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  • Writer's pictureThomas McCrossin

110 Fetter Lane Combined Chiller & Trigeneration Optimisation

Project Aim

To meet carbon friendly planning targets, the original design of 110 Fetter Lane included a Combined Cooling, Heating, and Power (CCHP) system; the site team identified that it was not operating to expected efficiency levels.

A CCHP system, also known as trigeneration, incorporates an absorption chiller that utilises the waste heat from the CHP component to generate chilled water (CHW). Despite being difficult to commission and at times challenging to operate, it is an efficient and worthwhile process. One potential complication is that the absorption chiller may "steal" more heat from the LTHW system than is generated by the CHP. This happens when the boilers operate to top-up the heat in the system so that CHW can be generated; this is inefficient.

In preventing this issue at 110 Fetter Lane, the existing control strategy was overzealous and significantly reduced the trigeneration operation; Cavendish Engineers were tasked with fixing this.

Several other issues were identified during the project; a lack of CCHP & boiler modulation, poor CCHP & boiler efficiencies, disingenuous demand, and limited control capabilities. Correcting these was incorporated into the overarching project goal – to ensure that the systems with 110 Fetter Lane operate as efficiently as they possibly can.

Project Achievements

Please see below for results breakdown and supporting consumption data.

For the 12 months post the projects Aug 2017 completion, raw sub-metering consumption data (not normalised for temperature) captured the following changes:

• CHW electricity consumption reduction of 200,129 kWh (or 14%)

• Trigeneration electricity generation increase of 70,247 kWh (or 20%).

• Boiler gas consumption increase of 106,653 kWh (or 4%).

For the 12 months through to the latest available data, Feb 2019, raw sub-metering consumption data (not normalised for temperature) captured the following changes:

• CHW electricity consumption reduction of 206,862 kWh (or 14%)

• Trigeneration electricity generation increase of 51,182 kWh (or 15%).

• Boiler gas consumption increase of 17,975 kWh (or 1%).

As stated, these figures have not been normalised for temperature. Page 4 of the Appendix captures both Cooling Degree Days and Heating Degree Days.

What is interesting to note is that the most recent 12 months (through Feb 2019) has seen the number of CDD (18°C) increase by 63% yet the amount of energy used to generate CHW at 110 Fetter Lane has decreased. Considering this we are expecting the CHW electricity consumption decrease to reach approx. 20% once temperatures normalise with the baseline period.

Completed in 2010, 110 Fetter Lane (better known as the Rolls Building) is the world’s largest commercial courts complex for the resolution of financial, business and property litigation.

The primary occupier is Her Majesty’s Court Service which pre-let approx. 17,200 m² prior to construction, resulting in a tailored design to provide 31 courtrooms, including three "super courts" for high-value cases and four landscape-oriented courtrooms for multi-party cases. An additional approx. 7,900m² of commercial space is also available over the top floors and basement for lease and these spaces are typically occupied by law firms.

With a BREAAM rating of “Excellent”, the sustainability strategy for 110 Fetter Lane incorporated innovative and energy-efficient solutions (such as the trigeneration) however design shortcomings resulted in less than ideal energy performance.

The energy objectives for the 110 Fetter Lane Combined Chiller & Trigeneration Optimisation project was two-fold; a clear 100,000 kWh per year reduction over the chiller side of the project and an equivalent improvement in the output over the trigeneration side of the project.

The optimisation of the combined chiller and trigeneration system at 110 Fetter Lane has resulted in clear evidence-based improvements. Sub-metering data captures a sustained 15% reduction in chilled water electricity consumption; this equates to 200,000 kWh (or 70,000 kgCO2e) per year. Also, the electrical output of the CHP was increased by 15%, meaning that 50,000 kWh (or 17,500 kgCO2e) less per year is required from the grid. This was all done without increasing the amount of gas required to power the system, which in turn results in lower CO2 emissions into London’s already polluted air.

Since its completion this project was shortlisted in the Energy Management Initiative category at the 2019 CIBSE Building Performance Awards. At a cost of roughly £87,000, the initiative’s ROI is trending towards 3-4 years.

Working with the site team, Cavendish Engineers performed the following:

• Installed inverters over multiple LTHW/CHP/Transition pumps and programmed them to modulate based on individual system requirements.

• Replaced the CHP heat exchanger plates to maximise heat transfer into the LTHW circuit and reduce reliance on boilers.

• Installed back-end valves on the boilers to direct flow into the CHP unit and prevent boiler losses.

• Connected communications from locally controlled assets into the existing BMS system thus allowing more informed control inputs/outputs between the systems.

• Installation of BACnet control and communication with the chillers.

• Identification and removal of disingenuous building demands to ensure all system operation only occurred to meet a meaningful end of line demand.

Many of the challenges found on site were overcome through smart and hard work, along with constant dialogue between the site team, George Birchalls, and Cavendish Engineers. Ongoing tuning and optimisation would have been impossible without the site team constantly helping through adjusting controls, tweaking dead bands, and relocating sensors to remove disingenuous demands.

One particular challenge for the project was to utilise (where possible) existing connections to minimise costs. Where equipment was found to be the cause of poor operation, research of suitable alternatives that matched these existing connections was a demanding yet necessary (and sometimes fruitless) task. Other major challenges included:

• The longer than usual and irregular operating demands of the building resulted in any changes to the services requiring planning well in advance to avoid disruption to the occupants' sensitive activities.

• Where controls were found to restrict performance, introduction of alternative technologies for the controls (on equipment designed 10 years before the project) required Cavendish Engineers to write complex algorithms to achieve an output that comes standard on most modern equipment.

• The boilers hydraulics were the source of the largest system performance issues. The original design used full flow circulation through all 9 modules of the boiler system, which resulted in large heat losses and prevented the system from capturing CHP heat. To improve the transfer of heat the hydraulics needed to be adjusted and after investigation, retrofitted control valves were introduced on each array of boiler modules to reduce the total flow.

• Due to the lack of automated meter reading (AMR) data, reviews of building operation needed to take place through the BMS. Trend logs were added to key points and these found that the HVAC plant was actually operating 24/7, mostly due to unnecessary CHW demand. This in turn was the starting point for the chiller optimisation part of the project.

As the frequency of comfort complaints (often encountered when large scale CHW and LTHW modifications are performed) did not change through the duration of the project, we are confident that our preventative measures and advanced planning paid off.

Post project completion was supported through a Soft Landings program. This involved continuous support post project completion to ensure that a) the system is operating as intended, b) any new or related issues can be addressed to further improve the performance of the system, and c) the site team have access to guidance from those.

Successful completion of this project as further strengthened Cavendish Engineers’ view that buildings in the UK are seldom optimised in terms of operational performance, especially those involving landlords, managing agents with complex supply chains, and tenants.

The technical nature of the 110 Fetter Lane Combined Chiller & Trigeneration Optimisation project means it is difficult for an individual or even an engineering site team to understand how to approach (their equivalent) problem.

As every building has its own unique sizing and hydraulic demands, trying to communicate how this combined chiller & trigeneration optimisation can be applied at other sites is not easy - each site team needs to assess its applicability on their own.

This is a worthwhile exercise given the potential financial savings associated with efficient trigeneration operation, as well as the benefits experienced by the local community and environment through lower electricity consumption and carbon dioxide emissions.

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