Building service and building service energy modelling

Lecture 06: Solar thermal system

Solar thermal is used in various parts of the world in different ways. In Europe it

involved a significant industrial effort until 2010-2012, then the market share was

greatly reduced, solar thermal systems in the world today are used above all in China,

for the production of DHW; Europe represents a <10% slice of the world market. It is

possible that the technology will make a comeback in the coming decades. In the civil

sector they are used mainly for the production of DHW, less importantly for the

production of heat. From 2011 to today, the growth rate of solar thermal has been

degrading over the years, unlike photovoltaic and wind power, even if the presence of

solar thermal remains significant. With the increase in the large-scale market there has

been a huge reduction in prices on photovoltaic, causing solar thermal to lose places.

A growth in solar thermal on centralized systems cannot be ruled out. Large solar

thermal systems, with different industrial production for the most important

dimensions, remain a technology that can have application in the future because the

cost remains absolutely competitive compared to other technologies. The energy

demand of buildings is not necessarily in phase with the Sun, it must be accumulated,

but the accumulation of electrical energy costs more than the accumulation of thermal

energy, so there is a greater cost. Another issue concerns densely populated areas,

where there is competition for available surfaces. In this case PVT systems are usually

used. Photovoltaic-Ther mal (PVT) collectors combine the production of both types of

solar energy – solar heat and solar electricity – simultaneously in one collector, thus

reaching higher yields per area. This is particularly important if the available roof area

is limited, but integrated solar energy concepts are needed to achieve a climate-

neutral energy supply for consumers, such as in residential and commercial buildings.

The PVT market is gaining momentum in several European countries. In recent years,

a growing number of specialized PVT technology suppliers have entered European

markets. By the end of 2018 more than 1 million m2 of PVT collectors were installed in

over 25 countries.

Distribution by Collector Type

The world market is distributed by technology classes, there are collectors that use a

component as a heat transfer fluid where solar radiation is transformed into thermal

energy, it is connected to a circuit. The fluid can be water-based or air-based, which

are not widely used. In the civil sector, air-based systems are not used because in

Europe almost all systems are hydronic. In Australia hydronic systems do not exist,

there are machines that heat the air. The solar radiation is absorbed by the collector,

which heats the heat transfer fluid; if I want to increase efficiency I have to ensure that

the fluid does not disperse quickly.

Non-glazed collectors are

unprotected, so they are used for

working at low temperatures; in the

USA they are used to heat

swimming pool water because even

if the non-glazed collector were to

disperse thermal energy it is not a

problem because the temperature

level of the swimming pool is very low. Non-glazed collectors cannot be used for high

temperature levels. Since the dispersions are UA∆T everything depends on the

temperature difference, therefore I cannot use non-glazed collectors for harsh

climates. Another typology is that of flat plate collectors and that of evacuated tube

collectors. The evacuated tubes are an evolution of the flat collectors, the advantage

lies in the fact that inside the volume (glass tube) there is no air and there is the solar

radiation absorber. In a flat plate collector I have air inside the box containing the

absorber. By removing the air the convective H decreases and therefore I have less

dispersion. Evacuated tube collectors account for 70% of the world market because

the Chinese have invested heavily in this technology. In Europe, flat collectors are

mainly used, but Europe represents very little compared to the global market.

Fundamentals: the solar radiation

The solar collector converts electromagnetic radiation into thermal energy. The Sun is

a black body as a physical model, that is, it is an ideal absorber and emits as a

function of its absolute temperature, with a distribution on wavelengths given by

Planck's law, which contains the temperature of the black body. Having measured the

radiation that reaches the Earth, by counting backwards, the temperature of the Sun

(5777 K) emerges. If we were to stand on the Sun-Earth connection with a measuring

instrument on the external surface of the atmosphere, we would measure a maximum

radiation of 1367 W for every square meter of surface of the atmosphere. This value is

defined as the solar constant. The solar constant is used to calculate how much

radiation we can exploit. If I put myself in the best condition on the external surface of

the atmosphere I collected 1367 W for every m2 of receiving surface. The volume in

which the Sun is enclosed is not homogeneous, it has heterogeneous densities and

different temperatures. It can be divided into zones (core, which is ¼ of the radius;

concentric bands which in turn have temperature and density levels and move in

different ways). Sun radiates in the universe about 3,8·1014 TW ( 1 TW = 1 billion kW)

of power as electromagnetic waves as consequence of the

various nuclear reactions take place in it. The Estimated

internal temperature is between 8 and 40 milions K and

The black body equivalent surface temperature is 5777 K.

It is a simplified model but sufficient for the purposes of

our engineering accounts.

The solar radiation is the electromagnetic energy emitted

from the sun. Average annual power density outside the

atmosphere of 1367 [W/m2] (solar constant) and on the

earth surface about 1000 [W/m2]. Wavelenght of solar

radiation is between 0,3 a 2,5 micrometers peak max 0,5

µm. The integral of the average density is the distribution curve of solar radiation,

similar to that of a black body. The distribution of energies on the wavelength is

peculiar, there are intervals of wavelengths in which the radiation is lower (in orange

the radiation that reaches the Earth is represented)

Scattering and absorption phenomena

The solar radiation which reaches the earth surface, going through the atmosphere, it

is modified (reduced intensity), both in each wave band and total, due to two

phenomena:

scattering (molecular and particulate), due to: air, water and atmospheric

 particulate

absorption, due to: O3, H2O, CO2, etc



The path that the radiation goes through from the external surface of the atmosphere

to the observer. It tis described by the parameter “Air Mass” Ma. Therefore the

phenomena of scattering and absorption depend on Ma. Referring to the components

of the atmosphere, we observe that the individual contributions are characterized by

specific absorption pockets: ozone absorbs almost completely for low wavelengths;

methane has two absorption peaks; CO2 initially lets everything through; summing all

the contributions we find a global absorption spectrum, which has a free part between

0.3 and 0.8 and beyond certain frequencies it is a wall. The problem of climate change

is at this wall since the more CO2 in the atmosphere increases, the warmer the

atmosphere becomes. If I superimpose the black body curve with these absorption

curves I obtain the curve that is recorded on Earth. The radiation we measure on Earth

is reduced in intensity on some specific wavelengths due to the phenomenon of

absorption. The phenomenon of scattering overlaps with this theme.

The macroscopic effect of scattering is:

Back reflection toward the space of patrt of the radiation incident on the

 external surface of the atmosphere;

The deviation of part of the extraterrestrial radiation which reaches the earth

 surface from all the directions, named as solar diffuse radiation or diffuse

irradiance, Gd (W/m2).

The radiation which does not interact with the atmosphere molecules maintains

 the incident direction on the external surface of the atmosphere solar direct

radiation or beam irradiance, Gb (W/m2).

To do the engineering calculations for both solar thermal and photovoltaic, in the

climate files of the various locations there are always at least two radiation data which

serve to identify the direct radiation component and the diffuse radiation component.

To do the calculations, reference is made to data influenced by temperature, humidity

and wind speed provided for hourly data throughout the year. I must therefore treat

the direct radiation component separately from the diffuse one. The sum of the two is

necessary to do the math.

Solar radiation availability

The solar radiation availability on the earth surface for the conversion in other energy

forms depence on: Extraterrestrial radiation available, Atmosferiche conditions,

general and microclimatic (atmosphere transparency), the Relative sun position in the

sky (altitude α and azimut γs), which varies: dayly and seasonally (declination δ); and

depending on the site; the Site altitude; and the Sun hours (day duration). Sun

radiation is usually measured with a Piranometer. The Pyranometer equipped with

sensor and transparent cap is positioned on the reception surface. To get the two data

I put a pyranometer with a cap and one without, with a band that only shades the

sensor. The first measures the diffuse radiation, the second the total radiation and by

making the difference between the two I obtain the direct component. Looking at a

thematic map you can see that the radiation in the North is lower than in the South,

where the installation of solar thermal is more advantageous. The inclination that

collects the most radiation is usually that of latitude. In reality, the variation in energy

collected in a year as the azimuth or inclination angle varies is not that important. The

collectors can be installed on the roof, integrated into it, or placed on the façade (on

low-rise buildings to facilitate maintenance). The air ones are easier to integrate into

the facade but it is not easy to move the air around.

Solar thermal collector

We can see a solar collector like a heat exchanger between the environment and a

thermal fluid (e.g. water):

We have learned how to characterize the environment (��, ��, ��, ��m�,Tsky) and

the collecting surface (area �� and orientation �, �). We will see how to characterize

optical and thermal losses. Solar collector is an insulated case, where a transparent

glass with low content of iron oxides is placed. Below there is an absorber, i.e. a

surface that absorbs solar radiation as much as possible and above it the tubes with

the heat transfer fluid are welded, which when heated cool the absorber. The thermal

energy moves away by convective motion between the glass and the absorber if there

is air and by radiat