Going green with Combined NatRef system solution

A walk-around freezer in assembly. Image credit: © ACS Consulting Engineer

By Ricky Savvides, HVAC and building services engineer at ACS Consulting Engineers

This project, for the largest retailer in South Africa, comprised several HVAC and refrigeration aspects as the planned new benchmark for future sites, ticking the boxes of efficiency and low environmental impacts.

ACS Consulting Engineers (ACS) were appointed by Massmart to design the HVAC, refrigeration, and plumbing and drainage aspects for their new store in Parsonsvlei, Gqeberha. This would be the eleventh new Makro store designed by ACS, this one however with a major twist.

Designs for the project began in 2019 and construction commenced in early 2020 – although due to the COVID -19 pandemic it was subsequently delayed. The project thus reached completion in April of 2021.

Project specifications and facility requirements

The client brief was to provide an energy efficient system for the facility’s HVAC and refrigeration requirements that is sustainable, that uses low ozone depleting potential (ODP) and global warming potential (GWP) refrigerants, that provided a combined system that would offer economy of scale including thermal storage, and which would produce hot water from free heat captured from the system.

The HVAC & R system solution and rationale

The twist of this particular facility was the first occurrence in the eleven sites handled by ACS in the requirement for a design to combine the HVAC and refrigeration systems onto a set of common CO₂ chillers/packs using two-stage transcritical booster packs with parallel compression operating above and below the critical point of 31°C and above the triple point of 38°C, and, at high compressor discharge pressures between 7 500-10 000 Kpa as the heart of the two systems. The site further incorporated the now standard Massmart request for thermal storage for the HVAC portion, the low (LT) and medium temperature (MT) cold rooms, freezers, and cabinets on the refrigeration installation, and to provide hot water for domestic use and winter space heating.

At time of completion, ACS believes this was the first combined project of its kind in South Africa, especially because of its capacity and the fact that ice thermal storage was used for HVAC, domestic hot water, that winter heating is provided from the same system, and that the refrigeration requirements are met.

Two racks were installed – each with two packs incorporating medium, low and HVAC Bitzer compressors (each for 50% capacity), electrical distribution boards and controls. Each rack comprises four HVAC compressors, four MT compressors, three LT compressors and three intermediate compressors.

LT compressors discharge into MT compressors and the lead compressors incorporate VSDs.

Refrigeration capacity was approx. 400kwR for MT and LT applications per pack. HVAC capacity was approx. 470kWR in day mode and 370kw in ice mode per pack. All compressors discharge into a common header towards the gas coolers and share common liquid receivers.

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The maximum HVAC required capacity was for the January month at 1 493kwR, and the maximum thermal energy was 14 774kwhr to be shared equally by the ice tanks and HVAC/refrigeration packs with HVAC packs only operating if the internal load required them to operate.

A CO₂ detection system with alarm monitoring is installed in the plant room and on walk around freezer.

The ACS team researched the application of CO₂ for a few years prior to this project and especially in order to combine the HVAC requirements with the refrigeration requirements.

The system’s HVAC ice thermal tanks. Image credit: © ACS Consulting Engineer

Previous, Makro projects used hydrofluorocarbon (HFC) refrigerants such as R134A for the HVAC installations and several stores were already using CO₂ for the refrigeration installations on low and medium temperature cold rooms and cabinets.

With the worldwide drive to combat global warming, ACS and Massmart decided to use CO₂ refrigerant which has a zero ODP and a GWP of merely 1, for the combined chiller packs.

CO₂ is increasingly becoming the preferred refrigerant in many refrigeration installations with comparable coefficient of performance (COP 2.8-3) with other refrigerants, but also a refrigerant that produces high temperatures thus making water heating a very viable solution.

Massmart/Makro  are one of the leading companies in South Africa that are trying very hard to reduce carbon emissions and reduce global warming by using natural refrigerants, and gladly accepted the challenge on this project.

With the clever design of the chillers/packs, gas coolers and with CO₂ /water-ethylene glycol heat exchangers, the thermal storage system produces -7 degree C water/glycol at night for ice production, and +7 degree C water for daytime use on the HVAC system.

Due to this design, ACS managed to produce favourable electrical energy savings for the site with all HVAC aspects, and refrigeration elements in operation – including free water heating for domestic use and the winter space heating needs.

HVAC system and installation

The heart of the HVAC, refrigeration and plumbing installations’ equipment are located at the back of the store in dedicated plant rooms or exposed to the elements as required.

The HVAC system is a traditional Makro ice thermal storage system of two large vertical steel tanks that use spherical nodules that are frozen at night when the utility rates offered to the client are lower than daytime rates. This method shifts the energy demand to nighttime thus significantly reducing energy costs. The water/ethylene glycol solution is circulated through the ice tanks and CO₂/ethylene glycol heat exchangers, which enable the nodules to freeze at -7°C.

The HVAC design philosophy is to use ice storage as much as possible and only use the chillers when the demand requires them to assist with the cooling load. On this project the chilling is provided by four South African built refrigeration packs/chillers using Bitzer reciprocating CO₂ compressors with parallel compression – generating the ability to offer low and medium temperatures for the refrigeration aspects.

It must be noted that this is a transcritical CO₂ operation which uses high pressure refrigerant that requires gas cooling. The benefit is that high water temperatures are also possible to gain for domestic water and space winter purposes. This improves the COP and overall operating energy costs, because no electrical elements are used to produce any hot water in the building.

The four packs are installed inside a purpose-built plant room and the four hybrid gas coolers or condensers are air cooled units placed on the roof above the plant room.

Traditional air handling units (AHUs) with EC plug fans, chilled water-cooling coils and hot water heating coils are used to condition the internal spaces for summer and winter.

Fabric ducting with appropriately designed diffuser nozzles or orifices have been used as much as possible to supply conditioned air to the internal spaces. This being more cost effective for this application type than traditional sheet metal ducting and diffusers.

Steel piping with insulation is used to circulate the chilled solution to all AHUs and fan coil units.

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Two chilled water/ethylene glycol pumps as duty and standby are used to circulate the solution in the three modes required, namely ice built, ice only and chiller-assist – thus eliminating the use of secondary pumps.

Similarly, two space heating water pumps are used for the winter heating of the store.

The main trading floor and liquor store are air-conditioned by six AHUs placed at the back of the store on a raised steel platform. Offices are provided with individual ceiling mounted hydronic cassette units or ducted hide away units as preferred by the client. All individual cassette units in individual offices are wired through motion sensors that offer additional energy saving when occupants are not stationed in their offices.

A dedicated building management system (BMS) is used to control all aspects of the HVAC and plumbing systems, with a separate system by Danfoss to control the refrigeration aspects.

HVAC and refrigeration dry coolers with ice tanks in background as well as the rainwater capture tanks. Image credit: © ACS Consulting Engineer

Refrigeration system and installation

The refrigeration system consists of the combined chillers/packs and hybrid gas coolers, various blower coils in the main freezer and cold rooms, walk around freezers and fridges, etc. All Insulated panels for the cold rooms were built on-site, and some island type freezers were provided for the butchery section.

Stainless steel piping is predominantly used for all refrigerant circulation from the plant room all the way into the store at high level which then drops down to all blower coils which are scattered around the building in various freezers, cold rooms and fridges.

Danfoss expansion valves are used as required by each blower coil arrangement with appropriate oil traps in strategic locations.

A refrigeration main distribution board (DB) is located inside the refrigeration plant room which supplies power to all the refrigeration packs and all other remote DBs that include power for blower coils, power for electronic controllers for the expansion valves, power to all glass doors on fridges and cold rooms which incorporate heating strips built into the glass, and humidistats for anti-condensation purposes should this be required.

A Danfoss central controller installed on the main DB indicates all important features of the refrigeration systems and is connected to a remote touch screen monitoring system located in the maintenance managers office.

Difficulty in meeting design specifications

One point of note for meeting the requirements of this project is that there was not a single solution in the form of a chiller that is available as yet for CO₂ systems and so a custom designed and built system was required to meet the client’s specification.

Project efficiency and sustainable

This project encompasses several aspects of efficiency and sustainability form the choice of CO₂ as the refrigerant of choice to the use of off-peak energy tariffs, as well as the use of free heat for domestic-use water heating.

Also, the application of a single system to achieve the entire facility’s needs is in itself very efficient and shows the clients strong support of sustainability.

ACS Consulting Engineers would like to thank all role players including the clients, Massmart, ITD Group and CRS/ Matador for their contributions, commitment, and excellent performance for a satisfactory completion of this project – on time and on budget.

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