Energy Considerations for Buildings
The energy consumption of a building is determined from the very first design decisions through to final demolition.
Conception and Design
In the very beginning of the design process, many architectural choices can be made to significantly increase, or decrease, the energy consumption of a building. For example, large un-shaded windows that face the afternoon sun can greatly increase the cooling load. Alternatively, the same windows, facing north produce a relatively small cooling load.
It is at the early design stage that the mechanical designer should become seriously involved in the building design as a whole. Historically, the architect would design the building, and then send a set of plans to the mechanical designer to design the HVAC. This model does not work well to produce energy-efficient buildings, because many early design choices can facilitate energy conserving design or make them totally impossible or uneconomic.
Consider this Example:
In cold climates, a perimeter hot water heating system is often used to offset the heating losses through the wall and windows. Because modern windows are available with insulation values, that approach the insulation value of traditional walls, if the architectural design specifies walls and windows with higher insulation values, the perimeter heating system requirements could be avoided. However, this is a suggestion that would typically be made by the mechanical designer, and the choice can only be made very early in the project. If the mechanical designer suggests a more energy efficient design, this could have a negative impact on the mechanical design fee. Why? Building owners often contract with the design team members for a fee that is based on a percentage of their individual portion of the building cost. In the example just given, the fee for the mechanical designers would include a percentage of the cost of the perimeter heating system. As a result, if the mechanical designers suggest that the perimeter heating be omitted in favor of higher priced windows, the they could be forfeiting a substantial portion of their fee. Hardly an incentive to the engineer to suggest the idea!
Since this method of calculating the mechanical design fee does not encourage energy conservation, what other alternatives are available?
Imagine an alternative fee structure, where the total design fee for the mechanical design would be calculated as a percentage of the cost of the completed building, rather than of the specific mechanical design elements. Then, the mechanical designers could make design suggestions that would not have a negative impact on their design fees. Furthermore, imagine what would happen if the contract also specified that an objective of the building design included energy savings, and provided the entire design team with financial bonuses based on achieving the energy savings. Then the design team would have an incentive to spend time on designing energy efficient buildings!
How could this bonus incentive be structured? Consider what would happen if the bonus represented half the energy savings that were achieved during the first five years after the building was completed, (based on the estimated energy costs for a conventional building design). In this case, the design team would have an incentive to design for maximum energy savings. The result would be that the operating expense for the owner would be reduced by half the energy cost reduction during the first five years, and after the first five years, the owner would receive the benefit of all future energy-related savings. In this scenario, the owner could save money by setting up the contract to encourage desired behavior! Notice that there is not necessarily any additional capital cost to the owner, only the likelihood of operational cost savings: a huge return based only on some contract wording.
In case you are thinking it would never work, you should know that many owners are willing to contract to have energy conservation specialists come back, after construction is completed, and to pay them a significant fee, in addition to retrofit costs, to fix what could have been achieved as part of the original design at a fraction of the cost. We will discuss energy conservation that can be achieved through retrofit in the section entitled: “Turn it in.”
Construction
The best possible building plans can be made a mockery by poor construction. If windows and doors are not sealed to the walls, and/or if insulation is installed unevenly and with gaps, the air-leakage can be costly in terms of both energy and building deterioration. The mechanical plant must be installed and set working correctly. Many systems are surprisingly robust, and gross errors in installation can go undetected, making the building less energy efficient —and less comfortable—than it was designed to be.
Operation
If the staff does not know how a system is meant to work, there is a very high probability that they will operate it differently and, more than likely, not as efficiently. It is really important that staff are taught how the systems
are designed to work and provided with clear, easy to understand, written instructions for later reference. A pile of manufacturer’s leaflets may look pretty but it does not explain how all the bits are meant to work together.
Maintenance
With limited maintenance, even the best equipment will falter and fail: Controls do not hold their calibration and work indefinitely. Control linkages wear out; damper seals lose their flexibility; cooling towers fill up with dust; the fill degenerates; and chiller tubes get fouled with a coating which reduces their heat transfer performance. The list of maintenance requirements is very long, but critical for maintaining energy-efficient building performance.
Three Ways to Save Energy
The mantra of energy savings is: Turn it off. Turn it down. Turn it in.
Turn it off
This is the simplest and almost always, the least expensive method to imple- ment, and it has the highest saving. If a service is not required, can it be turned off? There are usually several alternatives that can be considered to shorten the running time to the minimum.
Opportunities to “Turn it off” can be found at the design phase and at the operational phase of a building’s life cycle.
Let us take a simple example of stairway lighting in a mild climate. For this example, we will ignore any local issues of safety or legislation:
A four-storey apartment building has stairs for access. If the stairs are fully enclosed, the lights must be “on” all the time for people to see their way up and down the stairs.
The first alternative for energy savings can be identified early in the design phase of the building: Designing the stairs with large windows allows the lights to be turned off during daylight hours. The light switching can easily be done with an astronomical clock, or better still, a photocell. The astronomical clock allows for the changing lengths of the day, while the photocell senses the light level and switches on and off at a preset light level.
At both the design and the operation phase of the building’s life cycle, a second savings opportunity exists. To discover it, consider asking the question: “What is the objective of having the lights on?” The lights are to provide illumi- nation for people to go up and down the stairs. The next question is, “Is there a way to provide illumination when it is required, and yet not have the lights on when it is not required?” Several solutions come to mind. A low tech solution could be the installation of a pneumatic push-button timer switch at each level. Then, people entering the stairwell could push the button and turn the lights on for, say, ten minutes. The advantage is that, now, we have a simple system that provides the required service when it is required. However, there is an education requirement with a system like this. People need to be shown where the light switch is located. And they need to be taught that, even if the stairwell has been illuminated because an earlier person turned the switch “on,” they still have to reactivate the switch, in order to provide continuous illumination while they are in the stairwell. For example, if one person has entered the stair- well and depressed the switch, the stairwell will be illuminate for ten minutes. Nine minutes later, a second person, enters the stairwell and, because the light is “on,” does not look for a switch. While that second person is in the stairwell, the lights will go off, leaving that person in the dark. As a result, graphics- based signage would be required, to manage issues based on language and reading skills. Therefore, to alleviate these signage issues, as an alternative, the switch could be wired to detect and respond to the opening of the lobby door or motion detectors could be used to turn the lights “on.”
The above example illustrates how a building design choice, in this case, windows, allowed a substantial reduction in operating hours. Then thinking about “What is the objective?” allowed a further, large, reduction in operating hours.
Determining design parameters based on a requirement to “turn it off” may seem extreme, but it is the norm in many parts of the world. You would probably be surprised at how many opportunities you could find in your own experiences where things could be turned off, and energy could be saved, if the focus was on providing only what is needed.
Now let’s go on the second approach, which tends to be more complicated, and therefore more costly, to work out and implement.
Turn it down
“Turn it down” meaning reduce the amount of heating, cooling or other process while still providing the required service. In Chapter 4, when we covered CO2 control of ventilation air, we discussed the idea of only providing the required amount of a service at the time it is needed. As you recall, CO2 was used as a surrogate (indicator) for assessing the room population and deciding how much outside air was required for the current occupants. Using CO2 as a surrogate allowed the amount of outside air to be turned down when the room population was low.
There are numerous examples of using “turn it down” as an energy conservation tool. Two that are commonly implemented include:
Heating reset: In Chapter 8 we discussed resetting the heating water temperature down, as the load drops. This reduces piping heat losses and improves control. However, on a variable speed pumping system, lowering the water temperature increases water volume required and so increases pumping power. The issue is finding the best balance between temperature reset and pumping power.
Chilled water temperature reset: The chilled water system is designed for the hottest and most humid afternoons that happen a few times a year. The rest of the time the chilled water system is not running to full capacity. Except in a very humid climate, where dehumidification is always a challenge, the chilled water temperature can probably be reset up a degree or two or more. This improves chiller performance and generally saves energy.
Turn it in
“Turn it in” means “replace with a new one.” This is the third way of saving energy. It is almost always the most difficult to justify, since it is the most costly. For example, your building may have a forty-year-old boiler with a seasonal efficiency of only 50%. A modern boiler might raise the seasonal efficiency to 70% and provide a fuel saving of 28%. Although the percentage saving is substantial, it can be frustrating to find that it would take 12 years to pay for a new boiler out of the savings. Typically, a 12-year payback is too long for the financial officer to accept.
It almost never pays in energy savings to replace building fabric. For example, replacing single pane windows with double or triple pane or replacing a roof with a much better insulated roof usually have energy savings that pay for the work in 30 years or more. However, if the windows are going to be replaced because they are old and the frames have rotted, then it almost always worth spending a bit extra on a higher energy-efficient unit. Here, one is comparing the extra cost of better windows against the extra energy savings, and it is usually an attractive investment.
While it almost never pays to replace building fabric, we should also note that it is usually economically worthwhile to repair the building fabric, particularly where there are air holes. For example, many industrial buildings have concrete block walls up to the roof. Over time, the block walls may well drop a bit, leaving a gap between wall and roof. Plugging this gap with expanding foam is a simple task and can reduce the uncontrolled flow of air into, and out of, the building. In a humid climate, this can substantially reduce the dehumidification load; in a cool climate, it could provide substantial heating energy saving.
It is exactly the same for the plant. The boiler may be 40 years old but it will work better if the burner is regularly serviced.
Chillers are another area of consideration. Due to the regulated phase-out of CFC refrigerants, many owners are being forced to consider chiller replacement. If the chiller is to be replaced anyways, it is worth taking the time to calculate the extra savings that are available from a high efficiency unit as compared to the extra cost for the unit. It is highly likely that the difference in cost for the high efficiency chiller will have a speedy payback in energy savings.
Having introduced three ways of saving energy – Turn it off – Turn it down – Turn it in, let’s move on to a Standard that sets minimum requirements for energy saving in new buildings and major renovations.