Tesco’s goal is to reduce the total CO2 emissions of its DC portfolio by 50% by 2020 against the baseline of a DC installed in 2004. As part of the drive to achieve this it set up an Environmental DC Project team in 2010.
STS was invited to join a team which included Atelier Ten, RAU and OPAi. Its remit was to examine a wide range of potential strategies for reducing the environmental impact of DC construction and operation. While the majority of the strategies described in the report related directly to reductions in carbon emissions from energy use, many wider aspects of sustainability were also addressed. These wider considerations included reducing the embodied energy of materials and construction processes, limiting impacts on local biodiversity, reducing pollution and addressing the many social aspects of sustainability.
Displacement Cooling Study
As part of the Environmental DC Project a number of alternative ways of delivering cooling to the +1oC & +12oC chambers were explored. One that looked potentially attractive was the utilisation of the displacement ventilation concept. This would replace the existing standard method of using gantry mounted coolers to cool and distribute air in what is termed a “mixed” ventilation system. The displacement ventilation concept would supply cold air at floor level at a temperature closer to isothermal and at a rate that matched the pluming effect of heat loads in the room in order to maintain the required temperature in a stratified layer only as high as necessary to accommodate the product. So a temperature gradient would be generated through the height of the chamber, which in, for example, the +1oC Chill would go from around +1oC to possibly as high as +10oC at roof level.
The high level assessment undertaken during the Environmental DC Project indicated that the application of the displacement cooling concept had some significant potential benefits, which would deliver lower heat loads; initial assessments indicated in the region of 30% to 50%; leading to smaller refrigeration plant – saving on capital costs – and requiring lower power consumption – saving on operational costs and reducing the site’s carbon footprint.
STS was appointed to develop the concept further, to confirm the technical viability, assess the capital costs and provide a total cost of ownership evaluation. The study addressed a range of questions related to this novel cooling approach for DC’s, including:
- Was the concept of a temperature gradient of circa +10oC from floor to ceiling acceptable legally or operationally?
- If vertical ductwork were used, would the installation of approximately 50, 1.5m diameter diffuser columns in the +1oC Chill, with an equivalent density in the +12oC Produce be acceptable operationally?
- What were the revised heat loads?
- How would air-flow patterns behave in practice? Detailed CFD modelling of the various air supply options was carried out to verify this.
- What would a workable design look like, in terms of component selection, ductwork design, building structure, positioning of air-handling units and diffusers?
- What were the capital costs for a workable design, compared to the existing Tesco DC refrigeration system design?
- What were the operational cost savings?
- Did the combined potential capital and operational cost savings against present design deliver a big enough cost benefit?
- Were there additional H&S issues requiring consideration?
- Were there additional operational issues, eg maintenance, requiring consideration?
The study confirmed that the concept would work and that there were significant energy savings compared to the existing working practices. However, the additional capital costs did not provide an attractive payback on the investment.
While the study did not provide justification for implementing the displacement cooling concept it did provide findings that are being implemented into the next Tesco DC project and will deliver a significant reduction in capital costs and an improvement in operational efficiency.