Alpen Steel | Renewable Energy

~ ST-5 Powered Electricity Generating System

Cogeneration - Total Home Energy System

The heat from your ST-5 powered electricity generating system can be recovered and used in a number of ways for space and domestic water heating. The following is an example of one possible system, to show you how this is feasible. It has been installed at the home of one of Stirling Technology's founders. This system comprises two parts, one for generating and storing electricity, the other for recovering and storing heat.

Electricity is generated by attaching a 4kW AC alternator to the ST-5. The engine has been sized to provide up to 3.5 kW of electricity, enough to take the surge loads of induction motors used in washing machines and other domestic appliances. One option would be to run the engine for 4 to 6 hours a day, during which period high and intermediate draw appliances - washing machine, freezers and power tools - can be used. At the same time, a bank of batteries can be charged to provide electricity for domestic lighting and other small load equipment when the engine is not operating.

A bank of 4 deep-cycle, extra heavy duty batteries, interfacing with the alternator through a commercially available battery charger should be sufficient. When deciding upon the type and number of batteries needed, the rule of thumb is that the battery bank should store about twice as many amp hours as will be used between recharging. This is essential to prolong the life of the batteries and get the best possible service from them. With the tremendous advancements in battery technology, it is now possible to get batteries capable of holding charges of over 600 amp hours. The life on these batteries is extremely long, with constant, uninterrupted service of 20 years recorded by some users. It is recommended that 6 or 12 volt batteries with charging capacities of 400 amp hours or more be used; 8 volt batteries are not recommended, as there could be problems finding matching inverters and other compatible equipment.

To view a simple diagram of an ST-5 powered electrical circuit . . . Click Here

The length of time it takes to charge batteries depends on the amount of charge necessary and the rate of charging. The slower the rate, the better it is for the life of the battery. The safest way to charge your battery bank would be to run the AC current from the alternator through a battery charger. To strike a balance between the time required and the rate of charging, it is recommended that a 200 to 300 amp charger be used. The latter would enable a 660 amp hour charge within 5 hours. (A 660 amp charge for a bank of four 12 volt batteries is about 8 kWh of stored electricity.)

Since Watts = volts x amps, a 12 volt battery with a 400 amp hour charge can deliver 12 x 400 = 4800 Watts for an hour, or 4.8 kWh. Again, the batteries should not be discharged to less than 50% of their capacity, so using a maximum of 2.4kWh per day per battery is optimal. Four batteries, then, would provide for usage of up to 9.6 kWh between charging.

The next step is converting the DC current to AC and feeding it into the electric line for the home. This is accomplished by means of a DC to AC inverter. These are available in sizes up to 12,000 Watts and in various types. The best type would be one with high surge capacities and a sine wave output. Sine wave inverters eliminate the hums and flutters experienced from appliances when using square wave inverters.

The correct size is arrived at by estimating the usage and surge load, and type of inverter. Some inverters allow for surge loads of up to 4 times their constant draw. These inverters could start most motors with a constant draw equal to the size of the inverters. If the inverter does not provide for surge loads, its size would have to be large enough to handle such loads. To be absolutely safe, the inverter should be able to handle the maximum possible load you anticipate. This, however, has one drawback. Inverters are most efficient (85 to 90%) while operating at or near their capacity. If the draw is below 40% of the capacity, efficiency drops rapidly. Hence, if the inverter is too large for your normal usage, you would be wasting a great deal of stored electric energy. Another possibility is obtaining modular inverters. This development in inverters lets you use a small size inverter for lower draws and merely hook on additions to increase the draw capability. Efficiency can be maximized by this means.

The connection between the direct AC line from the alternator to the house and the line from the batteries and inverter is in the form of a switch, which enables either one or the other source of electricity to be used. While the engine is in operation, the switch enables the current to be used directly from the alternator, while the batteries are charged. Otherwise, the switch is thrown to draw the current off the batteries.

A word of Caution: If you are connected to the grid and are considering using the ST-5 for emergency or backup power, observe all precautions mandated by your local utility.

In order to minimize the load on the batteries and the size of the battery bank, every attempt should be made to use as much power as possible while the ST-5 is in operation. Running the washing machine, freezer, vacuum cleaner, power tools etc. while the ST-5 is operating and using the energy stored in the battery bank for lights and other low draw appliances will prolong the life of your system while decreasing fuel usage.

 

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