Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 219

6.1.5 Economic Analysis

To plan and project energy systems such as solar cooling systems, economic con-
siderations form the basis for decision making. The costs in energy economics can
be divided into three categories: capital costs, which contain the initial investment
including installation; operating costs for maintenance and system operation; and the
costs for energy and other material inputs into the system.
The analysis presented here is based on the annuity method, where all cash flows
connected with the solar cooling installation are converted into a series of annual
payments of equal amounts. The annuity a is obtained by first calculating the net
present value of all costs occurring at different times during the project, that is by
discounting all costs to the time t

= 0, when the investment takes place. The initial

investment costs P ( t 0) as well as further investments for component exchange in

further years P ( t ) result in a capital value CV of the investment, which is calculated
using the inflation rate f and the discount or basic interest rate d . The discount rate
chosen here was 4% and the inflation rate was set at 1.9%:

=

P ( t ) (1

f ) t +

CV = (6.11)

d ) t (1 +

Annual expenses for maintenance and plant operation EX , which occur regularly
during the lifetime N of the plant, are discounted to the present value by multiplication
of the expenses by the present value factor PVF . Thermal chillers today can expect a
lifetime of 20 years:

N

1

1 + f + f

PVF ( N, f, d ) = 1 − (6.12)

d − f d − f

In the case of solar cooling plants, no annual income is generated, so that the net
present value NPV is simply obtained from the sum of discounted investment costs
CV and the discounted annual expenses. It is defined here with a plus sign to obtain
positive annuity values:

NPV = CV + EX × PVF ( N, f, d ) (6.13)

Annual expenses include the maintenance costs and the operating energy and water
costs. For maintenance costs, some standards like VDI 2067 use 2% of the invest-
ment costs. Some chiller manufacturers calculate maintenance contracts at 1% of the
investment costs. For large thermal chillers, some companies offer constant cost main-
tenance and repair contracts: the costs vary between 0.5% for large machines (up to
700 kW) and up to 3% for smaller ones. Repair contracts are even more expensive
at 2% for larger machines and up to 12% for a 100 kW machine. In the calculations
shown here, 2% maintenance costs are used.