By: Ilana Koegelenberg – senior staff writer, in collaboration with George Rouse/Etienne Theron (Spoormaker & Partners) and Sebastiaan Jacobsz (Orenge Environeering Services)
In 2009, Spoormaker & Partners were approached by the University of Witwatersrand (Wits), in Johannesburg to design the mechanical services associated with the proposed new Undergraduate Science Centre (USC) on the Braamfontein Campus West. This new R179 million complex is part of the University’s capital programme, with the majority of the funding provided by the Department of Higher Education and Training, and additional generous donor support from AECI and Bateman. “The USC was built with the aim to increase the teaching space required for the science course expansion at the university,” George Rouse, project manager of Spoormaker & Partners, explains.
Conceptualised by PJ Carew consulting engineers and implemented by Spoormaker & Partners in association with Orenge Environeering Design, the Wits Undergraduate Science Centre (USC) has a footprint that includes 4 500m2 in lecture rooms and 3 500 tonnes of activated structural concrete mass.
This new laboratory building consists of chemistry, physics and biology laboratories spread over three floors and incorporates the following mechanical services systems:
Heating and cooling for comfort;
Fresh air and extract air ventilation systems;
Fume cupboards; and
Natural gas and compressed air systems to work stations.
In terms of the details related to accommodation, the client’s requirements were as follows:
The conversion of the underutilised Charles Skeen athletics stadium into large-scale auditoria venues seating 250, 350 and 450 students in a separate block.
The development of:
One new chemistry laboratory on the ground floor, accommodating 376 laboratory bench spaces, complete with a services bay, instrument suites, preparation rooms, storerooms, staff offices and a staff common room;
One new physics laboratory, divided into two sections accommodating 216 and 192 laboratory bench spaces respectively, complete with equipment stores, an office and workshop;
One new biology laboratory, accommodating 300 laboratory spaces, complete with storerooms, technician’s office and cold room;
Shared facilities such as tutorial rooms, laboratory sample preparation rooms, a staff office and common room; and
Multiple ablution facilities.
Mechanical design concept
Initial intentions were to design schemes using an ‘all-air’ system due to the lab space type of application, however this system had multiple limitations. “Not only are they energy inefficient due to the large amounts of fresh air required, but large plant spaces are needed for the air handling plant and the associated ducting,” Rouse elaborates.
From the outset, Paul Carew, the environmental design engineer, had a major influence on the mechanical services design by implementing a more energy efficient design than was originally considered. Carew introduced the following initiatives: daylighting in lab spaces; energy efficient light fittings in the labs wired to daylight sensors; and energy efficient heating and cooling.
Thus an alternative scheme was selected for the labs, consisting of a thermally active slab system with tempered ventilation and separate fume cupboard rooms. Prior to going ahead with the scheme, an in-depth expenditure analysis was carried out along with a life cycle costing to verify the correct scheme was utilised.
“Fortunately the client was sufficiently convinced of the scheme’s viability to proceed with a new means of heating and cooling in South Africa and, after the design’s health and safety aspects were analysed in terms of legislation, project design commenced.” Rouse explains. Orenge was then appointed to design the TABS system.
The TABS challenge
“Thermally Activated Building Systems (TABS) are considered an engineering heat transfer expression in its most fundamental form,” Sebastiaan Jacobsz, managing project engineer at Orenge Environeering Design, explains his decision to implement this alternative solution.
According to Jacobsz, TABS can basically be described as the technique by which the concrete structure of a building is used as an energy store. It acts as a heat sink when the building gains heat, and a heat store when the building loses heat. The building’s concrete structure is thermally activated by circulating water through Uponor Pex-a pipes embedded in the structure at the required temperature. The change in internal energy of the concrete structure then allows heat to transfer between the room space and concrete surface by means of radiation and convection.
The heat conducts primarily in the transient state between the two surface boundaries – the concrete surface to the room side and the pipe inner diameter embedded within the concrete structure. Heat is then further transferred primarily by porced convection between the inner pipe surface and liquid fluid circulating inside the pipes.
The large activated surface area allows for smaller temperature difference between fluid and room temperature, realising high temperature cooling and low temperature heating which considerably improves the refrigeration cycle efficiency. “This essentially defines TABS as one of the most energy efficient comfort heating and cooling systems with superb thermal occupant comfort,” Jacobsz says.
Off-peak heating and cooling can be realised due to the thermal heat storage capacity of the concrete structure which realises off-peak night time heating and cooling, using commercially advantageous electricity rates subject to the building’s electricity scheme. Free convection between the room air and concrete surface eliminates the need for fans to transfer heat via forced convection.
“The pick-up tendency towards thermal building design in Europe has increased considerably during the last 15 years due to realised benefits,” says Jacobsz. “Wits USC is only the second project to take thermal building design on board in South Africa.”
The design brief required the implementation of a high mass thermal based comfort climate system in conjunction with mechanical fresh air supply only. The specified annual comfort temperature profile for the building was 20°C-25°C with a maximum internal relative humidity of 60 percent. “One of the feature design criteria was that the building structure could only be conditioned during the night-time cycle in order to utilise cost effective off-peak electricity tariffs,” Jacobsz explains. “This implied that sufficient concrete storage mass had to be activated in order to yield a stable room temperature profile during the day, adhering to the annual comfort temperature profile as per the design brief.” (See graph 1)
|Graph 1: TABS behaviour during summer conditions.
*Readings subject to sensor positions.
|Graph 2: TABS auto regulation from cooling to heating.|
Detailed commissioning of the system had to be performed in order to ensure design specifications were achieved without any compromise. Sequential detailed commissioning of the controls was performed as well to prevent equipment miscommunication which would result in system failure.
“Challenges facing the design team included high internal load density thanks to the use of Bunsen burners and other equipment used for educational experiments,” Jacobsz explains. This required a detailed heat transfer analysis to be conducted on the concrete slab sections to determine the concrete’s ability to diffuse heat without the formation of steep isotherm gradients within the concrete structure.
Once detailed analysis on the concrete slab sections were complete, the dynamic room temperature profiles for both winter and summer conditions could be quantified in order to evaluate the feasibility of the design brief.
Sophisticated control algorithms had to be developed to dynamically control the building’s room conditioning equipment, preventing over cooling on mild summer days and allowing auto regulation between the centralised plant’s heating and cooling mode. (See graph 2).
The building’s management behaviour was modelled with a customised software program, allowing for the evaluation of the building’s controllability with fluctuating weather conditions as well as changes made to any of the control inputs. This allowed for predicting the control performance of the building which proved to be of great importance for aligning the set-up during the final commissioning procedure.
“The detailed control strategy for the building has proven to be one of the most significant features required for successful implementation of the project due to the delayed response time of the building's heavy concrete structure,” Jacobsz says. Graphs 3 and 4 represent a period over two weeks in May where the building space maintained itself in a state of thermal equilibrium without any mechanical system interference required. “This remarkable behaviour is explained by the thermal inertia of the exposed concrete structure and the building has been allowed to express its natural behaviour due to the intelligent control strategy implemented.”
|Graph 3: TABS state of thermal equilibrium.|
|Graph 4: TABS state of thermal equilibrium.|
“The design technique for high mass thermal based buildings is still considered a non-commercialised technique requiring detailed heat transfer calculations to validate feasibility of such systems at concept phase of any particular project,” Jacobsz says. “The inherent heat storage capacity of the exposed concrete structure becomes a significant design aspect in proving design feasibility prior to implementation of such type designs.”
Thermal buildings also have specific architectural requirements. They require exposure of the building’s concrete structure with potential limitations to certain type surface finishes. “It is generally found that high mass thermal comfort systems are most successfully implemented on projects where the professional team is aligned towards the intent of such type design,” Jacobsz explains. “From an engineering perspective the associated engineering risks are managed by proper quality assurance procedures implemented during the construction phase, eliminating potential system failure further down the line.”
Ventilation scheme design
The laboratory ventilation scheme consists of two major components: the main laboratory’s central ventilation and fume cupboard ventilation. Rouse explains that as part of the overall integrated scheme’s design, the fresh air ventilation system was designed according to the following criteria:
Maximum human occupancy – based on 7,5l/s per person minimum;
Natural gas emission ventilation – based on the maximum time of burner operation obtained from specific course data and worst case scenarios for gas leakage from pipe fittings on the benches; and
Operation during fire conditions in parallel to the motorised windows.
Heating and cooling is supplied from the same heat pump that provides cooling and heating to the concrete slabs within the building. At night the heat pump runs at temperatures dictated by the Building Management System (BMS) for the slab cooling/heating. During the day the heat pump provides heating or cooling for a supply air condition of 18°C-24°C from summer to winter.
According to Rouse, this system was designed with the following benefits in mind:
Once through fresh air supply;
Motorised windows to exhaust vitiated air at high level and for smoke relief;
Average supply air temperature of 18°C from summer to winter;
Optional de-humidification if slab temperatures reach dew point;
Reverse cycle heating in winter;
Minimised heating and cooling to the building by using the same heat pump as TABS;
Only supplies air when the labs are in use; and
Can be turned on in the event of a fire signal.
Ventilation system control
“Each space is fed from the central system. However, from the outset the decision to allow occupant operation was necessary to control the use of energy to the space in heating or cooling format,” Rouse explains.
To start the system, wall mounted switches are located in each lab and seminar room to allow air only to spaces in operation. The fresh air can either be manually turned off or the BMS will set the ventilation to turn off after 18:00.
At the same time the ventilation fan is started, the associated motorised windows are opened to allow spent air to be discharged into the atmosphere.
Fume cupboard ventilation
“As part of the chemistry lab, fume cupboards are a necessity with particularly special requirements associated with first year undergraduate sciences,” Rouse explains. “However there were a number of challenges that required a customised solution for the building.”
The fume cupboards were designed taking into consideration the following client design criteria:
Health and safety requirements, designed to ASHRAE standards;
Specific course experiments for first year students;
Large quantities of students doing the same experiment at the same time;
Use of low concentrations of chemicals; and
Close supervision of students.
Multiple work station fume cupboards in dedicated ‘fume rooms’ were required to reduce the fume cupboards’ footprint and yet maintain the health and safety (airflow) of each fume cupboard while limiting the tempered air supply’s energy consumption.
Customised supply and extract fume cupboards were designed by the supplier to allow 75 percent of incoming air to be un-heated fresh air supplied directly into the fume cupboard.
The room temperature in winter is controlled by the 25 percent make-up air supplied directly into the room for human habitation as well as the fume cupboard extract’s balance.
External gas and electrical shut-off controls located outside the cupboard at all work stations for individual control.
Materials used consist of polypropylene sheets welded into shape for the cupboard as well as the complete fume extraction system.
Computational fluid dynamics (CFD) analysis of the cupboard with sash in 25 percent, 50 percent and fully open positions which led to design modifications early on, prior to manufacture, as well as confirmation of design configurations during commissioning; and
Prototype factory testing using a test rig to verify the cupboard equipment’s airflows and physical operation, for example sash operation.
Notwithstanding the early design efforts, the commissioning process did require careful monitoring specifically the room pressure control/airflow fluctuations compared to adjacent spaces by use of adequately sized relief grilles as well as people movement in front of the cupboards by moderating the face velocity into the fume cupboard.
Prep room unit
“A separate custom designed item requested by the user group was a fume cupboard to allow concentrate chemicals to be mixed in a separate room to the lab,” Rouse explains.
Due to the high concentrations of chemicals used, this system was equipped with a dedicated fume extraction system with scrubbers prior to exhausting to atmosphere.
The entire system was again fabricated from polypropylene and specifically designed to incorporate:
Chemical spillage containment inside the cupboard for future safe collection;
Heavy duty stirring devices for 20-litre drums; and
Clear plastic curtaining for ease of loading and unloading the cupboard.
As part of the integrated approach to the building solution, the natural gas and compressed air installations were also installed in a novel manner.
Yellow Alu-Pex gas piping was enclosed in the concrete slab to provide the gas and air to each work station. “Johannesburg’s fire officer was approached to confirm that this would not contravene regulations and provided all joints are accessible, the use of the piping would be accepted,” said Rouse. “Consequently, there are no joints in the concrete and only at junction boxes serving the stations where potential points for leaks are likely. Thus the systems can be monitored on a regular basis to comply with regulations and provide a low maintenance solution to the application.” Otherwise Schedule 40 welded piping is used throughout for the gas piping installation.
Achieving the goals
According to Rouse, the underlying set of complex design criteria were met thanks to:
The latest technology in terms of heating and cooling to the building;
Energy efficiency of operation via design philosophy and control;
Integrated design solutions built into the structure;
Special needs solutions for fume extraction; and
Continuous reference to the mechanical services’ budget.
Budget and deadlines
The final contract sum, excluding VAT for this project amounted to R14 241 915,10.
The project started in October 2009, with the completion of the site works being in March 2011. However, the final completion of commission was in July 2011. According to Rouse, the project was physically completed on schedule, but due to the complexity of the system and need to commission a semi-operational building, the final commissioning was only completed around four months later.
A happy client
Emannuel Prinsloo, director of campus development and planning at Wits, stated: “As a client we are extremely pleased with the innovative, sustainable energy efficient outcome of collaborations with our consulting partners, which delivered a state-of-the-art energy efficient building as part of the on-going R1,5 billion capital development and refurbishment programme currently being undertaken by the university.”