Biomass Installation Feasibility Tool

Heat Demand Model

The first step in sizing of a biomass boiler is calculation of heat demand of the building where this boiler is going to be installed. The estimation of the heat demand shows the amount and the monthly variation in energy that needs to be supplied to the building.

The model was created with the priority to be simple and easy to use but with the highest possible accuracy. The parameters that have to be set initially are the basic dimensions of the building (length, width, height), the number of floors and the number of air changes per hour. The external surface and the volume can be automatically calculated. After that the number and size of the doors and the percentage of the glazing surface are set.

Figure 1

# Heat losses

## Design Heat Loss

The Design Heat Loss, Q, for any building is

Q = (Σ(U∙A) + Cv)∙ΔΤ

Where

U : transmittance coefficient of every building component (W/m2K)

A : area of every building component (m2)

Cv = ventilation conductance (Cv = N∙V/3, N: number of air changer per hour, V: volume of the building)

ΔΤ : Difference between inside and outside temperature (oC)

The internal temperature was set at 21 oC. The outside temperature was the mean average temperature that was found from the 20 years data (1998 – 2007) of metoffice (www.metoffice.co.uk). The ground temperature was taken as 0oC.

Table 1

 Average Mean Temperature 1998 - 2007 North Scotland East Scotland West Scotland January 3.3 2.98 4 February 3.15 3.06 4.1 March 4.34 4.38 5.22 April 6.37 6.48 7.18 May 8.82 9.3 10.03 June 11.12 11.79 12.27 July 12.83 13.57 13.9 August 12.81 13.47 13.9 September 11.35 11.75 12.31 October 8.16 8.25 9.04 November 5.39 5.19 6.24 December 3.22 3.07 3.99

Figure 2 - Average Mean Temperature 1998 - 2007

As it can be noticed from above the design heat loss for every building depends, not only on the construction and size parameters, but also on the location where it is built.

## Hot Water Supply

The hot water provision (HWS) was found from CIBSE guide depending of the total floor area and the kind of building and not of the number and the activities of people inside it.

Table 2

 Building Type HWS (W/m2) Office - 5 or 6 day week 2 Shop - 6 day week 1 Factories - 5 day single shift 9 Factories - 6 day single shift 11 Factories - 7 day multiple shift 12 Warehouse 1 Residential 17.5 Hotels 8 Hospitals 29 Education 2

## Casual Heat Gains

The main casual heat gains come from the number of people, the number of PC’s, the intensity of the lighting in the internal rooms and the solar radiation through the glazing surface. Again, from CIBSE guide it was found that every person emits an average of 130 W while every PC emits 75 W. A proper intensity of lighting contributes 10 W per m2. Finally, for Great Britain, the average monthly heat gains from solar radiation is approximately 13 kWh per m2 of glazing surface.

For the sizing of the boiler and the buffer that is going to be used, the casual heat gains were not taken into consideration, since the worst case scenario had to be set. On the other hand, for the calculation of the energy demand, the casual heat gains were included.

Energy Demand

Energy demand is the amount of energy that is needed for a building to satisfy its thermal requirements. It is equal to the energy for the space and water (hot water supply) heating minus the energy from the casual heat gains.

ED = ESH + EHWS - ECHG

## Space heating

The energy for space heating is proportional to the design heat loss (Q) and was calculated using the equation

Where

Q : Design heat loss (W)

ΔΤ : Temperature difference (oC)

SDD : Standard Degree Days

Standard Degree Days is the rate of heat loss from a building, (related to the building fabric) and the temperature difference between the inside and outside of a building - the greater the temperature difference the more heat will be lost. Heating degree days are a measure of the severity and duration of cold weather, the colder the weather in a given month the higher the degree day value.

The base temperature used to calculate degree days in the UK is 15.5oC. At this temperature most UK buildings can heat themselves without the need for supplementary heating, due to the internal gains from occupants and equipment and the solar gains through the building fabric, i.e. the walls and windows.

The Standard Degree Days for the four different regions of Scotland (North East, North West, East, West) is shown in the next table and figure.

Table 3

 Standard Degree Days in UK for the last 20 years (averages for 2007) West Scotland East Scotland N. West Scotland N. East Scotland January 339 352 318 350 February 303 310 295 316 March 293 297 301 306 April 227 238 242 248 May 154 174 195 183 June 85 93 120 104 July 51 56 79 60 August 54 56 69 61 September 96 97 104 105 October 183 181 184 191 November 268 274 248 276 December 347 357 314 356

Figure 3 - Standard Degree Days in UK for the last 20 years (averages for 2007)

In order to estimate the energy needed for space heating it has to be an alteration of the buildings between the ones that need continuous space heating and the others that need intermittent space heating. Examples of continuously heated buildings are hospitals, clinics, residential homes, nursing homes, workshops, airports and factories on three shift operations. This differentiation is primary due to the fact that there are buildings which are not continuously occupied. The next three tables show the correction factors that need to be taken into account (be multiplied with the final outcome of the energy demand) according to the type of building.

Table 4

 Occupied period (days) Correction Factor 5 0.8 7 1

Table 5

 Occupied Period (hours) Correction Factor 4 0.8 8 1 12 1.12 16 1.22

Table 6

 Type of Heating Correction Factor Continuous 1 Intermittent-responsive plant 0.7 Intermittent-plant with long time lag 0.85

Like it is shown in the next figure, which is part of the heat demand model, the final correction factor is the multiplication of each of the correction factors of the three tables above and depends on the occupied period and the type of heating of each building.

Figure 4

## Hot water supply

The energy for the water heating is calculated setting the kind of the building and the number of days per week and weeks per year.

Figure 5

It can be noticed that, according to CIBSE guide, every different kind of building needs a different amount of hot water supply per m2. Also the energy demand is calculated in kWh/m2.

## Casual heat gains

The amount of energy added to the building can be estimated by multiplying the casual heat gains that come from the occupants, the PCs and the lighting with the appropriate number of hours per day and days of each month. Another source of energy addition is the solar radiation through the glazing surfaces which was found from the CIBSE guide expressed in kWh per glazing area.

Figure 6

## Energy Consumption

The energy consumption is the total amount of energy that is finally needed to be supplied to a building in order for meet its heating demands. It is equal to the energy demand divided with the overall efficiency of the heating systems.

The overall efficiency depends on the type and the different parts of the heating systems but mostly of the boiler that is being used. For a conventional oil or gas boiler the overall efficiency is 75% to 85%. For a biomass boiler this efficiency is not that different as well.