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Executive Summary
The purpose of this report is to recommend to the Financial Services Company the best way to develop its planned expansion business park. This will be done as a consultant to the FSC sustainability manager. The Sustainability Manager will take into account energy use, carbon emissions, and climate change in his considerations as to what energy efficiency solutions to employ. The overall goal is to obtain a building with lower emissions and higher environmental credentials than the standard building set. However, these goals must be balanced with budget constraints realities. Hence there is little provision for experimental or unproven technologies.
The paper is from the point of view of an energy and environmental design specialist who has been appointed by the FSC to provide advice on the options for a new building with low carbon emissions. In addition to characterizing the contents of proposal A, the specialist will also design methods to improve the building along with the desired matrices while keeping in mind that there is a limited budget to implement such changes.
The end goal is to provide a report to the FSC’s senior management to summarize the design options for a low-carbon building and the corresponding implications.
Introduction
Energy usage is an important aspect of the recommendations presented to the FSC. Energy usage is linked to the insulation properties and the levels of ventilation. Another factor that will be considered is Ventilation. Ventilation is essential for the provision of a comfortable and healthy indoor environment for the occupants of the building. The main purpose of adequate ventilation is to provide a system of fresh air at an acceptable temperature, moisture content, and flow rate while concurrently removing stale, unwanted air. If the ventilation system is inadequate indoor air will become damp and stale. As more indoor air is breathed by the people inside and it is not replaced via proper ventilation oxygen levels decline and the air becomes ‘stuffy’ or ‘stale’ The CIBSE recommends fresh air flow rates of 8 liters per second per person (half a cubic meter every minute). If there is an inadequate fresh air supply this can lead to maladies ranging from dry nose and throat to headaches and even nausea. While these appear to be minor complaints in the long term this can result in absenteeism and what is known as sick building syndrome.
Since the building is located in the London area the ventilation system can also be used to keep the building cool. Thus, during the summer months, the ventilation requirements for cooling are actually the dominant factor in determining the ventilation rate. A part of the ventilation requirement can be afforded by natural ventilation. For the purposes of the client’s building, low-energy mechanical solutions may be required.
Sustainability Options for the FSC Building
It is the task of the environmental design specialist to achieve the following two goals:
- Achieve much lower emissions and higher environmental standards than previously, as well as improving natural ventilation and mechanical cooling.
- Identify the main areas for improvement in energy performance and carbon emissions, taking into account the implications for occupancy comfort.
In order to accomplish these two tasks, it is important to take into consideration the architectural characteristics of Proposal A. Potential advantages of Proposal A will be judged on its thermal performance, heat loss, and internal gains. Recommendations will be given on how to improve upon these characteristics.
Proposal A Characterisation
Fabric of Proposal A
Analysis of Proposal A. Heat Loss Computations
Heat loss due to conduction from the building:
It is assumed that the outside and inside temperatures are -1oC and 20oC respectively.
Conduction heat loss is computed as U-value x Area x Temperature Difference.
Assuming an outdoor temperature of -1oC and the desired room temperature of 20 oC and using the area and U-values derived from the brief the following computation is derived.
21.0 x (0.25 x 775 + 1.8 x 4.8 ) + 10 x 1 x 415.2 = 8402.19W
Conduction loss is = 8402.19W.
Ventilation Heat Loss
Building Volume = (10.5 x 35 x 25 ) + (10.5 x 5 x 5) = 9450 m3
The ventilation heat loss is 1/3 of the air change rate x volume x temperature difference
As a private office, the air change rate, in this case, will be 4 each.
Ventilation loss = 1/3 x 4.0 x 9450 m3 X 21.0 = 264,600W
This suggests that if the desired air change rate is achieved there will be significant heat loss as a result of air conduction.
Ventilation Requirement Analysis
The building exposed fabric area is;
2 m3/h.m2 is the leakage flow rate of the external fabric area under normal conditions. However, for testing purposes, we will use 10 m3/h.m2 to represent a pressure test. This test will be done under a 50pa pressure difference.
10.5 x [(20 X 2) + (35 X 2) + (5 X 5) x 2] (Walls (35X25) + (5x5x2)] = 2605m2
2605m2 x 10 m3/h.m2 =26,050 m3/h
26,050 / 3600 = 7.236 m3/s or 7,236 l/s
7.236 m3/s or 7,236 l/s is the leakage of the building fabric under test conditions.
2605m2 x 2 m3/h.m2 = 5210 m3/h
5210 / 3600 = 1.447 m3/s or 1447 l/s
1.447 m3/s or 1,447 l/s is the leakage of the building fabric under normal conditions.
Since the building is a low rise structure the 50pa will be divided by 30
5210 / 30 = 173.67 l/s
The building volume, is = (10.5 x 35 x 25 ) + (10.5 x 5 x 5) = 9450 m3
The infiltration rate is 173.67 * 3600 / 1000 = 625.21 m3/h
In terms of volume chance rate this is equivalent to = 625.21 / 9450 = 0.067 ac/h
The building, without any modifiers, has very poor air circulation.
Since the building has 2025 m2 of office space and an expected 10 m2 of space per worker this
The minimum fresh air ventilation rate should be 12.0 l/s per person.
203 (occupants) x 12 l/s = 2436 l/s
When compared to air change rates this is equivalent to;
2436 x 3600 / 1000 gives total employees at 203.
5209.3 = 1.683 ac/h note that this figure is only the amount of air change needed by the people in the building and does include the ventilation needs of the computers and other electronic equipment. However, when compared to the actual air change rate of the building via infiltration (0.067 ac/h) without any other means of ventilation the people inside are likely to suffocate.
The associated energy demand per degree is 1.2v or in this case;
1.2 x 2436 = 2923.2 W/oC.
With respect to the infiltration rate, the energy demand would be
1.2 x 173.67 = 208.4 W/oC.
Heat Gains Analysis
In order to give easily comprehendible recommendations to the client, it is appropriate to normalize each type of heat source gain to a 10m2 area which is the occupancy per usable floor area.
Total Gain = 433.8 W per 10m2. When normalized to a per square meter basis the total average internal gain in the office working space is 43.4 W/m2
Normalizing this to a per square meter basis the total average internal gain in this building is 43.4 W/m2
Server Room Heat Gains
Heat gain for the computer rack = 20 X 120 = 2400 W
Normalized to a square meter = 2400/25 = 96 W/m2
Including lighting gains = 96 + 5 = 101 W/m2
Once all the factors are considered, Proposal A appears to focus on heating losses and heating energy instead of overheating. Had the building been located in a temperate climate there would be a greater need for cooling. But given local conditions in London cooling is of less concern.
Based on the figures found above, the computers (187 W per 10m2) are the most significant source of heat gain followed by the heat generated by the workers (90W per 10m2). As a large open plan office, these two gains are predominant and in order to maintain optimum working conditions. Since there is a 10m2 cubicle workspace allotted to each worker and each worker has a computer the amount of heat generated would be considerable. Furthermore, the server room is another area of very high heat gain (101 W/m2) in fact the heat gain in the server room is almost double that of the rest of the building.
The conventional VAV or Variable Air Volume air conditioning system with air-to-air condensers and air handling plant is definitely needed. The VAV system provides a centralized system with air distribution. The cool air is generated from a central point and distributed on an as-needed basis via a local thermostat. If all zones have low cooling requirements the system reduces the output from the central fan. Heating coils are also present to provide heating during the winter. VAV is an effective but complex solution to the building’s needs.
Proposal A is sound, cost-effective, and readily available. It is an acceptable starting point from which further improvements can be suggested.
Energy Efficient Modifications
Alternative Plan B – Hybrid Ventilation
Plan A already deploys a form of mechanical ventilation in the form of a VAV air conditioner with air to air condensers. In order to lower the building’s energy requirements, it may be feasible to employ a natural ventilation system that relies on wind and the buoyancy effects of temperature differences. As the building is located in the suburbs of London this strategy becomes even more feasible. Given the size of the building and the number of people using the building on a daily basis, Natural ventilation alone may be insufficient to meet the building’s ventilation needs. The offices also have high heat gain and require a clean environment for the computer equipment. An alternative plan can be designed around the use of a hybrid ventilation system. This will exploit the benefits of both natural and mechanical ventilation systems while avoiding their potential limitations.
Plan A employs a form of natural ventilation in the form of the Atria. The atria have an effect similar to a stack in assisting ventilation. At day time it introduces a major space into the design of a building which can also be used to collect solar gain and thereby enhance the ventilation flow by inducing a vertical flow in the atrium which draws air through the adjoining spaces. By installing windows that can be opened, the spaces around the atrium can open onto it. At a high level, these openings allow warm exhaust air to pass upwards.
To implement this plan, two changes to the building will be made. The area around the Atria will be designated the Core area. In the Core area, high heat emission equipment will be placed. These include the server room and the pantry equipment that will be installed to supplement the vending machines. By placing them next to the Atria the heat they generate will be quickly flushed out of the building. The middle lobby will also contain a small pantry area where cooking equipment can be placed. In both cases, the goal is to allow the heat generated by these facilities to be removed quickly. By allowing the waste heat to drain into the Atria and be expelled there the building plan allows for a reduction in energy consumption from having to vent the heat energy via the outer walls or relying on conduction from the air conditioning system entirely.
Both the exterior walls, leading to outside the building, and the internal walls leading to the Atria will have windows. Not only will they provide the benefits of improving air circulation, they will also provide a fail-safe should the air conditioning systems fail.
Mechanical ventilation will be used for the outer sections of the building with airflow from the air conditioners focused towards blowing the air to the Atria section. The Areas near the Atria will use natural ventilation using slanted windows that will allow cool air from the lower levels to flow in while allowing warm air to flow out. At the top of the building, mechanical blowers will be installed at the Atria roof to help facilitate the movement of warm air out of the building. The Atria’s stack effect will also be supported by the glazed roof which will aid in heating the air in the Atria and further promote its quick exit from the building itself.
This way the air conditioning system is only required to supply cool air into the building. Natural ventilation will take care of flushing the warm air into the Atria where it will finally be expelled out of the building. Only in the cooking area and the server room will mechanical heat recovery devices be employed.
Alternative Plan C – Replace VAV
VAV and the heating condensers are an adequate solution to the building’s heating and cooling needs. However, it requires a large amount of energy to operate and some efficiency is lost because of its centralized nature. One alternative to the VAV is Evaporative cooling.
When water evaporates, it takes heat from its surroundings thus resulting in a cooling effect. Direct cooling results from spaying the water directly into the supply airstream. However, it can also be sprayed into the exhaust air, which is then passed through an air-to-air heat exchanger to cool the air supply. This system avoids changing the humidity of the air supply.
Indirect cooling is where the airstream is separated from the evaporating water by a heat exchanger. It can take the form of a cooling tower, from which water can be used as chilled water. Indirect cooling has the advantage of avoiding the possible microbiological contamination of the supply or exhaust air.
The primary drawback of evaporative cooling is that performance is lowest when it is needed the most, specifically during the summer months, when the outdoor air is least able to absorb the evaporated moisture. In other words, it needs to be supplemented by the hybrid ventilation system suggested under Plan B.
Alternative Plan D Replacement of VAV and Hybrid Ventilation
Both Plans B and C have their merits in improving the performance of the building. Both are attempts to improve the baseline found in Plan A When combined the improvements offered by the replacement of the VAV with a lower energy alternative like the Evaporative Cooling system and the improvements in the hybrid ventilation system will provide the best improvements over the status quo plan A.
Conclusions and Recommendations
Perhaps the best-case scenario would be to employ both replacing the VAV (Plan C) and Hybrid Ventilation (Plan B). Since the main goal of the ESC is to improve the performance of the building including both enhancements to the design would be the best course of action. Hence Plan D is the recommended plan of action. Not only will overall power consumption go down by employing a hybrid ventilation system but the mechanical component will use less power because it will employ an evaporative cooling system.
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