Pathfinder - UCD Belfield District Heating System Heat Pump Retrofit

Objective

The 2023 Climate Action Plan reaffirmed several targets for public bodies that had been originally introduced in previous iterations of the plan.  These include a requirement that all public bodies must reduce their GHG emissions from burning fossil fuels by 51% by 2030, compared to a 2016-18 baseline. They must also reduce their electricity emissions in line with anticipated supply-side reductions.  SEAI's pathfinder programme has been working collaboratively with public sector organisations to develop scalable solutions to meet these targets, for benefit across the public sector and the wider non-domestic sector. 

UCD proposes to progress a dual strategy relating to thermal energy: 

• Reducing thermal demand on campus through the retrofit of buildings 
• Phased decarbonisation of thermal energy. 

This pathfinder project focusses on the second strand of the strategy.  By integrating a heat pump into the Energy Centre, the carbon intensity of heat supplied to all buildings on the district heating is reduced.  With the knowledge and experience gained through this project, UCD can decarbonise the Energy Centre on a phased basis with the installation of additional heat pump capacity over time and the decommissioning of the natural gas boilers and CHP units as they reach end of useful life.  Learnings are also relevant for large buildings with large standalone gas-fired boilers. 

Background

The UCD Belfield Campus District Heating System (DHS) was installed as part of the original campus construction phase c. 1968-1971.  The original system was turf-fired.  The system has been upgraded over time through the addition of modern types of heating generation (with turf-firing changing to heavy fuel oil, and to natural gas).  Since 1999, gas-fired CHP has been the primary heat source.  The energy centre also incorporates a 900 kW biomass boiler, and two condensing gas-fired boilers, used for peak and back-up load. 

Project Description

The Pathfinder project involved the integration of a 1 MW air source heat pump into the Energy Centre, which supplies heat to several buildings on campus via the district heating system.  The heat pump is a two-stage hydrocarbon air source heat pump manufactured by Solid Energy in Denmark.  The heat pump adds to the 1.2 MWe and 0.8 MWe CHP units, the 0.92 MW biomass boiler and 2 x 1.75 MW condensing gas boilers which were already installed in the Energy Centre. 

The operating philosophy for the Energy Centre is for the heat pump to run as lead plant, followed by the biomass boiler and CHP, with the gas boilers running as lag-lag, providing for peak heat demand.  This significantly reduces the requirement to run the gas boilers, with associated reduction in fossil fuel usage and carbon emissions. 

Careful control of the supply and return temperatures of the DHS is required to ensure that the heat pump can operate reliably.  The heat pump is positioned within the Energy Centre to ensure that the return temperature supplied to the unit is as low as possible, thus ensuring efficient operation. 

A high level of metering has been incorporated with the installation, both associated with the heat pump, the Energy Centre as a whole, and ambient conditions in the vicinity of the external unit.  This will allow further insights to be drawn regarding the operation of the heat pump installed within a mixed source system and will inform future installation of additional heat pump capacity. 

To progress with the procurement process as efficiently as possible with a significant lead time for the heat pump, UCD opted to procure the heat pump in advance of detailed engineering designers being appointed. This approach also allowed for closer control over equipment specification and selected technology, and allowed UCD to use 20- year life cycle costing as a scoring criterion for heat pump selection, with tenderers populating a SCOP calculator. 

The approach has necessitated high levels of involvement by UCD Energy Unit staff and might not suit an organisation with less in-house expertise. 

Performance

The project timelines have been longer than originally anticipated.  While delays may be partly attributed to the timelines for the project coinciding with the Covid pandemic and the war in Ukraine, there have been subsequent delays with the procurement of parts and material from overseas.  The lack of in-country experience and expertise with this type of installation has also resulted in a steep learning curve for all parties involved. 

Harmonic filters were fitted by UCD as part of this project given the scale of the compressor variable speed drives.  They recommend that the requirement for additional harmonic filters be confirmed with suppliers, particularly for projects involving heat pump systems with large inverters (VSDs). 

There have been significant learnings around the impact of domestic hot water (DHW) supply on the operation of the DHS.  Health and safety guidelines to protect against Legionella require that the water temperature be regularly raised to 60oC, limiting DHS efficiency gains during periods of low thermal demand.  BMS control strategies for the Energy Centre have required updating to ensure that hot water requirements are maintained. 

UCD plans to remove domestic hot water supply from the DHS.  This will allow the system to be optimised for space heating.  DHW demand is currently being metered to allow appropriate sizing of local hot water systems.  In many buildings, calorifiers have been found to be significantly oversized - the specification of standalone DHW systems will lead to further savings as the systems are downsized to meet actual building demand. 

UCD anticipates a multi-year programme to reduce return water temperature of the DHS, including;

• weather compensation of the DH temperatures, which will allow lower flow and return temperatures in mild weather 
• a programme to improve balancing in each building supplied by the DHS 
• in the longer term, to improve the thermal performance of buildings on the DHS to allow heat emitters to operate at lower flow-return temperatures. 

The net impact of this programme, in conjunction with greening of the electricity grid, is that carbon savings relative to the baseline will increase year on year.  The greening of the electricity grid will also reduce the carbon benefits associated with CHP operation over time; this would lead to an increase in carbon emissions towards 2030 for the baseline case. 

For the period April 2023 to 2024 (the first year of operation);

• the Energy Centre carbon emission factor was 0.14 kgCO2/kWh of delivered heat (compared to 0.21 kgCO2/kWh for the baseline year) 
• the heat pump operated at 1,679 full output equivalent hours (lower than anticipated) 
• the Seasonal Coefficient of Performance for the ASHP was 2.58 
• 192 tonnes CO2 was avoided through the operation of the ASHP. 

The monitoring and verification programme will continue over two more years to fully assess the impact of the heat pump and to track improvements.  UCD Is planning to continue its program of District Heating Heat Pump optimisation works with additional projects planned to help improve the synergy of the heat load to the Heat Pump which should help to increase the operating hours of the heat pump in the next reporting period. 

Insights

The project highlights that legacy design approaches are not always fully compatible with heat pump technology. Heat pump operating range flexibility (i.e. the ability to operate at different temperature ranges) is very important if integrating with larger legacy heating systems for which high to low operating temperature changes may be staged over several years.  

Another key learning from this project is the importance of decoupling DHW systems from the space heating network in systems where a heat pump has been integrated. This will facilitate optimal operation of the heat pump, ensuring maximum energy and carbon savings are achieved. In decoupling the DHW system, surveys of actual hot water demand may allow significant downsizing of newly specified standalone systems, leading to further savings.