BEMS Building Energy Management System

BUILDING ENERGY MANAGEMENT SYSTEM - BEMS

PLANT CONNECTIONS AND CONTROLS:

Single or multiple boilers should always be connected to either a mixing header or separate flow and returns headers. This allows heating circuits to be independently connected to the boiler plant and facilitates the maintenance and breakdown of individual circuits and boilers. It is for this reason and indicator of good engineering practice.

Heating systems with CVVT (Constant Volume Variable Temperature) control are particularly subject to low return temperatures in mild weather. This can have a detrimental effect on the boiler by inducing condensation in the flue gas. Low return temperatures can also cause differential expansion within some boilers and consequent stress in the boiler metal. A by-pass protection is necessary so when the thermostat senses low temperature water in the boiler return, it energizes the pump and cuts it out on a predetermined higher temperature.

Condensing boilers are specifically designed to operate on low return water temperature. In the process, further sensible and latent heat of condensation from the flue gas are given up, therefore increasing the thermal efficiency of the boiler.

Temperature controls

In HVAC systems the heat emitter must be sized to meet the peak heating load (at design conditions) of the room. This involves determining the design steady-state heat loss rate and applying a plant ratio factor to allow for the thermal capacitance of the structure where intermittent heating is used.

The emitter will be required to provide full heat output at start-up, but solar gains and internal gains due to occupants, lighting and equipment will quickly reduce the heat load.

It is essential that the output rate of the emitter can be efficiently controlled both to ensure thermal comfort and to reduce energy consumption.

The output rate of any heat emitter can be achieved in one of two ways: modulating the water flow rate whilst maintaining the flow water temperature or modulating the flow water temperature whilst maintaining the water flow rate.

Modulation of water flow rate:

This is usually achieved by means of a two-port or three-port valve which throttles the water flow rate in response to a room temperature sensor. For radiators and radiant panels, thermostatic radiator valves (TRV) are invariably used. These are low cost and self-actuating valves requiring no wiring. Be aware that the response curve of the system is highly non-linear and such non-linearity requires a valve characteristics which will produce a large reduction in flow rate for a small valve stem movement, but they are relatively expensive.

Flow temperature modulation:

Flow temperature modulation is achieved by the blending of return water with the flow using a three-port mixing valve. The response of this control is very nearly linear, so good control is achieved. Unfortunately, such valves and control systems are relatively expensive so this system is used for controlling large groups of emitters.

Combined flow rate and temperature modulation:

Good control is achieved by combining the two methods. Flow temperature modulation can be carried out centrally by scheduling it to outdoor air temperature. Local two-port valves, controlled by the local room temperature, then modulate the water flow rate to each emitter in response to changing solar and internal heat gains. This combination of central and local control is specified in the Building Regulations.

Control systems in building energy management systems

Traditionally many building services systems are controlled using either pneumatics or electric/electronic and mechanical devices such as the five elements in CVVT control: valve body, valve actuator, immersion thermostat, outdoor detector and controller.

Direct digital control and supervisory control can be more user-friendly and can give the user more control over the building services systems either locally or remotely via a modem to a building energy management system (BEMS). The capital costs and advantages of a BEMS depend upon whether the user has the time and commitment to use this facility and take full advantage of the technology.

The local area network (LAN) might include BEMS outstations or original equipment manufacturer's (OEM) outstations. This would be linked to a modem if the final control and monitoring location is remote. Software is generated and dedicated to operate the controls and relay system conditions such as temperature, relative humidity, pressure, pressure drop, and status such as duty plant operation, standby plant operation. These conditions can be called up on a visual display unit (VDU) or monitor, and will include system logic diagrams. The way a BEMS is connected to a LAN is called the topology, of which there are basically three: bus, star and ring.

The keyboard and central processing unit (CPU) complete with visual display unit (VDU), collectively called a HMI Central Station, are connected to the LAN via modem.

Outstation functions:

These are split in three levels:

1. High Level: remote communication, user interface, optimizer control, cascade control, maintain trend logs, maintain event logs.

2. Mid Level: proportional plus integral plus differential control (PID), main data control (MD), alarm check, alarm communication, calendar/clock control, program defined interlocks.

3. Low level: hard-wired interlocks, timer control, check input limits, sensor interface, scan inputs.

Interlock systems:

Interlocks can be considered as "don't until" statements, and are of importance in defining the control strategy and in detailing the schematic.

An example of a system of interlocks of plant operation might be:

- Don't start primary heating pump until the time is right.
- Don't start the lead boiler unless primary pump is energized.
- Don't start secondary pump on zone 1 until boiler primary circuit is at 80 degrees Celsius ...

Supervisor station and central station functions:

- Plant supervision.
- Maintenance supervision.
- Security supervision.
- Energy monitoring.
- Environmental monitoring.
- System development.
- Plant executive control.
- Reporting.
- Data archival.
- Design evaluation.

Control strategies for heating systems:

These are a few recommendations relating to controls for heating systems:

1. The boiler primary circuit should be pumped at constant volume and be hydraulically independent of the secondary circuits.

2. Domestic hot water (DHW) should be provided by a separate system.

3. The temperature to each zone should be compensated according to outdoor temperature (clearly this cannot apply to fan convectors or unit heaters).

4. The zones themselves should be selected on the basis of:

- Solar heat gain (building orientation).
- Building exposure (multi-storey buildings, horizontal zoning..).
- Occupancy times.
- Building thermal response.
- Types of heating appliance.

5. Frost protection during off periods.

6. Sequence control of multiple boilers.

7. Control of demand.

8. Summer exercising of pumps.

9. Flue gas monitoring and alarm.

10. Graphic display of operating times and off times.

System operation method statement:

An important part of the schematic or logic diagram control is a written statement that explains how you intend the systems to operate.

The Method Statement will include:

- Details of zoning by time scheduling and control of temperature.
- Description of plant and circuits, which would include space heating, ventilation, hot water supply.
- Description of interlocks.
- Client interface (how the client can use the system).
- Specialist interface (adjust/monitor/refine and maintain).

Further reading and information in:

CIBSE Guide H: Building control systems
CIBSE Guide B1 Heating

PLC CONTROL SYSTEMS

PLC CONTROL SYSTEMS

The development of low cost computer has brought the Programmable Logic Controller (PLC). PLC´s have gained popularity on the factory floor and industrial control process, most of this is because of the advantages they offer:

- Cost effective for controlling complex systems.
- Flexible and can be reapplied to control other systems quickly and easily.
- Computational abilities allow more sophisticated control.
- Programming is easier and reduce downtime.
- Reliable components make these likely to operate for years before failure.

There are different types of control such as continuous and discrete controls.

In the continuous control systems the values to be controlled change smoothly and they can be linear or non linear. Linear means that can be described with a simple differential equation (e.g. PID), while in the non-linear systems the mathematics become much more complex (e.g. MRAC or FUZZY LOGIC).

In the discrete control systems, they could be conditional or/and sequential. Conditional or logical means that the value to be controlled are described, with BOOLEAN and EXPERT SYSTEMS, as on-off. The sequential control is a logical control that will keep track of time and previous events, event and temporal based using TIMERS and COUNTERS.

Logical and sequential control is preferred for system design. These systems are more stable, and often lower cost. Most continuous systems can be controlled logically.

When a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators. That means that the process will change over time because the actuators will drive the system to new states or modes of operation.

The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any other computer this does not happen instantly. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. If the PLC passes the sanity check it will then scan all the inputs. After the inputs are stored in memory, the ladder logic will be solved using the stored values (not the current values). When the ladder logic is complete the outputs will be scanned and changed. After this the system goes back to do the sanity check and the loop continues indefinitely.

Many PLC configurations are available such as rack, mini, micro, but the most essential components of all them are:

Power Supply: this can be built into the PLC or can be an external unit. Common voltages required by the PLC are 24 Vdc and 220 Vac.

CPU: Central Processing Unit, this is the brain where ladder logic is stored and processed.

I/O Units: These are the input/output cards provided so that the PLC can monitor the process and initiate actions.
Input cards can be digital or analog, and common input sensors can be proximity switches (using inductance, capacitance, light or mechanical mechanisms), temperature sensors PTC´s, potentiometers for measuring angular positions...
Output cards have typically 8 to 16 outputs and these outputs can be relays, transistors or triacs. Relays are the most flexible but they are slower and they will wear out after millions of cycles, relay outputs are often called dry contacts. Transistors are limited to DC outputs, and Triacs are limited to AC outputs. Popular actuators are solenoid valves, lights, motor starters, servomotors...

Indicator lights: These indicate the status of the PLC including power on, program running and a fault.

Distributed Generation (DG)

Distributed Generation:

Nowadays, a key change is the movement from a relatively small number of large and centrally controlled power stations connected to the transmission system towards a greater number of generating plants connected to both the transmission and distribution systems.

Generation connected to the distribution network is called Distributed Generation (DG). If you install DG you can use the electricity you produce on-site, hence lowering your electricity demand and bills. You can also sell electricity to customers, suppliers or, depending on the size of the generation, on the wholesale market.

Some roles and components of the traditional power sector that you have to notice are:

- Generating plant: composed by the Generators in charge of producing electricity. Most electricity is generated between 11,000 and 33,000 volts AC by a different kind of technologies (high speed steam turbine alternator, wind turbine, wave motion, hydroelectric, oil, gas turbine, nuclear technology...).

- Transmission system: composed by the Transmission System Owners (TSOs) whose commitment is transport power from generating plants to distribution networks. Once generated, the electricity is transformed in value to 132, 275 or 400 kV and delivered to the National Grid

- Distribution network: called Distribution Network Operators (DNOs) and they distribute power from the transmission system to customers. National Grid substations provide the means to reduce the power taken from the grid by transforming it back to 11 kV again. The reduced voltage is then transmitted over short distances across country and routed to community transformers where it is further reduced for industrial, commerce and domestic voltage (400/230 V).

The transmission and distribution systems are owned and operated by regulated monopoly business.

Electricity cannot be stored and so demand has to be balanced with generation on a second by second by the System Operator. National Grid Electricity Transmission (NGET) is the System Operator in Great Britain. To match generation with demand, the System Operator could ask generators to increase the output of their plant. Conversely, some large customers on certain contracts can be asked to reduce their demand.

Until relatively recently, the design and operation of most electricity distribution networks has been based on the key assumption that power always flows from higher voltage systems to lower voltage systems. The increased penetration of DG is changing this landscape.

Benefits and impacts of Distributed Generation:

There are a variety of benefits to having Distributed Generation, on top of the financial revenue you could have from selling some or all of the electricity that you generate, others include:

- DG can be a renewable generating technology (e.g. wind, solar, CHP, tidal..). This means the DG does not rely just on fossil fuels, so it is sustainable in the long term and does not produce or reduce emissions.

- The introduction of local generation in businesses and communities can lead to greater awareness of energy issues.

- The use of CHP plants result in higher efficiencies than generating electricity and heat independently.

- There is a reduced need for the distribution and transmission infrastructure, therefore transmission and distribution losses are reduced.

As well as introducing benefits, the increase of DG in distribution networks also poses challenges:

- DG changes the current flows and shape of the load cycle and this could cause thermal ratings to be exceeded.

- DG can cause system voltage to rise beyond acceptable limits.

- DG could cause reverse power flows that means that the power flows in the opposite direction to which the system has been designed.

By taken this picture as a summary of the new energy technology challenges that the future have for us. Enjoy and share with us the creation a new community of knowledge.

Saving energy is less expensive than buying it.

Wellcome!

The objective of this blog is to create a base community of people interested in promoting and exploring the fantastic and amazing field of energy engineering.

All the suggestions, questions and resources are welcome.

This is a free space of knowledge and opinion. Don´t forget it!