With the inherent flexibility of PLC architecture, it is now possible to implement intelligent HVAC control algorithms as well as provide network connectivity not available with traditional commercial HVAC solutions.
In the past PLCs were not considered for HVAC controls based on prohibitive cost compared with commercial HVAC products. However, the cost-effective hardware of the Siemens S7-1200 platform has created an opportunity to apply PLC technology to this particular field. The result to the customer is decreased energy costs and the ability to remotely monitor and control the HVAC systems within their telecom network.
The objective of an HVAC system is to control the temperature, relative humidity, air movement, and air cleanliness within a particular space. These factors individually and collectively affect the performance of electronic equipment. It is therefore essential that the environment within telecommunications shelters be strictly controlled to maximize component operation and performance and to minimize potential equipment failures.
As part of an initiative to improve the operation of these shelters, a major wireless company created the “Shelter of the Future” standard. This company is responsible for wireless service and equipment infrastructure across the United States. The new standard required improved energy efficiency and remote connectivity to the HVAC system, so Marvair, one of the primary suppliers of telecom shelter HVAC, turned to Siemens’ solution partner Prism Systems (Prism) to help with the controls to meet these new requirements.
What is a telecom shelter?
A typical wireless site consists of a tower with radio antennas and a small building— a shelter— at the base of the tower. Shelter walls can be up to four inches thick and consist of a cast concrete outer shell, metal intermediate wall, insulation, and plywood interior. Ambient temperature has less than 5% effect on the internal temperature of a shelter.
Shelters house radio equipment, rectifiers, DC battery systems, and network components— all of which continuously generate heat and require year-round cooling. Two wall-mount HVAC units installed on the outside of the shelter provide internal cooling. Units are redundant and under normal operation function in lead-lag mode.
Requirements for the new shelter
Traditional shelter HVAC control consists of either a dedicated thermostat for each HVAC unit or a proprietary standalone HVAC controller. Each HVAC unit runs on either a 230V single-phase or 208V three-phase AC supply. If there is no AC power or no thermostat or no controller, there is no cooling. There is also no connectivity with other equipment either inside or outside the shelter.
The customer’s requirements for control of the HVAC units in the new shelter were straightforward:
- Energy efficiency in both mechanical and external air-cooling.
- Ethernet connectivity to the Network Operations Center (NOC).
- Remote control and monitoring over SNMP.
- A built-in web page.
- Suitability for new builds and end-of-life replacements.
From these requirements, Prism and Marvair recommended a hybrid control solution with a new HVAC controller, a new HVAC unit control board, and updates to the standard HVAC units. All primary control components would be powered by 48VDC from the DC battery plant to ensure continued operation in the event of utility power failure.
The new central HVAC controller based on the Siemens S7-1212 PLC replaced the proprietary HVAC controller hardware. A new intelligent control board with Ethernet communications to this PLC was installed in each HVAC unit.
The S7-1212 controller provided a single point lead/lag and DC free-air control for multiple HVAC units, an integral SNMP agent and web interface, an expandable platform able to incorporate alarms and controls for non-HVAC equipment, and a Siemens HMI panel for local control and alarm indication.
The schematic below shows the CoolLinks controls architecture:
The intelligent control board can fully operate the HVAC unit with or without the S7-1212 controller. With the PLC connected, it is a slave operation; when disconnected, it operates in standalone mode. A single Ethernet connection to the S7-1212 simplifies onsite installation and wiring. Further, the intelligence provides an additional level of equipment redundancy over traditional HVAC unit control boards.
In the HVAC units, DC motors replaced the traditional AC evaporator blower motors. A DC-operated full-flow Free-Air economizer was built into the HVAC units to allow the introduction of outside air to cool the shelter.
A DC evaporator motor powered by 48 VDC is more efficient than its AC counterpart. The DC motor is brushless with an operational life in excess of 70,000 hours, or eight years of continuous operation. A 100% full-flow economizer allows same quantity of outside air as the rated air flow of the HVAC unit, while the DC blower and DC Free-Air damper together provide emergency ventilation.
Proving the Concept
Marvair and Prism proved the concept in Q2 2013 by installing two “Shelter of the Future” systems in the Houston, TX area with a power monitoring system installed in a third shelter to serve as a baseline. All systems recorded total shelter power consumption, HVAC power consumption, indoor temperature, outdoor temperature and humidity, and HVAC run times. Data was recorded at one minute and one hour intervals.
The results of the test— HVAC power consumption was reduced from 37% of the total shelter power to 27%. Actual total utility costs were reduced by 26% through reduced actual energy consumption costs and reduced demand charges.
We’re often told the future will be really cool; the Shelter of the Future is not only helping make that promise a reality for wireless providers but also at a significantly lower operating cost.Have an Inquiry for Siemens about this article? Click Here >>