Sydney Water has been utilising biogas to operate cogeneration assets, which produce electricity and heat, since 1999. The renewable energy program has expanded over the last 15 years, and the cogeneration plants now produce around 15 per cent of the organisation’s energy needs, providing significant financial benefits from reduced electricity costs, a hedge against future power cost increases, as well as a green commodity value.
Biogas is produced in anaerobic digesters at wastewater treatment plants across Sydney and contains between 55-65 per cent methane.
Sydney Water now has cogeneration units at eight wastewater treatment plants with an installed capacity of 9.8MW following the installation of new cogeneration assets in 2009 and 2014.
Sydney Water has followed a staged approach in implementing cogeneration. The technology was first implemented at Sydney Water in 1999 and in 2007 a renewable energy program commenced. Several sites were identified where cogeneration was calculated to be financially feasible and facilities were installed at these sites during 2009 and 2010.
Optimisation programs were undertaken between 2010 and 2012, aiming to realise the full potential of the new and existing cogeneration fleet.
A second program examined sites with excess biogas or sites with anaerobic digestion and no cogeneration unit. This resulted in further expansion of cogeneration in 2014 at Cronulla and North Head wastewater treatment plants.
Some sites currently remain unviable and will be reviewed again as key success factors change. Sydney Water will continue to investigate ways to get more value from cogeneration in the future. This includes gaining a better understanding of co-digestion and other technologies including the Organic Rankine Cycle.
Sydney Water has gained significant value from cogeneration and continues to invest in it. While the primary benefit of cogeneration is the financial returns achieved, ongoing value from cogeneration also extends to the environment and plant performance.
The installed cogeneration plants have a capacity to generate over 60,000MWh of electricity per year, reducing over 60,000 tonnes of greenhouse gas emissions, which is the equivalent saving of removing 15,000 cars from the road.
Lessons learned – installation
Fifteen years of planning, installation, maintenance and operation in the cogeneration space has provided Sydney Water with significant insights. These insights have been amplified due to the technical expertise provided by Sydney Water contractors.
Alex Sanbrook, Renewable Energy Generation Manager at Sydney Water said “Prior to installation it is important for utilities to have a full understanding of all capital investment requirements to ensure value from cogeneration.
“Consideration must be given to ensure appropriate engine sizing, plant type, biogas supply/optimisation and measurement, biogas pre-treatment requirements and to the high voltage connections for large engines.
“We learned pretty quickly that engines below 200kW, are generally not financially viable for Sydney Water.
“Allowance must also be made for site factors such as corrosion, emission requirements, access and maintainability of engines.
“Understanding maintenance costs and developing maintenance strategies compared to those recommended by the manufacturers, as well as any auxiliary equipment costs required and the complexity of the operation are also key considerations.
”Correct assumptions during the evaluation process around costs, onsite power utilisation, minimisation of energy export, electricity price, green commodity value forecasting, as well as accurate prediction of engine downtime and the timing for implementation are keys to success.
“Identifying additional value from cogeneration assets beyond financial benefits from electricity production is a valid approach. A focus on optimising gas production can lead to better biosolids management”, said Ms Sanbrook.
Lessons learned – operation
According to Ms Sanbrook, during an optimisation program between 2010 and 2012, Sydney Water increased generation from cogeneration by 10 per cent in the first year and a further 5 per cent in the second year.
“We have continued to incorporate improvement planning into our management of renewable energy assets,” said Ms Sanbrook.
“Root cause analysis showed that downtime is the greatest cause of lost generation. Without standby units, any downtime represents lost revenue.
“Maintenance planning remains the biggest challenge in reducing downtime. A high level of unplanned maintenance is required on biogas engines and coordinating these maintenance needs across ten sites with a small team is very challenging.
“Technical expertise of the maintenance provider is essential. The provision of fast diagnosis and innovative solutions are important elements in reducing downtime. Coupled with this, parts management is crucial because waiting for delivery of engine parts can cause significant delays.
“Good performance monitoring provided us with the foundation for identifying and monitoring improvements. Performance was constantly challenged. We soon discovered that cogeneration units running on biogas do not all respond equally. This created differences in maintenance requirements depending on engine model and biogas composition.
“Biogas pre-treatment for removal of hydrogen sulphides and siloxanes needs to be considered on a site-by-site basis to determine whether the cost of biogas cleaning is offset by reduced engine maintenance.
“Optimisation of digester gas production and accurate gas flow measurement is important and can be done before cogeneration is installed to ensure appropriate sizing of engines and that the full potential of the asset is realised.
“Stakeholder engagement can improve performance. Operator training and sharing of lessons can assist in identifying causes of lower biogas production”, said Ms Sanbrook.
As electricity exported to the grid receives a much lower return than the savings generated by offsetting imported grid electricity, there is significant benefit in maximising the onsite usage of electricity produced from cogeneration.
Electrical load studies have provided data to allow Sydney Water to reduce export over the last few years.
Practical limitations on operating and maintaining cogeneration units are now well understood, including operator capability, realistic downtime and major periodic maintenance requirements. These limitations can now be built into business cases for future investment in cogeneration units, which gives business owners greater confidence in achieving a positive financial return.
Utilising excess biogas – a case study
In 2012, Sydney Water carried out an investigation into the feasibility of new or additional cogeneration capacity at four wastewater treatment plants – Shellharbour, West Hornsby, Cronulla and North Head.
Installation of cogeneration at West Hornsby was not considered feasible as the plant had low gas production and Shellharbour is not currently cost-effective as the result of low biogas production (<200kW) and low electricity rates. However, a future business case may be submitted if a more cost-effective solution or a method to increase biogas production can be implemented.
A $6million project was approved to utilise excess biogas at Cronulla and North Head. The project upgraded the existing cogeneration unit at Cronulla from 475kW to 835kW and installed an additional 1MW cogeneration unit at the North Head Wastewater Treatment Plant. The project was aligned with Sydney Water’s Energy and Greenhouse Gas Mitigation Strategy to provide cost effective reduction of greenhouse gas emissions by on-site generation of renewable energy. The project’s business case provided a net present value of $3.5million.
Sydney Water’s lessons from installing and operating cogeneration were incorporated into these business cases along with an understanding of electricity and green certificate forecast rates, government policy, current price trends and electricity demand forecasts.
Minimising export at North Head was an important consideration as the site had two existing renewable energy assets; a 1.4MW cogeneration unit and a 2.1MW mini-hydro plant. The cost of an 11kV ring main unit to enable more versatile utilisation of power from the new engine to the site was incorporated into the business case to help minimise the export of renewable power.
The North Head project was delivered under a Design and Construct contract during 2014. This method of project delivery and preparing an advanced concept design prior to tendering helped manage capital costs.
Treatment process improvements are being implemented as a result of digester audit recommendations. Studies are underway to improve gas production, storage and distribution at some sites.
Sydney Water is trialling the co-digestion of trucked organic waste with sewage sludge. This involves receiving trucked organic waste for a fee and adding it to a sludge digester to increase biogas production. Glycerol is currently being trialled at Bondi. Other waste materials including beverage waste, pulped food waste, dairy, bakery and fats, oils and grease will be trialled at other sites over the next three years. Co-digestion has the potential to double biogas generation, which could see some wastewater treatment plants become energy neutral and make cogeneration cost-effective at plants that were previously not viable.
The Organic Rankine Cycle technology makes use of waste heat from the cogeneration units to produce additional electricity. This process requires higher temperatures and is more suitable from exhaust heat recovery rather than the lower temperature engine jacket water. Sydney Water is currently investigating the technology. One study showed that an additional 100kW could be generated using heat from the new 1,000kW cogeneration unit at North Head.
While the primary benefit of cogeneration is the financial returns achieved, ongoing value from cogeneration also extends to the environment, plant performance as
well as contributing to a liveable city.
Any current limiting factors such as achievable production rates and the market value of energy are constantly changing. Decisions on sites where cogeneration implementation or expansion is not currently feasible should be regularly reviewed.
For further details contact Alex Sanbrook, Renewable Energy Generation Manager at Sydney Water via email at [email protected]