Instituto Superior Técnico 18/12/2012

Introdução à Investigação Manuel Nascimento - 52294

Presentation Summary

Solar thermal power components and setups

The main component for thermal power setups is the collector, whose function is to “capture” the solar irradiation, absorbing its energy. They’re composed of tightly packed, multiple crisscrossing absorber tubes where the solar rays enter and their energy is thermally transferred to the HTFs (heat transfer fluids) that flow in the tubes. HTFs will later transfer collected energy to potable water in a fashion depending on the setup. The most common HTFs in use are water and glycol-water mixtures. Water is cheaper and easy to use but can freeze during winter in some locations. Using glycol-water mixtures prevents freezing and provides greater thermal absorption. The most widely used collectors are flat panel collectors which are flat insulated boxes with glass surfaces on top through which the solar rays penetrate and where the cylindrical absorber tubes are. The insulation also provides a “hot house” effect, further contributing to solar energy “capture”. Also popular but more expensive are the CPC (compound parabolic collectors), that use parabolic-type mirrors below the tubes in order to concentrate the solar radiation (within their optical critical angles) into the absorbers. These can be inside an insulated box or by themselves. Studies show that while CPCs are not as efficient as flat panel when HTF temperature is equal to ambient temperature, they lose efficiency more slowly as the temperature rises.

There are two major types of Solar Thermal Power setups: Termosyphon Circulation and Forced Circulation. In the Termosyphon setup, the simplest and cheapest, the hot water tank is mounted directly above the collectors, on the outside of the building (usually roof). The sun heats up the HTF in the collectors making it less dense, making it flow naturally upwards to the water tank above. There, through

a heat exchanger component, its heat is transferred to the potable water connected to the buildings’ piping system. The cold HTF then returns to the bottom of the collectors, continuing the cycle. Usually, the water tank has an electrical resistance inside to provide additional heating when the solar irradiation is not enough to meet the demand. Because in this setup the water tank is on the outside, it’s more prone to energy losses. Also, it’s only possible to have smaller capacity tanks (< 200l). Since the HTF flow occurs naturally, there is no possibility of control. If the solar irradiation is not enough (clouds and cold weather), the HTF may not be hot enough to heat the water but still flow to the tank and exchange heat with it, effectively cooling the water.

In the Forced Circulation system, the water tank is placed inside the building and the HTF flow is controlled through automatic pumping. This way, the HTF is only channeled to the water tank’s heat exchanger if its temperature is high enough. Because the tank is inside, fewer energy losses are expected and there’s a possibility of using other types of systems for auxiliary heating in the tank, such as conventional gas or electrical boilers. Forced Circulation systems are more complex and expensive, but more scalable, powerful and versatile. Larger systems can provide additional hot water for house heating and cooling through the walls, floor, radiators and air conditioning.

There are various systems of these available in the market. A Portuguese company, Rigsun (www.rigsun.pt), sells a wide array of “kits”, entire setups made up of all the different necessary components and dimensioned for different consumption needs. For instance, the Sani line can be used to heat water and the Poli line can be used to heat water and also the inside of the house or building.

ABSTRACT The aim of this presentation was to characterize the existing solar thermal power systems to heat water in domestic or small building environments and describe how to calculate the energy they provide and how much money can be saved on traditional water heating systems (gas, electricity). Firstly, the main components and installation setups for these systems are reviewed and some commercially available kits are presented. Then, the required information and methodology for determining the supplied solar energy is briefly reviewed. Lastly, a case study is considered for a 4 person family using one of the mentioned kits. Using SolTerm software, we explore how to calculate the solar energy supplied and how much money can be saved, discussing the results obtained for our project. Further savings in energy bills using these systems for ambient heating are briefly discussed. It can be concluded that solar thermal systems are efficient for hot water heating, particularly in places with better sunshine. Installing a kit at home is profitable even using a bank loan and will reduce energy bills and lower CO2 emissions. Purchasing with one’s capital, the invested capital will be recovered well before the end of systems lifetime of 20 years. Software solutions are invaluable tools to study the potential implementation of these systems and to help tailor and optimize the system’s components for specific locations, building characteristics and hot water consumption profiles.

CONSUMER SOLAR THERMAL POWER

The main consideration one has to take into account is the number of people in the house that the solar system is intended to work in. The panels should be installed facing directly South and with an inclination equal to the latitude of the location. However, it can be demonstrated that small offsets from these parameters (within 10º) will not compromise the performance significantly. Lastly, as a general rule, one should design the system to have a solar fraction of around 90% in the Summer, that is, to provide about 90% of the energy necessary to heat the water.

Calculating the supplied energy

To determine the supplied energy from the solar system, basically it’s necessary to evaluate the solar energy absorbed by the system and subtract the heat losses in each component. However, those quantities depend on multiple variables and there is a multitude of factors to consider. Firstly one needs to know in detail the average solar irradiation profile (for all the solar radiation components: direct, diffuse and reflected from the Earth’s surface) for each day of the year which in itself is a extensive problem. Then the thermodynamic properties of the solar system components are considered and the energy balance is established. In the first item of the bibliography there’s a detailed explanation of all the steps, but going in detail was outside the scope of this work. More importantly, there are software solutions available to design these kinds of solar systems and analyze the results, in terms of energy and of economics. For instance, SolTerm is a software designed by LNEG (Laboratório Nacional de Engenharia e Geologia) that has all the solar irradiation data necessary for Portugal and has the equations for innumerous typical solar setups (thermal and photovoltaic), as well as a library of many components and systems on the market in Portugal. Inputting the required thermodynamic quantities for each component of the system (manually, consulting the manufacturer’s technical data or via the internal library), and also the average hourly hot water consumption profile for each month, the software does the calculations and presents an analysis in respect to energy and economic considerations (using current and predicted energy tariffs). It also takes into account the costs of maintenance and substitution of parts in percentage of purchase price (percentages can be selected). The Software can also suggest improvements on the setup by trying to optimize one of the different energetic criteria at a time, like solar fraction, solar energy waste, usage of auxiliary energy and collector orientation.

Case study using SolTerm

SolTerm software was used to analyze the viability of installing a forced circulation type “Sani 300-2” solar thermal system with a typical gas boiler as the auxiliary heater in a 4 person family home in the Lisbon area. The system comprises 2 solar panels (4.7 m2 in total), a 300L water tank, costs 4200€ (VAT included) and has a rated average solar fraction of 60% to 80%. A hot water consumption plan was considered involving 2 showers in the morning, 2 in the late afternoon and water for lunch and dinner cooking and dish washing. It can be assumed a typical water usage of 95 L/person.day, because a typical shower/bathtub flows 6L/min and, considering a 15

minute shower, that makes 90L. The remaining 5L can be considered for cooking, dish washing and other activities. Consulting the technical specs from Rigsun’s website, the solar setup was replicated in the software and results were obtained.

Regarding the energy results, the single most important and useful figure obtained is the solar fraction. An average global yearly solar fraction of 81.8% was obtained, strongly suggesting that using this system would be effective and profitable. In detail, about 3974 kWh of energy would be saved. If the solar system would replace a conventional gas boiler for water heating (“esquentador”), about 5300 kWh of natural gas would be saved. That means 1.3 ton of equivalent CO2 not emitted to the atmosphere, effectively lowering the ecological footprint.

With respect to the economics, using reasonable values for inflation and TAEG interest rate, two cases were considered for the purchase of the solar system: contracting a 5 year bank loan (bill of about 93€/month) or using one’s own capital. Regardless, the savings in the natural gas bill will be of about 572€/year during the 20 years of typical life-time of these systems. When using a bank loan, the real total profit after 20 years would be about 4700€ . During the first 5 years there would be expenses of about 50€/month, but after that and until the end of the 20 year expected life time of the system, there would be savings of about 42.7 €/month. Using one’s own capital, because there aren’t interest rates to be paid, the profitability is even higher: a real profit of 6136€, meaning a profitability rate of 7% a year, which is higher than a high-yield financial investment. In 12 years the invested capital would be recovered.

By using a different system that could also contribute for house heating, like a Rigsun’s Poli line kit, additional savings could be obtained by replacing (if only partially) conventional heaters (let’s consider electrical radiators). With a solar fraction for ambient heating of 50% (Poli line rates at 30%-50%), using heating half the year (colder months) and recalling the present electrical tariff (about 0.21€/kWh) one could expect savings of up to 255€/year.

Conclusions

It’s easy to see that solar thermal systems are effective and efficient in replacing traditional hot water heating, especially in places with high solar irradiation. Installing a kit at home is profitable, even if a bank loan is needed, providing cuts in energy bills and lowering CO2 emissions. If no bank loan is needed, one will get the invested money well before the end of the system’s lifetime and with profitability higher than a good financial application. Software tools like SolTerm are essential to design and test the systems’ configuration, making it possible to tailor and optimize the installation for specific locations, building types and hot water consumption profiles. This makes sure what you choose to install will meet your needs effectively. Since the solar irradiation availability in Portugal is quite good, only a lack of awareness and the financial crisis can explain why there isn’t a wider proliferation and investment on these systems.