22.4
22.1
(a)
(b)
22.2
(a)
(b)
(c)
(d)
22.3
(a)
(b)
(c)
(d)
22.4
(a)
(b)
22.5
(a)
(b)
(c)
(d)
Exercises
During the winter, the inside of an average house is maintained at 20 °C, while the outside temperature is 0 °C.
Assuming that the only mechanism of heat transfer is conduction, the walls are 10 cm thick and the heat
conductivity of the walls is 0.5 W/(Km).
Calculate the heat flux from the room to the surroundings in W/m 2 .
To reduce the heat loss through the walls, the material should be changed to an insulator material. The
new overall conductivity will be 0.1 W/(Km); the thickness of the walls is maintained. Calculate the
reduction of the heat flux through the walls compared to the initial case.
What is the most important heat transfer mechanism in domestic solar water heating systems?
Conduction.
Convection.
Radiation.
They are all equally important.
What does it mean when the water heating system is in an open loop?
The solar water heating system is used for heating instead of power production.
The fluid that is heated in the collector is directly used to cover the heating demand.
The flow of the collector liquid is caused by natural convection.
The fluid of the collector liquid is caused by forced convection.
A solar collector with an area of 1.5 m 2 is installed on the rooftop of a house. Assume that the radiative energy
arriving from the sun is 1,000 W/m 2 . The collector reflects 10% of the energy arriving on its surface. Also, the
collector is not perfectly insulated, and losses occur. The collector has a heat transfer coefficient h of 2 Wm −2 K
−1 . The side areas of the collector are assumed to be negligible. The ambient temperature is 20 °C and the
collector is assumed to be at a temperature of 50 °C. Consider that this temperature is constant throughout the
whole collector. The collector is assumed to behave like a black body.
What is the power output of the collector?
What percentage of the total losses is caused by radiation?
The Smith family have already installed PV in their house. Now, they also want to cover their needs for warm
water with solar energy. For this, they are considering having a solar thermal water heating system. They need
100 L/day of warm water and the water has to be heated from 10 °C to 60 °C. The specific heat capacity of
water is 4.18 J/gK. Assume a solar irradiance of 1,000 W/m 2 for three equivalent sun hours and an efficiency of
70% for the solar thermal installation.
How much heat does the system need to generate per day to meet the warm water demand?
How much collector area will be needed to cover the demand?
If only half of the required hot water has to be stored, what would the minimum size of the storage tank
be?
The cost of the solar collector is estimated at € 120/m 2 , and the extra costs for the water tank and piping
are € 6 per L of storage. How much will the whole system cost?
22.1
(a)
(b)
22.2
(a)
(b)
(c)
(d)
22.3
(a)
(b)
(c)
(d)
22.4
(a)
(b)
22.5
(a)
(b)
(c)
(d)
Exercises
During the winter, the inside of an average house is maintained at 20 °C, while the outside temperature is 0 °C.
Assuming that the only mechanism of heat transfer is conduction, the walls are 10 cm thick and the heat
conductivity of the walls is 0.5 W/(Km).
Calculate the heat flux from the room to the surroundings in W/m 2 .
To reduce the heat loss through the walls, the material should be changed to an insulator material. The
new overall conductivity will be 0.1 W/(Km); the thickness of the walls is maintained. Calculate the
reduction of the heat flux through the walls compared to the initial case.
What is the most important heat transfer mechanism in domestic solar water heating systems?
Conduction.
Convection.
Radiation.
They are all equally important.
What does it mean when the water heating system is in an open loop?
The solar water heating system is used for heating instead of power production.
The fluid that is heated in the collector is directly used to cover the heating demand.
The flow of the collector liquid is caused by natural convection.
The fluid of the collector liquid is caused by forced convection.
A solar collector with an area of 1.5 m 2 is installed on the rooftop of a house. Assume that the radiative energy
arriving from the sun is 1,000 W/m 2 . The collector reflects 10% of the energy arriving on its surface. Also, the
collector is not perfectly insulated, and losses occur. The collector has a heat transfer coefficient h of 2 Wm −2 K
−1 . The side areas of the collector are assumed to be negligible. The ambient temperature is 20 °C and the
collector is assumed to be at a temperature of 50 °C. Consider that this temperature is constant throughout the
whole collector. The collector is assumed to behave like a black body.
What is the power output of the collector?
What percentage of the total losses is caused by radiation?
The Smith family have already installed PV in their house. Now, they also want to cover their needs for warm
water with solar energy. For this, they are considering having a solar thermal water heating system. They need
100 L/day of warm water and the water has to be heated from 10 °C to 60 °C. The specific heat capacity of
water is 4.18 J/gK. Assume a solar irradiance of 1,000 W/m 2 for three equivalent sun hours and an efficiency of
70% for the solar thermal installation.
How much heat does the system need to generate per day to meet the warm water demand?
How much collector area will be needed to cover the demand?
If only half of the required hot water has to be stored, what would the minimum size of the storage tank
be?
The cost of the solar collector is estimated at € 120/m 2 , and the extra costs for the water tank and piping
are € 6 per L of storage. How much will the whole system cost?