispersed sources will become a larger fraction of our electrical supply.” This quote is taken from the 2001 Future Energy Challenge, a national US student competition sponsored and set up by the Department of Energy and the IEEE, which spanned Fall 2000 through Summer 2001. This is one of the important economically improved domestical usage.
FUTURE ENERGY CHALLENGE
The Challenge sought to “. . . dramatically improve the design and reduce the cost of dc-ac inverters and interface systems for use in distributed generation systems . . . with the goal of making these interface systems practical and cost effective. The objectives are to design elegant, manufacturable systems that would reduce the costs of commercial interface systems by at least 50% to below $50 per kilowatt and, thereby accelerate the deployment of distributed generation systems in homes and buildings.
No new technology was used in the design—just an optimized combination of current technologies. The paper discusses technical aspects of the topology used to achieve the said objective, the rationale used in choosing this topology, detailed component selection which minimized cost, and the control. Other papers cover issues such as the educational aspect of the UW’s involvement and other technical aspects such as project management and heatsink optimization.
Finally some conclusions are made and a new total system-approach design using a high voltage fuel cell is proposed to further reduce the cost of the inverter.
BACKGROUND
The competition objective was to design and build a system, namely an inverter, as shown in Fig.1, that changed a fuel cell’s variable dc output voltage into a standard US domestic 120/240 Vrms split-phase supply. Table-I on the following page shows the inverter’s specifications for the Challenge.
The advantage of using a fuel cell to provide the chemical to-electrical energy conversion is its high fuel-to-electrical energy efficiency of about 40% including system losses. This can be boosted to as high as 80% by using the heat by product for home water & space heating or cooling. The particular fuel cell cited for the competition used Proton Exchange Membrane (PEM) cells and had a fuel flow regulation system. This type of fuel cell had two important characteristics:
(i) the loaded output voltage was nominally 48 V but varied from 42 V to 60 V (open circuit voltage ~72 V).
(ii) the fuel cell had a slow response time which can be modeled by a first order system with ~ 40s.
Input Filter
The particular fuel cell used required the input current to stay within certain bounds; bounds dependent on the fuel flow. Furthermore, the input current ripple must remain within limits or damage could result; the maximum ripple specification is shown in Fig. 6. To keep the current ripple within the required bounds two steps were taken. Firstly, the 120 Hz power ripple was removed by using input current control—see Section. Secondly, the dc/dc converter switching ripple was filtered using an LC input filter. The selected filter values were 150 μF and 6 μH. The filter inductor’s cost was substantial, due to its 230 A average current rating.
High Frequency Transformer
At this point in time a normal ferrite ‘E’ core transformer is the least expensive HF transformer. However, in large volume mass production a planar transformer built into the structure of the PCB may be cost competitive. The planar transformer used in the 10 kW prototype had one primary turn and 14 secondary turns. A photograph of the transformer is shown in Fig. 7. The TO-247 MOSFET shows how small the transformer is. The transformer had a calculated loss of 40 W at full load and cost $40.
Intermediate DC-Link and Transient Energy Storage
Since the fuel cell responds slowly the load power would not match by the power output from the fuel cell during transients; there would be a power deficiency or excess. The fuel cell could be damaged if more current is taken than it can supply, so current demand should never exceed available current.