Applications based on renewable energy are thriving. This is leading to a requirement for increased testing of devices that generate renewable DC power—devices like solar panels, fuel cells, and batteries, to name a few. This testing is typically by employing electronic loads, mostly programmable and with a design that can draw various specified amounts of power from the source. In the lab or on the production floor an electronic load is the most suitable instrument to characterize devices producing DC output.
Selection of an electronic load requires careful consideration of several options like the voltage, current, and power ratings; operating modes; cooling methods; transient response times; calibration techniques; computer interfaces; and protective features.
Starting with the choices for voltage, current, and power ratings, most users also look for subtleties like the need for a load capable of sinking high currents at very low voltages. The cooling method is typically based on power rating, either a water-cooled device or an air-cooled one. Air-cooled loads have the advantage of flexibility—they can be self-contained, capable of being moved anywhere in the facility without the need for plumbing. On the other hand, water-cooled loads are smaller and less expensive as compared to air-cooled loads of the same power rating. Moreover, water-cooled loads will not load the HVAC system with extra heat generation. Usually, the HVAC system may not consider a 1 kW air-cooled load as a burden, but a 50 kW air-cooled load will certainly tax the HVAC system.
A number of factors determine the exact power level above which a user might consider a water-cooled load as preferable. Apart from the application, this might include the space and facility available. Most programmable electronic loads employ field-effect transistors or FETs. According to a rule of thumb, the air-cooled design uses only 50% of the capacity of each FET, and a water-cooled design uses up to about 85%. This results in a 35% saving in the number of FETs at a given power level for a water-cooled load. Not only does this lead to a reduction is costs, but also space requirements. For instance, at a 7.5 kW rating, an air-cooled load can cost roughly twice as much for a water-cooled load.
On the other hand, water-cooled loads lack the flexibility that is inherent in an air-cooled unit. Moreover, to use a water-cooled load, the user must install a water-cooling infrastructure, such as a chiller and associated plumbing. Depending on the layout of the user’s facility, this might be a costly and difficult task. Moreover, a chiller may need an expansion in the future, and the plan must accommodate it.
Operating modes need consideration next. Broadly, electronic loads operate in two modes—constant current and constant voltage. The constant current mode allows the load to sink a specific current, irrespective of the input voltage, provided the load’s specifications are not exceeded. In the constant voltage mode, the load will sink variable amounts of current to maintain a constant voltage at its input. Some loads will also offer additional modes like constant-power and constant-resistance modes.