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Mini-project report

Analysis of a Rooftop Greenhouse

Alex Buckman

alex.buckman@sheffield.ac.uk

14th February 2012

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Abstract

The Seawater Greenhouse office in London is a three storey building with the top storey designed to be a working

greenhouse. This concept has been developed to reduce the heating load within the building whilst providing locally

sourced produce. This report will develop a 2D CFD model of the building to demonstrate the heat flow provided by

the greenhouse. Specific improvements will be investigated to improve the energy efficiency of the building. The

model will then be used to provide suggested improvements to the design, with a focus on use within a public

building.

Introduction

Many countries have set targets for the reduction of greenhouse emissions. In the UK a large focus is being put on

reducing energy demand by reducing the energy consumption of the building sector (Act on CO2, 2008). More

stringent regulations are being implemented on a regular basis and the rooftop greenhouse aims to provide an

opportunity to reduced energy consumption whilst allowing the ability to grow local produce, reducing secondary

energy consumption.

Modelling the Building

The modelling of the building will be carried out using Ansys Workbench. The geometry and mesh creation will be

carried out using Ansys and the simulation and solution will be carried out using Fluent. A number of assumptions

have been made to allow the development of a 2D model.

Geometry Creation

Figure 1 shows the dimensions of the model to be used in this

analysis. These dimensions have been taken from the architectural

plans of the building. This model has been chosen to represent the

key features of the building in 2D. The windows will therefore be

on the right hand side wall of the model and the extractor fan will

be to the left of the greenhouse roof on the horizontal boundary to

remove the possibility of undesired and unrealistic effects caused

by the modelling of radiation as a heat flux on the glazing.

Figure 1 - Dimensions of Simplified Building Model

3 Physics Specification

The flow is assumed to be fully turbulent and the κ-ε turbulence model is used. The energy equation needs to be

activated in the model and the Boussinesq approximation is used. In order to allow this induced motion within the

air gravity needs to be activated in the model.

All external walls, roofs and windows (except the greenhouse windows) were set to the external temperature as

there is a significant amount of insulation (u value ≈ 0.22) keeping the heat within the building. All internal walls and

floors were given fixed temperatures that correspond to the measured internal temperature of the building.

The greenhouse roof and window were given heat flux to represent the incoming solar gain. The gaps either side of

the windows have been treated as velocity and the gaps to the side of the extraction fan and roof light have been

treated as pressure boundaries.

Steady State Conditions

The steady state conditions to be used for summer and winter validation were selected through the filters shown in

Table 1.

Filter Summer Winter

Months June – August December Time Midday to ensure good angle of

incidence Midday to ensure as little shading as possible

Wind Under 0.3ms-1 Under 1ms-1

Radiation High (direct light) Low (diffuse light) Internal Temperature

18<x<24 to allow both fans to be working

Table 1 - Filter Factors

For the initial model I shall choose to use the summer conditions at midday on July 26th

Date Time Temp Out

Wind Speed

Wind Dir

Wind Run

Hi Speed

Hi Dir

Solar Rad.

In Temp

26/07/2011 12:00 18.3 0 E 0 1.3 NW 354 23.9

The winter conditions shall be

Date Time Temp Out

Wind Speed

Wind Dir

Wind Run

Hi Speed

Hi Dir

Solar Rad.

In Temp

21/12/2011 12:00 10.6 0.9 NW 1.61 2.2 NNW 38 10.9

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Validation

In order to validate the model the measured temperatures within the greenhouse will be compared to those in the

model. This will be carried out on two different dates in different conditions.

Figure 2 shows temperature contour plot for the summer steady state conditions.

Figure 2 - Temperature Contour Plot for Summer Steady State Conditions

The probe representing the temperature sensor can be seen in Figure 2 and is at a temperature of 24.3 degrees.

Celsius. This is 0.4 degrees higher than the steady state conditions and is close enough to validate the model.

The winter conditions were then used to further validate the model. Figure 3 shows the results from this validation.

Figure 3 - Temperature Contour Plot at Winter Steady State Conditions

The temperature within the greenhouse was measured to be 10.85 degrees Celsius, which is within 0.05 degrees of

the actual measurement. This demonstrates that the model is not only valid but is able to adapt to varying

conditions.

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Results

It is interesting to look at the results to see the mechanism of heat flow throughout the building.

Figure 4 - Velocity Vector Diagram with Both Fans Working

Figure 4 shows the varying velocity of air within the building in summer conditions. It can be seen that the pressure

boundaries on the skylight and extractor fan are acting as outlets, having originally been set at zero pressure

boundaries. This shows a negative pressure gradient across the boundaries.

Figure 5 - Streamlines for all Heat Flows

Figure 5 shows the warm and cold streamlines of air throughout the building. The airflows mix well and reach the

majority of the building. It can be seen that the heat leaving the ground floor fan rises immediately and would be

unlikely to have a significant effect on the heating within this floor when there is a free path for the air to flow

towards the higher floors.

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Discussion

Can the Extractor Fan be Removed?

The extractor fan in the greenhouse comes on when the temperature reaches 25 degrees Celsius and ramps up to

90% of its power by 29 degrees Celsius. As the temperature increases within the greenhouse the buoyancy of the air

will increase and would therefore exert a higher pressure within the greenhouse. It is interesting to look into how

effective it would be to utilise the stack effect in the building (or a building of similar scale).

The fan installed in the greenhouse has a maximum airflow of 8.45m3h-1 (Vostermans, 2012). At 25oC the fan will be

at 40% power.

Initially the stack effect at this temperature shall be calculated and compared (Moss, 1998):

( )

If a 0.3m2 opening was implemented on the ground floor and one in the flow rate through the house would be:

[

( )]

Where is the coefficient of diffusion and assumed to be 0.61. is the mean temperature between the exterior

and interior conditions.

[ ( )

( )]

The results shown here demonstrate that the effectiveness of the fan is very little in comparison to stack ventilation.

It has been noted that when the fan is activated there is a noticeable draft through the building. However, this may

well be due to the stack effect occurring through the open gravity shutters. The results show that replacing the

extractor fan with a sealed panel, controlled to open varying amounts as the temperature ranges from 25oC to 29oC,

and an equivalent sealed panel on the ground floor will adequately replace the fan’s function and save the energy

needed to operate the fan.

Improvements

The following aspects could be improved in the current Seawater Greenhouse building and any public buildings in

the future;

1. Temperature sensors implemented throughout the building would allow more accurate validation of any

models created.

2. A closable, air tight door/hatch to the greenhouse

3. Improve the air tightness of the building

7 The following improvements could be made when applying the technology to a public building in the future:

1. Use a reflective frit in the ETFE/polythene cushion (Poirazis, et al., 2009). This will allow the control of

radiation into and out of the greenhouse.

2. Make better use of thermal mass by using specifically selected, high heat capacity and moderate thermal

conductivity, materials for the construction of the greenhouse rather than a thermal wall in the cavity.

3. Excess heat could be used to preheat water within the building instead of being vented

4. Increase the percentage area of glazing on the greenhouse

5. Utilize the CO2 generated in a public building.

Future Work

The following areas would be useful to further research in light of this project.

Model including wind flow and fluctuating radiation conditions throughout a day

The effects of implementing suggested improvements

The optimum power of the fans needed

Assessing the energy saving differences between running both internal fans and using excess heat to preheat

water and thermal mass.

Inclusion of walls/floors in the model, which allow heat flow through them.

Conclusions

The rooftop greenhouse implemented by Seawater Greenhouse has been proven to distribute heat throughout the

building well in summer conditions when both internal fans are working. It has been shown that both fans need to

be used for good circulation of air, but the extractor fan could possibly be replaced with an active shutter device to

make use of stack ventilation.

Until further research into health is carried out, the air circulation from the greenhouse through a cavity wall should

not be implemented in a public building. However, the use of excess CO2 should be encouraged by integrating the

greenhouse within the building, as Seawater Greenhouse have done. This will allow the flow of CO2 rich air into the

greenhouse, as shown by the streamline diagrams.

The effect on reducing the auxiliary heating load when optimised in the ways mentioned above is significant and the

possibility of large amounts of locally produced food will save a lot of secondary energy, which would be particularly

of interest for supermarkets, who will pay directly for these costs in their supply chain.

In winter conditions, more measures need to be taken to ensure that the building will remain within the human

comfort zones whilst not using more heating than previously would have been used. This would be implemented by

using an insulated shutter to the greenhouse, stopping the flow of warm internal air into the greenhouse.

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References

Act on CO2, 2008. The UK Low Carbon Transition Plan, London: HM Government.

Firsttunnels.co.uk, n.d. http://www.firsttunnels.co.uk/subcategory.asp?catid=6. [Online].

Fluent, n.d. 13.2.1 Heat Transfer Theory. [Online]

Available at: http://iceberg2.shef.ac.uk/docs/fluent63doc/html/ug/node568.htm#86361

Incropera, F. P. & DeWitt, D. P., 1981. Fundamentals of Heat and Mass Transfer. Fourth ed. s.l.:Wiley.

Johnston, D. D., Wingfield, D. J. & Bell, P. M., 2004. Air Tightness of Buildings - Towards Higher Performance, Leeds:

Leeds Met University.

Lebens, R. M., 1980. Passive Solar Heating Design. s.l.:Applied Science Publishers Ltd.

Moss, K. J., 1998. Heat and Mass Transfer in Buildings. 2nd ed. Abingdon: Taylor & Francis.

Poirazis, H., Kragh, M. & Hogg, C., 2009. ENERGY MODELLING OF ETFE MEMBRANES IN BUILDING APPLICATIONS.

Glasgow, Arup.

Vostermans, 2012. Vostermans Ventilation. [Online]

Available at: http://ventilation.vostermans.com/en-v/multifan/fans/panel_and_wall_fans

[Accessed 13 2 2012].