Ballast or dump load
A ballast load is mostly an electrical resistance heater. It's sized to handle the
full generating capacity of the microhydro turbine. They're placed in air or water. If there is more electricity produced then consumed the charge controller uses this excess energy to generate heat.
Other but not common ballast load may be pumping water or ice production.
Load factor
The load factor is the amount of
power used divided by the amount of power that is available if the
turbine were
to be used continuously. Unlike technologies relying on costly fuel
sources,
the 'fuel' for hydropower generation is free and therefore the plant
becomes
more cost effective if run for a high percentage of the time. If the
turbine is
only used for domestic lighting in the evenings then the plant factor
will be
very low. If the turbine provides power for rural industry during the
day,
meets domestic demand during the evening, and maybe pumps water for
irrigation
in the evening, then the plant factor will be high.
It is very important to ensure a
high plant factor if the scheme is to be cost effective and this should
be
taken into account during the planning stage. Many schemes use a 'dump'
load
(in conjunction with an electronic load controller - see below), which
is
effectively a low priority energy demand that can accept surplus energy
when an
excess is produced e.g. water heating, storage heaters or storage
cookers.
Turbine types
[[Image:]]
A turbine converts the energy in
falling water into shaft power. There are various types of turbine which
can be categorized in one of several ways. The choice of turbine will depend mainly on the pressure head available and the design flow for the proposed hydropower installation. As shown in table 2 below, turbines are broadly divided into three groups; high, medium and low head, and into two categories: impulse and reaction.
Turbine
Runner
|
Head
pressure
|
|
High
|
Medium
|
Low
|
|
Impulse
|
Pelton
Turgo
Multi-jet Pelton
|
Crossflow
Turgo
Multi-jet Pelton
|
Crossflow
|
|
Reaction
|
Francis
Pump-as-turbine (PAT)
|
Propeller
Kaplan
|
|
The difference between impulse and
reaction can be explained simply by stating that the impulse
turbines
convert the kinetic energy of a jet of water in air into movement by
striking
turbine buckets or blades - there is no pressure reduction as the water
pressure is atmospheric on both sides of the impeller. The blades of a reaction
turbine, on the other hand, are totally immersed in the flow of water,
and the
angular as well as linear momentum of the water is converted into shaft
power -
the pressure of water leaving the runner is reduced to atmospheric or
lower.
Load control governors
Water turbines, like petrol or
diesel engines, will vary in speed as load is applied or relieved.
Although not
such a great problem with machinery which uses direct shaft power, this
speed
variation will seriously affect both frequency and voltage output from a
generator. Traditionally, complex hydraulic or mechanical speed
governors
altered flow as the load varied, but more recently an electronic load
controller (ELC) has been developed which has increased the simplicity
and
reliability of modern micro-hydro sets. The ELC prevents speed
variations by
continuously adding or subtracting an artificial load, so that in
effect, the
turbine is working permanently under full load. A further benefit is
that the
ELC has no moving parts, is very reliable and virtually maintenance
free. The
advent of electronic load control has allowed the introduction of simple and efficient, multi-jet turbines, no longer burdened by expensive hydraulic governors.
Controller:
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Load- or Flow- controller ensures that thepower output not exceeds the power demand.
If flow of water in a MHP-station is constant the energy output of a turbine/generator is constant as well. Power demand is usually fluctuating over the time (e.g. day/night). If supply is higher than demand, excess energy must be diverted / dumped. Alternatively the water flow can be reduced which results also in less power output.
In case of more power demand than supply the controller cuts of the of single users (clusters) in order to keep voltage and frequency constant (e.g. 230V, 50 Hz).
Load controller are placed between generator output and the consumer line.
Controller Types
Load controller:
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Electronic circuit, which keeps output power constant in Frequency- and Voltage- parameters.
Fluctuating energy demand requires a mechanism which either regulates the water input into the turbine (= flow control) or by diverting excess energy from the consumer connection (= ballast load).
Ballast load
usually electrical heaters in water or air. If energy demand is temporarily low the excess energy is converted into heat.
Flow control
regulates the amount of water into the turbine in order to match power output and power demand.
Nowadays flow control is done mostly via electronics (which steer a valve)
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