The purpose of this project was to bring CO₂ refrigeration to the BP estate and ensure the maximum value and benefit in terms of environmental impact was achieved

As key supplier Carter Synergy designed and provided an option for a fully integrated refrigeration solution the key aspects of which were:

  • To provide all refrigeration capacity
  • To provide a second temperature band for air conditioning
  • To provide all building heat through heat recovery

The design was presented and approved in late 2013 with the St Ives store nominated for a trial and the installation complete and running by July 2014.

Following the installation, a period of monitoring in direct comparison with a store of the same format in Cockfosters was initiated and continues to date. 

This document gives an overview of the scheme and details the findings of this monitoring so far.

General Installation & Componentry


Central to this scheme is a Green & Cool dual temperature CO₂ refrigeration pack, this is complete with PLC and plate heat exchanger/3 way valve arrangement for heat recovery. The two temperature bands in this instance are -7°C (SST) for MT refrigeration and 7°C (SST) for air conditioning. All Green & Cool systems are designed to contain refrigerant (system resilience) and as a result, PRV setpoints are 80 Bar liquid & 60 Bar suction to ensure refrigerant charge is held under power failure.

The pack is an external unit with an integrated gas cooler to offer a compact footprint suitable for this small format application.

Pack Controller, Remote Access & Interface:

The pack controller is a purpose-built PLC, the software is designed and written by Green & Cool directly and as a result, we have great flexibility in adapting it to many different applications such as heat recovery. The touchscreen user interface allows easy understanding for technicians with logical navigation around the system.

The PLC also allows remote access to the unit. This is an internet-based tool which can be provided through an on-site connection and accessed from any computer connected to the internet. There is also a phone/tablet application for access.

The controller links directly with the store HVAC controller through a 0-10V signal, in this instance the interface it with a purpose-built RDM solution due to the client’s preference of monitoring system however any system able to provide this signal would be compatible.

The plant controller also sends an alarm signal to the store monitoring system.


The coldstore was supplied and built by Carter Coldstore Services and is standard 80mm PIR panelling with glazed doors along one wall facing the salesfloor. Internal shelving is back-filled and stock is traded form the coldstore. This layout significantly increases shelving space without compromising refrigerated storage.

HVAC Distribution:

The distribution of sales floor and office heating & cooling is through 3 No. ducted fan coil units, these are Marstair units designed in conjunction with Carter Synergy and contain 2 coils for LPHW and DX CO₂ respectively. 

An LPHW overdoor has also been installed due to the high throughput of customers to these sites. 

All LPHW and CO₂ equipment is supplied by the refrigeration plant only.

The HVAC system is controlled by a purpose-built RDM controller with a programme developed with Carter Synergy to meet the needs of the specific system. The system reacts to changes in the store environment in a more measured manner than typical LPHW systems to ensure we maximise the free heat output from the plant.

Ducted fan coil units

St Ives and Cockfosters Comparison Exercise

The site at Cockfosters was selected as a typical refrigeration/HVAC installation for direct comparison with the St Ives integrated scheme. Sub-metering was fitted to all refrigeration and AC plant and associated equipment to allow detailed analysis of the two system performances. 

Energy Savings

Energy readings have been taken at regular intervals across the period and a more detailed breakdown of the findings is contained in the graph & table below however, the headline figures are as follows:

System ItemEnergy Saving kWhEnergy Saving %
Combined Refrigeration & HVAC68,747.07kWh49.73%
Refrigeration Plant Only7400.9kWh10.59%
HVAC Equipment Only61,300.56kWh89.78%


For the 12 months, the study has been carried out so far the store is demonstrating a 49.29% reduction in energy use for refrigeration and HVAC services, this represents a 25% saving in total store energy use. This is being received as a huge success and ensures that the payback on investment will allow significant benefits to BP across the life cycle of the equipment.