Progressive HVAC solution for Cornubia filling station

By Richard Gibson of Parsons and Lumsden Consulting Engineers

The new multi-use Shell filling station in Cornubia has raised the bar for projects of this kind, opting for a very efficient chilled-water ice storage HVAC system.

The Cornubia site is adjacent to Umhlanga, north of Durban, and is a mixed-use development comprising residential, commercial, and industrial sites. The area was previously earmarked for development and, since 2011, has seen exponential growth.

One of the recent additions to this precinct is the Cornubia filling station, which is operated by United African Group. This development started its design journey in May 2017 and the building was completed in November 2018.

The goal was for the filling station to be open for business during November 2018, and thanks to a great deal of coordination and co-operation by all who were involved in the project, this deadline was met.

Photos by Ilana Koegelenberg

 

Client brief

The client is a successful property developer and had set requirements that the HVAC system needed to meet.

Firstly, energy efficiency was of foremost importance. The client developed this building to own and operate in the future and, therefore, aimed to create a sustainable and efficient building.

Secondly, reliability was crucial, as any air-conditioning downtime would discourage tenants and the public from using the building. Thirdly, it was essential that the equipment would be easily maintainable and that if maintenance tasks were performed, they would be restricted to plant rooms.

Certain methods were used to satisfy this brief. A chilled-water system was determined to be the best system for this application. This system was to be used in conjunction with thermal ice storage. Chilled-water air handling units (AHUs), situated in dedicated plant rooms, were used in most areas. This ensured that any maintenance would be concentrated in a plant room and not in the public air-conditioned areas.

Photos by Ilana Koegelenberg

Fan coil units (FCUs) were used in limited areas where individual control and specific operating time schedules were required.

System description

The filling station is a multi-use building that consists of three levels.

The lower ground floor comprises a large parking area and a retail area, but the latter area has been left for future tenancy. The ground floor consists of more retail. One half of the ground floor has been occupied by the 24-hour supermarket concept, which is a concept first launched by the developer/operator. These retail activities form a collaborative effort with the filling station’s activities. The convenience store incorporates a general trading area, kitchen, coffee shop, and liquor store. The other half of the ground floor has been left for a 24-hour drive-through operation, which the developer is currently in the process of finalising.

The first floor contains the developer’s offices. This floor is divided into two tenancies with individual plant for each.

The HVAC system design needed to be flexible to adapt to each different area and occupancy, namely the portion of the building that will be occupied immediately and the balance to be tenanted at a later stage. The chilled-water system was sized to accommodate a fully occupied building and, therefore, chilled-water piping has been routed to all areas and blanked off for future connections.

Photos by Ilana Koegelenberg

 

The chiller, ice tanks, and secondary pumps are located in a spacious plant room on the lower ground floor. Chilled water is circulated via insulated steel piping to the various areas where it is required. There are two secondary pumping circuits. One pumping circuit is for the retail areas on the ground floor, which will be operating continuously. The other secondary pumping circuit supplies the offices on the first floor and the retail area on the lower ground floor where operating hours are likely to be from 7:00 to 17:00.

The chiller is designed to operate at night, during off-peak periods, when it will build an ice charge. During the day, and especially during peak periods, the chillers will be off and the ice reserve will be used as the cooling medium. 

Chilled water AHUs are situated in plant rooms for the various tenants. Energy metering is installed on each supply point so that the client can monitor and charge the various tenants separately according to the amount of chilled water used. Fresh air is individually supplied to each plant room to ensure compliance. Air-conditioned air is supplied to the different areas via insulated steel ducting and either through grilles or diffusers. Variable air volume (VAV) systems have been used where appropriate.

Product selection

An air-cooled chiller was selected for this project. This decision was made after Parsons and Lumsden reported to the client on the different chiller options that were available. The options, which included two small air-cooled chillers, one large air-cooled chiller, a water-cooled chiller, and an air-cooled chiller operating with ice storage, were presented with their respective advantages and disadvantages. 

Ultimately, the decision to go with one air-cooled chiller, linked to ice storage, was made, as it was the most economically sound option. By using ice tanks, the electrical load on the chiller is shifted to off-peak periods. Thus, the savings have been substantial and after approximately five years, the client will have paid off the initial capital that was required for the ice tanks. Thereafter, they will enjoy considerable operational cost savings.

(The payback period for an ice storage system will differ from project to project. Electrical price increases are uncertain, so estimations in the annual increases are made. Differing site locations affect the payback due to differing tariffs. One should also consider the interest on the savings that are accrued every year when working out a payback period.)

Four ice tanks were selected to provide the thermal storage required to run the plant on a partial storage basis. This calculation was made by careful heat load estimation based on the expected heat loads that the building would experience throughout the year. Load profiles were created for the building and a suitable number of ice tanks were then selected.

Secondary pumps, which operate with variable speed drives, were selected so that they only supply chilled water based on the building load. Two-way energy valves were used at each AHU and FCU.

AHUs were locally built and incorporate EC plug fans, cooling coils, and filters.

A building management system (BMS) has been used to monitor the chiller, ice tank levels, pumps, AHUs, energy valves, and the like. The BMS will also be used to monitor kWh cooling used by the various tenants. Space temperature monitoring has also been incorporated.

Cornubia 00 12

Chilled-water plant room Revit drawing. Image credit: Parsons and Lumsden

Cornubia 00 15

First-floor supply air Revit drawing. Image credit: Parsons and Lumsden

 Cornubia 00 18

Revit drawing of the HVAC system at the Cornubia filling station. Image credit: Parsons and Lumsden

Sustainability matters

Apart from the energy-efficient HVAC system, another sustainable aspect of the installation was the aspect of water usage. Water usage management was high on the client’s agenda, so a rainwater harvesting system was used on the site.
 
All rainwater that lands on the forecourt roof, as well as the building roof, is piped down into the lower ground floor where it is fed into a dedicated storage tank, which can store 58 000ℓ. This tank is also fed by a borehole that will only be used if there is insufficient rainwater. The water stored in this tank is then boosted into the building via pumps that supply all the WCs and urinals as well as certain taps located at bin areas and various landscaped areas for irrigation.

The HVAC system is robust and simple. It is designed to be able to adapt to future tenant changes with easy access to all the equipment. It is not often that a client would opt for an ice storage system due to the initial capital required, but in this case, the client being a progressive thinker was able to look beyond just having an air-conditioning system  meeting internal temperature requirements to also meeting these requirements in the most efficient way possible. 

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