Compiled by Benjamin Brits
Aptly named, this building located in Rosebank, Gauteng, is the product of a complete re-development of an old bank building that had substantial design changes mid-construction, and a solution to fit.
The design aesthetic celebrates the history of the original building. The deep facebrick façade created an impression of strength and resilience, which is softened by the various incorporated greenery. The building then de-fragments at the top into a minimalist crown-shape, which creates deep landscaped roof-gardens offering magnificent views of the Johannesburg skyline.
The project, having started hallway through 2018, was partially completed by the end of 2020, and, at the time of publishing, final completion of the entire project was scheduled for the beginning of 2021, pending no unforeseen delays.
The project scope covered the entire re-development of the property that was originally the four-storey bank (used by FNB). This led the owners to simply naming the new building – ‘The Bank’. The old internal structures were scrapped and gutted, with the new design and use-profile applied. The building design kept the three levels of parking as the basement.
The new design then called for a conversion into a high-rise building, and initially four-storeys on top of the original structure were to be added. What changed the development quite drastically midway, was that the owners decided on an even higher design and thus required an further three storeys to be included. So, this then became a 12-storey building – excluding the ground floor. The entire re-development also made use of the original building’s foundations.
{os-gal-155} Images by Benjamin Brits, RACA Journal
As a mixed-use or multi-purpose building, the conversion changed the ground floor into a restaurant and hotel lobby, and includes a few small retail shops. The first to fourth floors have been set up to accommodate standard office-space tenants, the fifth to tenth floors are a luxury hotel, and finally, the eleventh and twelfth floors are high-end luxury offices.
The HVAC system, design and energy supply
In considering an appropriate system for this project, VMG Consultants was appointed to co-ordinate the solution and technology for the building, making sure that what was to be provided matched the heat loads and cooling demands. They worked closely with HVAC contractor Tempkor, and power solution specialist Blackdot Energy, who were responsible for the proposals and installation of the tri-, and installed co-generation electrical system for the building.
VMG needed to look at the type of HVAC system best suited given the various criteria and limitations, and decided on the Mitsubishi Electric variable refrigerant flow (VRF) system throughout the building. Configurations were then split across the different types of VRF system that Mitsubishi Electric offer.
The VRF market has grown exponentially over the last decades, not only in South Africa but globally. Mitsubishi Electric has also always been seen as a ‘VRF company’ and are known for their huge amounts of research and development into this technology over the last 20-plus years, particularly the 2-pipe heat recovery system. What many people may not know is that this particular 2-pipe technology is owned by Mitsubishi Electric through a registered trademark as the only VRF system using 2 pipes for simultaneous heating and cooling. All other brands use a 3-pipe technology.
Mitsubishi Electric launched their hybrid VRF technology first in Japan and then it moved to Germany where it spent a further two years being perfected and was then opened up to the European market. It was then released in South Africa in 2019. Currently the Mitsubishi Electric HVRF system is compatible with either R410A or R32.
The Bank’s ground floor, lower office space and premium offices all incorporate a standard heat recovery VRF system, while the hotel adopted the continent’s first new chilled/hot water hybrid VRF system. This system combines the refrigerant HVRF technology with chilled water. This feature is also the flagship item on the project as the first in Africa with the six-floor hotel space benefitting with the newest technology in the VRF range.
The office spaces have been fitted with a mix of cassette and ducted hideaways because these allow far more flexibility in office-partitioning or layout changes. Installing diffusers then allows you to move around more easily, whereas the cassette solution is purpose-fit for particular offices. Taking this into account VMG worked towards the solution ratio that offered the most flexibility for any future changes.
{os-gal-156} Images by Benjamin Brits, RACA Journal
The hybrid solution was not implemented for the office spaces and ground floor because these areas didn’t pose significant risk to the occupants – the areas have set times of occupancy and if any refrigerant leaks occur, there is much more open space and ventilation means than in the hotel rooms. Any leaks would be picked up through an error code on the system in any event.
A major reason for pushing to implement the chilled/hot water hybdrid VRF system in the hotel area was because the technology has a significantly lower refrigerant requirement but also is a solution that is very environmentally-friendly. Secondly, you don’t want to run a refrigerant through any room where an occupant sleeps because if there is a leak in the pipe work, even a slow leak, it can be extremely dangerous.
Leaks are a reality in any AC system no matter what technology you choose. With the hybrid system the refrigerant runs outside of the rooms in the passageways between the hybrid box connector (HBC) controller and the outdoor units, all of the internal reticulation to the hideaway units in the rooms only contain water supplied via PEX piping which has no harmful aspects if a leak does occur.
Why this is important is because the EN378 standard: Refrigerating systems and heat pumps: safety and environmental requirements has now come to the fore and relates to the amount of allowed refrigerant within a confined occupied space. VRF system can carry up to 60kg of refrigerant, so they fall within this limit value immediately. If a leak occurs, you can land up with 60kg of refrigerant being dumped into a space. This would remove the oxygen because refrigerant is heavier, and the person in the room could suffocate and die.
EN378 further specifies that if the amount of refrigerant exceeds the limit-value, refrigerant sensors must be installed in the space. These sensors need to be calibrated, tested and further connected to the building management system (BMS) that will then in turn connect to a separate alarm system installed in every room, or facility. This naturally carries a lot more costs in the actual products required, but installation too.
The VRF solution comes with the BC controllers that are the ‘unique piece of the system’. Effectively how they work is that when one system is rejecting its by-product – for example a hot gas – the BC controller will capture that hot gas and then divert it to an evaporator that is looking for a heating function. Mitsubishi Electric took this design further realising the potential to make it water-based and went through years of development to get right. The only refrigerant in the system now is between the BC controller and the condenser. Everything between the BC controller and the evaporator, now commonly referred to as the fan coil, is either chilled water or hot water, importantly only using two pipes irrespective of the mode.
The BC controller can send chilled water at 5°C or hot water at 60°C, offering a wide range of operating conditions. The new hybrid technology has further eliminated two thirds of the refrigerant requirement in a VFR system, replacing this with water. The result is that the parameters and restrictions of EN375 can be bypassed, passing savings onto the client. Although the cost of the hybrid solution is slightly more expensive, the benefits are realised by not needing all of the additional sensors and control methodology.
The generator plant room showing the gas generator installed. Also indicated is the heat recovery system capturing waste heat to
generate the building’s hot water needs and control system. Image credit: Blackdot Energy
For the hotel section, all of the condensers are installed on each floor and hidden within the façade so as not to be visible externally. The refrigerant piping runs to the HBC controller (or mini-chiller / hot water heat exchanger) and then from there onwards, PEX piping is run for the water reticulation portion.
The use of PEX piping in the system reduced the risk of the usual loss-factor of stolen copper piping which is a major cause of losses on many construction sites. Pressure testing and vacuuming becomes quite simple as excessive long copper pipe runs are also eliminated. The use of PEX is further a low-skilled application so the contractor doesn’t need skilled welders or specialist heavy equipment on site.
A further aspect on VMGs selection of the hybrid VRF system for the hotel rooms was that there are no expansion valves. These can sometime be a bit noisy – in a hotel room you may sometimes hear the ‘hissing sound’ of the refrigerant though the unit. With the hybrid system this is eliminated because there is only water being pumped through, and these pumps are situated outside of the rooms in the HBC boxes, reducing noise levels.
Each installed unit or room has its own controller that can be set by the occupant. The building control that was installed is the Mitsubishi Electric energy management central controller. Set up on the ground floor it gives access to the service team or the building manager. This system is also connected to the network infrastructure of the building so it further allows the operation of the system to be viewed remotely.
Further, if an office tenant on the second floor wanted remote viewing of how their particular AC system was running, or the particular set points, they can be issued with their own IP access to their system. The control systems in this project were quite complex and allow monitoring of all aspects tied into the central control system. The systems were also connected to the primary gas and secondary diesel generators installed in the building, allowing the monitoring of all electrical usage. This is an important factor because the electrical demand on the building is highly sensitive due to the limited capacity that is supplied by the local council.
{os-gal-157} Images by Benjamin Brits, RACA Journal
If the building’s electrical supply, for whatever reason, increases to the point of threshold, which is reaching 1000kVA in electrical supply, load control on the air conditioning system is implemented reducing the entire system to 50% of demand to avoid the electrical supply tripping. The functionality also allows a timeframe to stabilise – running the AC system within a specific range until the electrical supply drops below the minimum range – then normalising back to 100% operation.
The fire relay is also connected to the central controller and rather than linking the fire detection signal into every single unit in the building, if fire detection does occur, the controller permits all of the ACs to switch off.
At the point of first phase completion these control ranges and the methodology were theoretically laid out. Once the building has operated fully, there may be adjustments in terms of the parameters in which the system running is carried out, and this can be changed and re-programmed to suit the client needs at any time.
Turning to the building ventilation system, the 11th and 12th floors, with each floor plate being over 500m2, smoke extraction was implemented for these areas which operate on motorised dampers in conjunction with window actuators that are installed across the façade. If smoke extraction is activated, certain windows are actuated open to allow smoke to exit the floors.
The three-level basement of the building is also fitted with smoke extraction, split into an extraction system and general ventilation on a low level variable speed drive operating frequency allowing fans to ramp to 100% capacity in a fire scenario to extract smoke out of the basement. These both discharge above the basement level and operate together with various push fans that are spread across the area to assist in smoke flow towards the intake points.
In the basement ventilation VMG also implemented the positioning of CO₂ and CO sensors across the different levels to monitor the levels of carbon dioxide and monoxide. Having a lot of vehicle traffic through these areas, it can then be determined to either increase the fan speed of the ventilation or reduce it to save on electrical use.
Another notable design element of this project was the fire pressurisation system. There are four pressurisation zones in the building – the main stairwell, the fireman’s lift, the normal passenger lift and the goods lift. Each of these, due to regulations and the height of the building, required an emergency pressurisation system that operates together with the fire detection systems. These zones were designed based on simulation to achieve a 50 Pascal pressure differential.
Another control feature VMG implemented was electricity management, accommodated when a power shift occurs between power supplies. The generator switches off and goes through a restart. Every time this happens, the supply is not capable of managing a full load immediately – there is approximately a three minute time period for the generator to be able to start up and reach full capacity. Through the central controller again, all of the AC units are limited not to auto-restart until the generator is at full capacity again, whereafter a signal is sent allowing the AC systems to continue normal functionality.
The condenser units installed on the roof plant area. Image credit: Benjamin Brits, RACA Journal
Challenges
Changing a building’s design midway through ongoing construction was one of the more significant challenges in this project. The design started off with five levels of hotel space, being changed to six levels and then adding an additional two floors on top of that. This also all happened after the tendering phase.
In terms of the HVAC system and accommodating this change, it was greatly advantageous that VMG chose a split phase VRF system as it allowed them to easily add more systems catering to the new levels. Had they opted for a central system such as a chilled water system, this may have compromised the initial design capacity. The VRF split phase option then also didn’t compromise the installation of floors below.
Another significant challenge of the building was power supply. There was a very limited power supply from the council to the building (600kVA), and that brought into the design brief the requirement for an alternative source of power, and thus highly-efficient systems, which the VRF system meets.
To solve the power scenario, and which was another technological advancement that was added to the building, was the implementation of a gas generator. The Rosebank area has existing gas lines, with gas supply by Egoli Gas. The gas generator was implemented to feed off this supply and provide extra power to the building to supplement the low electrical supply provided by the city.
The building was determined to require at least a 1200kVA supply. To provide this power requirement, a system was designed as a hybrid renewable co-generation solution consisting of a solar photovoltaic (PV) rooftop installation of 90kVA and a 416kVA gas generator supplementing the 600kVA city connection. System redundancy was also included through a 700kVA diesel backup generator in place should the city connection be lost.
The co-generation solution enabled the entire hotel’s hot water requirements to be provided from the generator’s waste-heat. This is recovered through heat exchangers from the engine block and exhaust. The heat recovery system enabled significant electrical baseload reduction through utilising electrical elements thus producing a substantial cost saving.
All three of these power sources feed through a synchronised supply into the building. So the primary means of power supply to this building is now a mix of the gas generator and Solar PV, and the shortfall provided by council.
A further challenge on this project was plant space requirements. Traditionally with high-rise buildings, you would find very often a central system located on the roof or in the basement is specified with a chilled water system that would allow you to install long distribution lines, with limited space in the plant rooms themselves. A VRF system across every floor gave this project a lot of flexibility in terms of pipe runs, but importantly plant space.
Maintaining the existing façade dimensions of the building, a service level midway in the building between the fourth and fifth floors (originally the roof space of the old building) became the plant space level, where some of the plant is installed. This area it is also air-bricked to allow for natural ventilation to supply the required fresh air to various levels. This level also holds a number of condensing units and various water storage tanks and pumps. On top of the roof, more condensing units are installed as well as the fresh air and pressurisation systems.
The hot water system for the hotel is also implemented on this services level. A 7 600 litre tank was installed which is heated from the second floor gas generator plantroom, and the buildings hot water ring main is circulated from here. This services level also houses the heat evaporation units that dump the remainder of the generator waste heat to the back of the building.
Weight limits were another challenge. Because the new building used the existing foundation that had a much lower elevation, the weight per m2 allowance was very low – approx 250kg/m2. When looking at plant placement, this also favoured the VFR system as you need a very small footprint to get high cooling capacity out of the system. Other solutions for this building would have either required additional platforms on the side of the building, screens for acoustics and aesthetics, or additional measures to manage water vapour released into the air based on the available placement areas for the plant type.
{os-gal-158} Images by Benjamin Brits, RACA Journal
Energy efficiency elements
Renewable energy was a core design element. The building’s entire available roof space was covered with a solar PV panel array, this power generation system feeds directly into the building’s main grid intended to supply the baseload power requirement for the day time HVAC system. The solar PV serves as a first phase of the solar cooling design that is strategically planned for the developers strategic roll-out to their other property portfolios.
The VRF solution is very efficient as it is and also contributes to the power consumption savings of the building. With the installed control system that comes with VRF solution, it further allows the capability to optimise the way the entire system runs such as load control and peak hour cutting, which were implemented for this building. Added to this, if load shedding occurs and the generators are running, the AC system has been set to operate in fan-only mode to ensure continual circulation of air and to minimise the load on the generators ensuring the maintenance of power for any essential services the building needs.
Installing a gas generator also opened up the perfect opportunity to make use of a typical waste by-product. Gas generators in their own right are actually highly inefficient with a large portion of heat rejection taking place. In the production of electricity, almost 70% of what goes in to a gas generator gets released as waste heat. This offers a process to harness the heat and produce hot water from it. This solution was implemented in the building producing a saving that offered an internal rate of return exceeding 25% of the project value.
The overall gas generator system installation has saved the building up to 20% of electricity costs compared to the city power supply costs. The benefits of greening in this way have proved phenomenal and when carbon taxes are implemented, there will be a significant further savings for the building. Choosing a gas plant or solar installation is much more feasible now than the regular power supplied by councils (given the availability of gas in the area). If more engineers are able to see this through actual data and installations – greening technology will be implemented much faster and in many more developments.
{os-gal-159} Images by Benjamin Brits, RACA Journal
Unique elements
As a first on the African continent, this already makes this project unique but coupled with the control technologies and co generation system installed while meeting the client’s requirements is worth noting for this project.
Tri- and co-generation power solutions are very new to the industry and there are also not a lot of companies that specialise in this field that can supply solutions based on the various needs of the client. Blackdot Energy is one of the scarce companies in South Africa who were able to provide a solution for this building.
This project went though a very long feasibility process in terms of the tri-generation system because it is new technology – there are only a handful of installations in the country currently, but catering more to the data centre environment and large university buildings. This system works extremely well when you have a constant cooling load and it was therefore a practical option as the gas generator is continually producing electricity and you then have the waste heat to use as ‘free’ cooling and heating. However the tri-generation solution is quite costly in terms of capital expenditure because an absorption chiller would need to be added to the overall system. The owners then decided to go with the co-generation solution, being power supply and using of the heat to create the hot water.
From an architectural point of view the entire building is fitted with double pane glazing and for those interested in what happens below ground, an interesting operating feature of the building is that the client installed car lifts throughout the basement so that they can double stack cars with a valet service, increasing the capacity, which is a feature that you don’t see in many projects.
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