Economic Aspects of Building Energy Audit

Samuel I. Egwunatum and Ovie I. Akpokodje

### Abstract

Within the practice of construction economics, cost-benefit audits are carried out by proprietary audits with the intention of reporting the adequacy of any action and decision taken, meeting planned objective of a project or by efficiency audit which requires a more concise and restrictive investigation (like energy optimization) for its reporting. The efficiency audit system is most appropriate for energy utilization and performance investigation since it seeks to compare actual level of energy uses as against planned targets. This economic audit system of building energy requires that information about the energy designs are collected by means of management information system (MIS), reestablishing the data collected, comparing potential energy financial parameters with actuals, establishing the possible causes of variance. This is often justified or validated by such techniques as budgeted energy cost variance analysis, present value depreciation method, profit variance analysis, and cash flow and financial criteria analysis.

Keywords: building energy, energy audit, thermal properties, cost variance analysis, profit variance analysis

#### 1. Introduction

The basic idea that heat is a form of energy that flows from one point to another as a result of difference in temperatures though governed by the laws of thermodynamics suggests that it can take a pattern of distribution in space according to the medium it travels through. Going through the space by way of transfer and interacting with the bodies it comes in contact with is a function of the phase and provided that there is no such change in phase, the heat required to raise the temperature of a mass of a building element (m) by a temperature (T) is associated by Eq. (1):

Q ¼ mct c ¼ being the specific heat capacity of the element (1)

where m = mass of building element; t = temperature of building element. At the point of temperature, increase Eq. (2) heat is absorbed or evolved when there is a phase change such that:

Q ¼ ml, l ¼ being the specific latent heat (2)

On the contrary, the absorption or evolution of heat causes heat loss to its surrounding, such that a reversal process of that nature makes heat to be loss at a certain rate with a typical building space. The rate of such heat loss brings about the cooling of the environment or space which is proportional to the excess temperature of the building space over the external temperature of its surroundings based on forced convection when the excess temperature is small.

In practice, the term is commonly referred to as R-value such that Eq. (6)

Another pseudo form of measuring thermal conductance in materials is the U-

<sup>U</sup> <sup>¼</sup> <sup>1</sup> Rth

The possibility that a building wall is layered with different materials and different geometries suggests that Fourier laws cannot be restricted to single layer with uniform thermal resistance [1]. HVAC systems carries different insulating and piping materials which calls for Eq. (8) cylindrical examination of Fourier law of steady heat conduction in cylindrical coordinates [3]. Under such consideration,

<sup>Q</sup> <sup>¼</sup> <sup>T</sup><sup>1</sup> � <sup>T</sup><sup>2</sup>

with ri = outer radius; ro = inner radius; L = pipe length; and K = thermal

As a general rule, to the effect of other geometries, a shape factor (S) is introduced Eq. (9) to accommodate any derived shape for the measurement of heat loss

Usually shape factors are derivatives of components meant in design to restrain

where L ≫ r; and D = buried depth in semi-infinite medium Eqs. (11) and (12).

ð Þ D=r

L ≫ r D , r

> D ≫ r L ≫ r

L ≫ r1r<sup>2</sup>

<sup>L</sup> <sup>≫</sup> <sup>D</sup> (13)

þ 0:54 (14)

<sup>S</sup> <sup>¼</sup> <sup>2</sup>π<sup>L</sup> Cosh�<sup>1</sup>

<sup>r</sup> <sup>1</sup> � In <sup>l</sup> ð Þ <sup>2</sup><sup>d</sup> In <sup>L</sup> ð Þ<sup>r</sup> 

There can also be conduction between two isothermal cylinder Eq. (13) buried in

1�r<sup>2</sup> 2 2r1r<sup>2</sup>

It can also take the form of conduction through two composite rectangular plane

sections with edge section of two adjoining walls having combined k-value and

<sup>S</sup> <sup>¼</sup> <sup>2</sup>π<sup>L</sup> In ð Þ 2D=r

<sup>S</sup> <sup>¼</sup> <sup>2</sup>π<sup>L</sup> In <sup>L</sup>

<sup>S</sup> <sup>¼</sup> <sup>2</sup>π<sup>L</sup> Cosh�<sup>1</sup> <sup>D</sup>2�r<sup>2</sup>

> <sup>s</sup> <sup>¼</sup> al Δx þ bl Δx

inner/outer surface uniform temperatures S-value of Eq. (14)

losses which are either isothermal cylinder Eq. (10) with S-value of

<sup>k</sup> <sup>¼</sup> AR (6)

In rð Þ <sup>o</sup>=ri <sup>=</sup>2πkl (8)

Q ¼ KSΔT ¼ KS Tð Þ <sup>1</sup> � Tn (9)

(7)

(10)

(11)

(12)

Rth <sup>¼</sup> <sup>Δ</sup><sup>x</sup>

value which is expressed as the reciprocal of the R value:

Economic Aspects of Building Energy Audit DOI: http://dx.doi.org/10.5772/intechopen.85490

of pipes in buried walls conveying hot fluid as:

infinite medium with S-value of

121

conductivity.
