Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 Page 9 Page 10 Page 11 Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Page 20 Page 21 Page 22 Page 23 Page 24 Page 25 Page 26 Page 27 Page 28 Page 29 Page 30 Page 31 Page 32 Page 33 Page 34 Page 35 Page 36 Page 37 Page 38 Page 39 Page 40 Page 41 Page 42 Page 43 Page 44 Page 45 Page 46 Page 47 Page 4826 JANUARY/FEBRUARY 2017 • FOGHORN COST OF ELECTRICAL ENERGY Unlike diesel fuel, there are generally three parts to the cost of electrical energy; basic charge, energy charge and demand charge. You’ll need to check with your local utility to verify exactly how they charge for the power you receive. The basic charge is small and applies to all customers in a certain rate schedule and does not vary with the amount of energy used. Energy cost is pretty much what you might guess; you pay so much for each unit of energy (KWH) consumed. This will vary tremendously be geographic region and source of the power. Demand cost is dependent upon the maximum amount of power you use in the billing month. You can use the same amount of energy in a month, but using a lot of power in a short amount of time will cost more than using less power over a longer period of time because of the demand charge. The reason for this is simple; the size of the equipment (transformers, cables, etc.) used to supply the energy is based on the maximum amount of power required. If you need a lot of power, then the utility must install larger trans- formers, cables, conduit, protective devices, etc. Understanding demand costs is very important when considering how to charge and size your batteries. OPERATING SCENARIO AND CHARGING OPTIONS SINGLE OVERNIGHT CHARGE Using the example we’ve started, if charging is performed once per day, the propulsion energy required to be 5117 kWh. Also, the total system is ap- proximately 90% efficient, so the total required utility energy is 5117 kWh/0.9 = 5685 kWh. Given we’re operating 18 hours/ day, which leaves six hours to charge the batteries, so: 5685 kWh/6 hours = 948 kW is required. At 480Vac, this would be 1140 amps; three 400A connectors At 690Vac, 793 amps; two 400A connectors Full recharge after each run (charge sustain- ing mode) As shown above, we need 284.3 kWH for each run, and have 20 minutes to charge, so: 284.3 kWh/0.33 hours/90% efficiency = 948.6 kW is required. Coincidentally, the same as the single overnight charge. And plugging in 2-3 400A connectors after every run would be extremely awkward for the crew. BUT, the battery size would only need to be around 600 kWh (assuming not more than 50% depth of discharge, DoD). This is MUCH less than an 11MWh bank required for single overnight charging (again, assuming a 50% DoD). PARTIAL RECHARGE AFTER EACH RUN (CHARGE DEPLETION MODE) If we conduct a partial recharge after each run, with a single 400A connector at 690Vac, for 20 minutes, this recharges 159.3 kWh of energy (of the 284.3 kWh used), which means we’ll actually be down only 124.9 kWh for each round trip, so we’ll need 124.9 kWh x 18 trips / 90% efficiency = 2500 kWh of charge during the night. This results in 417 kW of power required; much less than the single night charge TECHNOLOGY