Taking control of renewable energy
With the rapid growth in renewable energy technologies, effective control is vital in terms of maximising their potential. Mike Clinch and Yasir Sheikh of Trend Control Systems explain how building energy management systems (BEMS) are increasingly being used to monitor and manage biomass boilers.
The renewable heat and energy sector grew at its strongest ever pace last year and has attracted almost £30 billion of private-sector investment since 2010. This investment enabled it to sustain over 100 000 jobs in 2013, and initiatives such as the Renewable Heat Incentive (RHI), which offers financial rewards for investing in renewable heat, are driving up demand for biomass-based systems.
Biomass heating systems generate heat by burning organic materials — often in the form of wood pellets, chips and logs, but also animal and vegetable-derived material — in a boiler to supply central-heating and hot-water systems. Biomass is a theoretically inexhaustible fuel source and generally cleaner burning and more sustainable than fossil fuels. However, while the benefits of biomass are clear, maximising the efficiency and effectiveness of these systems requires the type of high-level monitoring and control functionality offered by a BEMS.
A BEMS can be configured for processes that are specific to biomass boilers, such as the following.
• cold start
• manual restart
• slumber mode
• burn-back protection
• automatic burn-back monitoring
• water dosing
• oxygen-level monitoring
• controlling variable-speed drives (VSDs) on flue fans
• stoker feed control
• cooling jacket pump control
Monitoring oxygen levels is also important.
If the oxygen level is too high and the boiler demand is also high, but the boiler temperature is still low, a controller can be used to add more fuel into the combustion chamber.
At lower boiler demand, a controller will permit a higher level of oxygen to avoid too much fuel being fed into the combustion chamber. This means that variable boiler outputs are achieved, and the boiler controllers will shift into slumber mode when the boiler flow temperatures are satisfied against their desired setpoints.
The installation of BEMS process control can reduce the amount of biomass fuel required without incurring a loss of heat output. Similarly, feed and ash auger motors can be protected with smart motor protection overloads that can stop the motors automatically when they sense overcurrent due to a jam. A control system can then allow an automatic attempt to clear the jam by reversing the motors for a few seconds before re-starting in the correct direction. Further prevention of wood jams can be further prevented by flush fitting proximity sensing probes with LED colour change indication mounted in a wood hopper. The visual indication on the probes will help to identify where a jam has occurred.
A BEMS can also play an instrumental role in reducing one of the most common issues relating to biomass boiler operation. Burn-backs are prevented by first automatically dosing wood chips with water through the pulsing water solenoid valves. Then, once the overheat sensors are satisfied, the stoker motors are reversed to draw back the wood chips from the flames, thereby preventing it happening again. As soon as the BEMS detects a stable condition, the process will attempt to restart.
Boiler dry cycling has been the cause of significant energy waste in non-domestic buildings and is caused by a boiler’s standing losses. It refers to the repeated firing of the plant to maintain its internal water temperature when there is no actual heating demand from the building. The problem is exacerbated if water continues to be pumped around the heating circuits, because the boilers then fire to compensate for thermal losses from the pipework system in order to maintain the desired fixed flow temperature.
To address this issue, a BEMS can be used to monitor a boiler’s flow and return temperatures and calculate the latter’s rate of decay. This gives it some measure of actual heating demand, thus enabling it to hold the boiler off for relatively long periods in certain conditions.
Although the use of renewable-energy technologies such as biomass boilers should be welcomed, optimising their use requires effective monitoring and control. Not only will this ensure that they operate as smoothly as possible, the use of a BEMS can also help reduce maintenance, solve any problems quickly, enhance the product’s lifecycle and improve overall efficiency.
