Hydro Systems Using LCBs

Paul Cunningham

For a given magnetic field, and driven by a water jet of given pressure and flow, a DC hydro generator will produce its greatest power at a certain combination of voltage and current. This combination is called the maximum power point". The problem of running a PM generator at near its maximum power point voltage while charging a battery at a significantly lower voltage has a simple solution --- a linear current booster or LCB.

Conditions 

Most hydro machines will only perform well under certain conditions and only perform at their best under one set of conditions. Using a variable field, as with automotive alternators, is one solution. These machines can be used with an RPM range of around l000 to 4000 or more. Although these alternators are low in cost and fairly reliable, they have low efficiency, typically 50% or less, depending on conditions. So with variable field strength, controlled electronically or with a rheostat, an optimum match between input power and output power can be made. 

Improving the situation 

Let's look at some ways of improving the situation. The automotive alternators have a place. But at low-head sites they work poorly or not at all. The problem is made worse because they not only are less efficient at low speeds, but more power is required to operate the field as the speed (head) is reduced. The only practical solution is a generator that uses permanent magnets (PM) for the field. This can be done using either stationary magnets with a rotating armature like DC motors have, or the rotor can contain the magnets with the armature and its coil of wire being stationary. Either way, the permanent magnets supply the magnetic flux that moves in relation to the output coils (where the power is generated). Because no energy is added to produce the magnetic field (and for other reasons) permanent magnet generators are significantly more efficient than their wound field counterparts. PM hydro machines can operate at very low heads and low rates of water flow because of their higher efficiency. 

Half Solved 

This is a step in the right direction, but the problem is only half solved, The field strength must be controlled (or some other techniques used) to produce optimum output. One way is to custom build each generator for each site (ARGH!). Another is to mechanically adjust the distance of the magnets from the armature (ARGH again!). In the case of stationary coils and PM rotors, it is possible in some designs to reconnect the output coils to vary the loading. But this cannot be done in small increments. And I won't even discuss mechanical drives like belts and pulleys for these very small machines. This is because of their complexity and losses.

Maximum Power Point Trackers 

Wouldn't it be nice if this could be done electronically so one machine could be used at widely differing sites? There are device called Maximum Power Point Trackers that do this. The  automatically seek out the best operating point of a power source and effectively match the power source to the load. The only ones I know of are very expensive. We aren't going to benefit if the operation is successful but the financial strain kills the patient. 

LCB 

Recently, I used a standard LCB (linear current booster) made by Bobier Electronics (type 3-4-8-T) with a permanent magnet, DC hydro machine and had excellent results. This machine (model DCT-1) could charge a 12VDC battery with a five foot head. I wanted to operate it at a 15 foot head. This meant that if the PM generator was connected directly to the battery it would run too slowly and the power output would decrease. The PM generator would produce a higher power output if the generator could turn faster which meant operating the system at a higher battery voltage. The optimum voltage increases in proportion to the speed of the PM generator. With an LCB the generator can operate at this higher optimum voltage and in a sense trade voltage for current thus charging batteries at their voltage. 

Easily Retrofitted 

An LCB can easily be retrofitted to a hydro site. If you have a PM generator, or In some cases an induction machine, you may benefit. With a PM generator, if the no-load voltage exceeds twice the battery voltage, a performance increase is possible. The installation of the LCB is very simple. It should be installed according to instructions as if it were operating in a PV system, see the figure below. 

The LCB should be mounted neat the battery bank. Then it can simply be adjusted for maximum output current. This is a nonstandard use of the LCB and you are advised to use an LCB with twice the current rating of the PM generator.

The Proof of the Pudding 

At the 15 foot head site, the no load voltage was around 47 VDC. This meant that the correct voltage under load should be about 23 Volts. By using a variable resistance, I determined that the maximum power point was to 22.1 VDC and 2.1 Amperes giving 46.4 Watts. Connecting the generator directly to a 12 VDC battery produced 3.0 Amperes and 12.5 VDC or 37.5 Watts. This is about 81% of the maximum that was produced at 22 VDC. Using the LCB in the circuit produced an output of 3.6 Amperes at 12.6 VDC giving 45.4 Watts. This means the efficiency of the whole system with the LCB is around 98%. It is important to note that the power increases will rise as the difference between generated voltage and battery voltage increases. LCBs are available from Bobier that are rated up to 250 VDC.

Other Benefits

There are other benefits from using an LCB. Whenever nozzles are changed, the machine can easily be re-adjusted for maximum performance. Another plus is that the generator voltage is increased which greatly reduces transmission line losses.

Bobier has just introduced new models of LCBs. Devices specifically designed for use with batteries must be ordered.

Access

Paul Cunningham, Energy Systems & Design,
P.O. Box 4557, Sussex, NB Canada E4E 5L7
(506) 433-3151

Bobier Electronics, 800-222-3982