Category Archives: Particle Image Velocimetry (PIV)


Three types of systems are investigated in this work. A large solar water heating system suitable for a block of 10 flats or similar use, a solar heating system for a house and an industrial process heat system. A generic schematic diagram of the systems considered is shown in Fig. 1. The same basic system applies to all cases with the options concerning location of auxiliary and house heating as shown.

Fig. 1 Schematic diagram of the solar hot water and space heating system

1.1 Large hot water system

The system consists from a collector array, a storage tank, solar pump and auxiliary. A differential thermostat is used to compare the temperature at the exit of the collectors and the storage tank and give a signal to switch on the pump. The auxiliary energy considered in this case is diesel. The specifications of the system are shown in Table 1. Such a system can supply hot water to blocks of 10 flats or to any other similar size system.

Table 1. Large hot water system specifications



Collector area Storage tank volume Load temperature Collector inclination





1.2 Solar heating system

This is very similar to the hot water system with the difference that the hot water is supplied to the house radiators circuit. The specifications of the system are shown in Table


Table 2. Space heating system specifications



Collector area Storage tank volume Collector inclination House UA value Room temperature




1200 kJ/hr°C 21°C

Cost Structure of Solar-Thermal Flat-Plate Collectors

Approximately 50 % of the production costs of a solar-thermal collector are related to the collector structure (figure 3; structure, glass cover, assembly cost). Furthermore, these components define the collector’s weight and its handling especially on the houses’ roofs. Since the collector manufacturers insist on the use of the expensive solar glass, reengineering activities have to concentrate on the collector structure.

Competitors Analysis / Types of Solar-Thermal Flat-Plate Collectors

Aluminium still is the dominating material of the standard frame structure designs (except for large-scale collectors where wood is still often in use for the collector structure).

Few companies (e. g. GREENoneTEC, St. Veit/A; thermolsolar, Landshut/D) offer collectors with deep-drawn aluminium troughs. The trough design, however, offers a considerable potential for cost reduction due to the minimum number of parts and thereby avoided processes of material handling, logistics and assembly.

Only Buderus, Wetzlar/D, currently uses plastic as structural material in a flat-plate collector (figure 4). The former ALLIGATOR Sunshine Technologies GmbH, Berlin/D, also made use of plastics for the structure of a roof-integrated collector (solar-thermal and photovoltaic) as seen in figure 5.

Combined Solar Heat and Power. A Future Solar Option?

Dirk Kruger, Dirk Mangold*, Klaus Hennecke, Ralf Christmann, Jurgen Dersch, Eckhard
Lupfert and Klaus-Jurgen Riffelmann,

Solar Research, Institute of Technical Thermodynamics
Deutsches Zentrum fur Luft — und Raumfahrt e. V., 51170 Koln
Tel: (49) 02203 601 2661, Fax: (49) 02203 66900, E-mail: dirk. krueger@dlr. de
* Solar — und Warmetechnik Stuttgart (SWT)
ein Forschungsinstitut der Steinbeis-Stiftung, 70550 Stuttgart
Tel: (49) 0711 685 3279, Fax: (49) 0711 685 3242, E-mail: mangold@swt-stuttgart. de

The high exergy of the solar radiation allows to produce heat in thermal collectors, and also to generate electricity as in large solar thermal power plants or with photovoltaics in smaller applications. Solar district heating systems can be enhanced by small engines converting the valuable part of the energy at high temperature into electricity, while the remaining fraction at lower temperature is still used for heat production. This will improve the benefit for solar district heating. In this paper a solution for producing electricity and heat from one solar system, a parabolic trough collector field in conjunction with a steam engine is presented (Figure 1).

Figure 1: Principle scheme of Combined Solar Heat and Power


In the municipal sector various solar installations for district heating, sometimes with seasonal storage, have been erected for domestic hot water and heating purposes. Combining such kind of systems with a heat engine to produce electricity is the principle of a solar combined heat and power solution. Process heat applications needing heat up to 100°C in conjunction with electrical power are also appropriate. The concept is interesting for small heat and power applications in residential homes as well.


Temperatures of 200°C to 400°C are desired in order to reach appropriate engine efficiency. Large parabolic trough collectors for solar power plants as the EuroTrough collector (Geyer et al (1), Geyer et al (2), Lupfert et al) can deliver heat at these temperatures efficiently, but they are not cost effective for smaller solar fields of up to several thousand square meters of aperture area. Existing medium sized parabolic trough collectors for process heat can deliver heat at 200°C and more, but their efficiency is fairly low at elevated temperature (Kruger et al). Assuming a small scale parabolic trough
collector would be enhanced by performance-improved features typical for solar power plant collectors as eg vacuum receivers and sufficient concentration quality, an annual yield described in Figure 2 could be reached according to simulations in TRNSYS. Especially a small collector, with a tilted north-south axis, could reach energy yields around 600 kWh/m2*a, when enhanced to an efficiency of the EuroTrough collector. Tracking from east to west provides a high output level for several hours on sunny days (Figure 3), as the collector is close to perpendicular irradiation all day.

Typical property of the solar heat is its discontinuous power and temperature level due to the day-night cycle and irradiance variations with weather conditions. This affects the selection of the appropriate heat engine.

Upper curve: High performance collector, tilt 35° to south

Centre curve: High performance collector, horizontal axis

Lower curve: Process heat collector of Industrial Solar Technology, horizontal axis

900 800 700 600 500 400 300 200 100 0


In principle, various types of engines can be used for conversion of solar heat to electricity: Steam turbines, ORC turbines, Stirling engines and steam engines. Steam turbines are
nowadays hardly available for the range below 500 kW electrical power. ORC turbines also start in the range of 500 kW electrical power. Stirling engines have been developed for small domestic CHP (Combined Heat and Power Production) and tested in combination with high temperature heat from parabolic dishes. The temperatures necessary exceed the temperature provided by parabolic trough collectors. New low temperature Stirling engines may be developed though. Steam engines are today only commercially available from the company Spilling, starting from 60 kW nominal electrical power with good part-load behaviour.

As the solar output of the collector varies with radiation and incident angle, a steam engine with its high part load efficiency is chosen for this study. Spilling produces a 120 kW machine, which can be operated by 210°C saturated steam.

Nominal thermal input power is 960 kW. According to supplier information, part load is possible down to 30%. Between 100% power down to 30% part load the gross electrical efficiency is almost constant at 12.5%. Parasitic power for pumps and assemblies amount to about 3 kW over full and part load. Outlet steam quality is wet steam at 110°C and 1.5 bar.


5 Watt-PV module is utilized for tracking solar oven concentrator system with 2.6 kWTH capacity and 250 Kg weight. The tracking system follows the Sun autonomously in altitude and azimuth using only 5 Watt-peak PV solar module as a tracking energy source. The tracking system is driven by means of two 12 DCV motors of 36 W each, and fed by electrolytic condenser with 78,000 pFd capacity charged properly by PV module. The PV based tracking system has two circuits in H Bridge configuration using N — and P-channel power MOSFET transistors. This electronic circuit commands DC motor rotation way, as a function of the optical sensors for altitude and azimuth position.

The proposed system must be designed based upon local technology and adopted to the needs. Simple design concept is one of the issues in this tracking system to reduce different troubles during its lifetime. The tracking system consists of electrolytic condenser storage, instead of conventional battery and its charge controller configuration. A couple of electrolytic condensers satisfy the total system energy needs. FIG.16 shows "H” configured basic electronic circuit for feeding two DC motors of 36W-each, one for solar altitude and the other for azimuth movements.

The design and construction of effective 2.6kWTH stand-alone solar concentrator oven tracking system was developed using 5 Watts-peak PV module. The objective is focused for simple and robust electronic tracking system for Mexican rural area application.

The generated power at PV module is coupled for charging electrolytic condenser. The maximum module voltage is 16V, and when the electrolytic condenser achieves 15V, the electronic circuit compare, and switches for discharging maximum of about 8.8 Joule of energy as (1/2 CV2), where C is the capacitance and voltage V provided by the module, the plot is shown in FIG. 17.

The energy delivered by the capacitors is conducted towards the selected DC motor according to the optical sensor decision. The DC motor has low internal resistance of ~2Q and considering PV module as a constant current source with about 340mA, the capacitor’s charging time lasts about 2 to 3.5 seconds depending of its charge state. The
electrolytic capacitor charging process for feeding low-resistance DC motor load is illustrated in FIG.17. The figure shows I-V and Voltage-Time curves for PV-module and capacitor charging operation with storage time.

Superposition of the Voltage-time axes indicates the energy charging process in the capacitor and its transference to the DC motor, using MPPES concept.

The energy stored in electrolytic condenser is discharged towards the DC motor by using MPPES (Maximum Power Point Energy Storage) concept. This is a DC/DC converter similar to MPPT (Maximum Power Point Tracking), feeding the load using maximum PV module power point. In the case of MPPES, the energy is temporally stored and discharged, repeating this cycle.

Respect azimuth and altitude mechanical traction, are driven by two independent pulleys connected to each DC motors through v-belts. The PV Module location on the solar oven is shown in FIG. 18.

This tracking configuration has some advantage, which prevents mechanical damage when is compared with the conventional “mechanical-gear” system. This tracking system driven by pulley and v-belts has great flexibility in movements but maintaining precise position.

The energy discharging process on DC motors for azimuth or altitude tracking is given by equation (a):

FIG 18 PV Module location at the top of the main structure of the solar oven


Rmotor T


From theoretical calculation for voltage-current discharge cycle:

v(t) = Vm exp(-t/t) and i(t) = Vm / Rl exp(-t/x)

Solving equation (a) and using t = 0.156 sec, the stored energy is equal to the consumed energy. If the sun position displaces about 1.5° every 6 minutes, it is enough time for charging DC motor supply energy [4].


Most of the solar concentrator cooking systems does not posses an autonomous tracking system. We have demonstrated how 5 Watt-peak PV Module can track 2.6KwTH solar concentrator cooking system by means of electrolytic condenser storage system using two DC motors. This cooking system avoids deforestation, one of the mayor rural problems. The PV-based stand-alone tracking system has big energy factor-merit of about 520 times, due to the reduced electrical energy consumption for obtaining high thermal energy. This is thought as the first time that MPPES (Maximum Power Point Energy Storage) concept is used for a stand-alone Sun Tracker system[7]. In addition to the last facts, the Redundancy provided by the use of two detection elements increases the Sun Tracker’s efficiency.

Solar oven reliability is now in their evaluation stage and the total cost is about US$2,500 (by March 2004). The following paragraph describes obtained important fact.

From 60% to 65 % global thermal efficiency (output power available over the incident solar power) has been obtained. The prices of the produced energy is estimated to be US ф 3.0 per kWh as equivalent to electric energy and US$ 1.3 W-peak for an installed total system considering 5.2 peak-hour [2] locally available direct solar radiation resource. This is based on the 30-year estimated system lifetime.

They pay back time is estimated in 3.5 years considering energy price at US ф 15 per kWh (June 2003). This solar oven contributes reducing 2.87 Ton/year of firewood combustion, which means 5.32 Ton/year of CO2 emission to the atmosphere [5, 6].

The cost due the use of the oven is around US ф 30.2 per day, during the 30 years lifetime system, this cost represents US ф 3.8 per individual a day considering 8 people per solar oven.

As a protection issue to the Environment, the solar oven implies big benefits. This prototype can be promoted as a green bonus for CO2, that UNEP (United Nations Environment Program), the GEF (Global Environmental Facility) and the World Bank Institute provide as a result of the Kyoto Protocol to reduce greenhouse effect.

Surface fitting

To determine the flux on an arbitrary surface two ele­ments are required, the en­ergy striking a region on a surface and the area of that individual region. The gener­ated data cube provides us with the first of these com­ponents, the energy. What is then required, is those energy values need to be compensate for the non­horizontal-planar surface en­ergy calculation and generate a curve that best fits all of these corrected data points.

Initially, from the data cube points in space representing 400 time the solar concentra­tion for planar surfaces below the focal point were extracted

an example of which can be seen in Figure 2. This provided a list of points in space of equal energy, and from this list a surface can be generated that best represents those points. For our simulations we chose to use only a quadric surface as described by [13]
and included for the readers convenience. This method describes the fitting of a quadric surface using the least squares method which has the general form

M2 + k2y2 + k3z2 + k4xy + k5yz + k6zx + k7x + k8y + k9z = 1, (1)

where x, y,z represent relative displacements about an orthogonal basis in three- dimensions and the nine coefficents ki(i = 1,2 9) define a unique quadric surface.

The input data to the fitting procedure is a set of 3D coordinates of the sample points (a series of (x, y,z) values). If there are m sample points, there are m( x, y, z) values. The­oretically, the nine unknown coefficients can be solved from a group of nine linear equa­tions of the form of Equation 1, each of which has one data point (xi, yi, zi) assigned to its corresponding variables, x, y and z. However, due to sampling errors such a result is not robust [13].

A more practical way to solve this quadric fitting problem is to find the least squares solu­tion of a group of m linear equations,


generated by substituting in each of the m, points defining regions of constant illumina­tion,

X0 = [k1 k2 k3 k4 k5 k6 k7 k8 kg],

representing the coefficients and

bmx1 [1 1 … 1] .

Normally, the least squares solution does not satisfy all equations in the group, but it min­imises the value of the residual error

er = 11 AoXo_Bo |І2,

and this solution can be considered the optimal least squares solution of the problem.

The solution X0, can then be computed by the normal equation

Xo = (ATAo)’1ATb.

Having determined the coefficients k, the surface normal at each of the points is defined by the vector [14],

[2k1 Xi + k4yi + k5Zi + k7, 2k2Yi + k4Xi + kaZi + ke, 2k3Zi + k4Xi + k5yi + kg].

The initial data cube can now be rescaled by dividing each of the corresponding points by the dot product of this surface normal vector and the z-normal vector (representing the deviation of the flux from striking a non-horizontal-planar surface). The actual surface rep­resenting areas of uniform flux can then be generated by iteratively applying these meth­ods to optimise the surface area and energy combination until it falls within reasonable tolerances.

Isothermal storage systems using latent heat

After the demonstration of the feasibility of direct steam generation in parabolic troughs [4] one focus of further research activities of this technology lies in the development of a suitable storage technology. The development of a cost effective DSG-storage concept (Fig. 7) is the aim of the recently launched DISTOR project funded by the European Community within the 6th Framework Programme on Research, Technological Development and Demonstration.

Regarding efficiency, a fundamental demand for thermal storage systems in power plants is the minimization of temperature differences between working fluid and storage medium. This requires isothermal storage systems for the DSG-process. An obvious solution is the application of latent heat storage materials. Fig. 8 shows the process in the T-s diagram: during the charging period, heat from the condensating steam is transferred to the melting storage material (Fig. 8, 2-3). During the discharge process, heat from the solidifying storage material is used to generate steam (Fig. 8, 6-7).

The selection of the latent heat storage material depends strongly on the saturation temperature resulting from the pressure in the steam cycle; the DSG-process with an operation range of 30-100 bar requires melting temperatures between 250°C and 300°C. Considering also economic aspects, candidate materials for latent heat storage systems are salts. Although this approach has often been suggested, only limited experience is available in this temperature range. Most problems result from the low thermal conductivity of salts, particularly in the solid phase.

Basically, there are two methods to overcome the problems resulting from the low thermal conductivity:

о reduce the specific resistance for heat conduction in the latent heat storage material

о reduce the average distance for heat conduction within the storage material Solutions based on both methods are investigated within the DISTOR project. The specific resistance for heat conduction can be reduced by embedding the storage material in a matrix made of a material with a high thermal conductivity, such as expanded graphite. This approach has been tested for low temperature applications and will be extended to the operation range of the DSG-process. This development aims at a composite material with an effective thermal conductivity in the range of 5-10W/(mK).

Reducing the average distance for heat transport in the storage material means an increase of the ratio of surface area to mass of storage material. By introducing an intermediate heat transfer medium between storage material and steam pipes the surface of the storage material can be extended while the mass of piping remains constant.

Within the DISTOR-project three basic storage concepts will be tested in laboratory scale. Based on the experiences gained with 10kW lab-units, one concept will be selected for the design of a 100kW storage unit that will be connected to the DISS test facility to assess the storage system under realistic operating conditions.

Design of Hot Water Storage Tank

2.3. Design of Heat Exchanger

Heat exchanger is required to transfer the heat without mixing system hot water with the chemicals (liquid) available in the process tank. Following points have been considered while designing the heat exchanger.

a. Quality of the chemical (for selecting the material of H E)

b. Temp. rise required in the liquid.

c. Heat transfer co efficient of The H E material.

d. Time available to increase the temp to set level.

e. Volume of the chemical tank.

f. Evaporation losses during heating from the top portion.

g. Make up liquid volume

2.4. Control System & Piping layout designing

i) Control System & instrumentation: The system is designed such a way that max. Heat is to be collected even during the rainy season when sunshine is not with constant intensity. So we have used the differential controller system with the difference set point. As per the above calculation procedure we come to the requirement of 95 collectors for the process of 60 trolleys per day. To save the space for solar collector installation it has been decided to fabricate a SUPER STRUCTURE on the asbestos roof of the factory shed.

There are two Temp. sensors (PT100) provided to sense the hot water outlet temp of collector outlet and at the bottom of Hot Water storage tank. When the temp. of water at the collector outlet crosses the difference set, the pump gets started and forces the low
temp water of the storage tank to the collector and the process continues till the difference comes down below the set point the pump gets off and stops the circulation. Suitable capacity pump is used for circulation of water with mechanical seal to handle high temp. Liquid.

ii) Piping layout Designing ( Лв per the schematic diagram )

a. Solar Collector & Tank Piping (Solar Loop): Solar collectors are connected in series and parallel and cold-water inlet is connected at the bottom of the each row of the collectors. Hot water outlet has been taken from the top of the solar collector of each row. There is temp. sensor is fixed at solar collector outlet and bottom of the hot water storage storage tank. The pump is used at the inlet side of the collector, which sucks the water from the tank bottom (lowest temp. ) and pushes to the collector inlet. The size of the piping is design to circulate the required quantity of water within the average sunshine hours.

b. Chemical tank and hot water storage tank piping ( Process loop ) : The function of this loop is to carry the hottest water from the tank and circulate through the heat exchanger of the three tank where chemicals have to be heated. The rate of flow is decided by the calculation of heat exchanger and accordingly the pump capacity and pipe size has been decided. The complete piping is insulated by rock wool with aluminum cladding on it to prevent heat loss. The tube type heat exchanger is used so that it can occupy the minimum space with max heat exchange as well as it should be removable to clean the scaling formed due to the deposition of chemical on the tube. The on-off valves have been provided at the inlet of the heat exchanger of each tank to control the hot water flow as per requirement. This has been done manually and temp indicators with electronic buzzers are provided to indicate the temp and removal time of each batch. Compressed air is also circulated to mix the chemical uniformly & to get uniform temp. at every point in the tank.

Development of a MaReCo-Hybrid for Hammarby Sjostad, Stockholm

Anna Helgesson, Peter Krohn, and Bjorn Karlsson Vattenfall Utveckling AB, SE-814 26 ALVKARLEBY, Sweden,

Phone no: +46 26 83500, Fax no: +46 26 83670, e-mail: anna. helgesson@vattenfall. com

Introduction: The main disadvantage of using solar electricity is the high cost. By using concentrators to increase the radiation, some of the expensive absorber material can be replaced by cheaper reflecting material. One disadvantage with concentration is, however, that the cell temperature rises, resulting in a reduced electrical efficiency. The performance can be improved by cooling the cells, and this is the background for the development of the so-called MaReCo-hybrid. This is a specially developed concentrating collector giving both heat and electricity. The MaReCo-hybrid has been developed at the Swedish company Vattenfall Utveckling AB, and has now been delivered for installation in a newly build residential area in Stockholm. The paper will present the collector in more detail.


Hammarby Sjostad is an old industrial area in Stockholm, Sweden. This area is now changed into a modern ecologically sustainable residential area. The company SBC Bostad has been planning to use solar energy for heating and electricity generation in one of its buildings. The use of solar energy will help to create a green image. The collector to be used is a so-called MaReCo-hybrid, developed at the Swedish company Vattenfall Utveckling AB. This construction gives both heat and electricity, and is therefore an interesting way of making an efficient use of the available roofing area. The utilisation of the heat can also be seen as a way to reduce the costs for the electricity generated.

During spring 2004, 30 m2 of the MaReCo-hybrid was delivered to Hammarby Sjostad. During the rest of the year, measurements will be made on the performance of the hybrid — system.


One barrier for an increased utilisation of the PV technology is the high costs and the low output. By using concentrators to increase the irradiation onto the solar cells, the use of PV can be made more cost competitive. In this way, some of the expensive solar cell area can be replaced by cheaper reflecting material. One disadvantage with concentration of the radiation onto the solar cells is, however, that the cell temperature rises, resulting in a reduced electrical efficiency. By cooling the cells with circulating water, the output from the PV-module can be improved. The heated cooling-water can then also be utilised for heating purposes, and this will result in an increased total energy output and a better performace/cost-ratio. A system giving both heat and electricity is called a hybrid system.

In this case, a special construction named “MaReCo” has been used. The abbreviation MaReCo stands for “Maximum Reflector Collector”, and the name refers to the purpose of the construction, which is to replace some of the expensive absorbing material with cheaper reflector material. The hybrid-MaReCo has been developed at the company Vattenfall Utveckling AB, and it consists of an asymmetric reflector trough and a specially designed hybrid-absorber. A large part of the development work has been made within the frames of two large Swedish RD&D-programmes concerning solar heating that have been
going on since 1996 (Helgesson et al, 2000 and 2004) and within a national R&D — programme concerning solar electricity managed by ELFORSK. The goal of the development of the MaReCo-hybrid has been that the installation cost shall not exceed SEK 2 000 per m2 glazed area, and that the construction shall give a yearly energy output of 50 kWh electricity and 200 kWh heat per m2 glazed area.

Industrial Process Heat System

Such a system is suitable for supplying hot water or low temperature steam to various industrial applications (e. g. food industry). The system consists of an array of collectors, a circulating pump and a storage tank. It includes also the necessary controls and thermal relief valve, which relieves energy when storage tank temperature is above a preset value. The system is once through, thus the used hot water is replaced by mains water.

Mean monthly ground temperature values are used for the mains water temperature in simulations. When the temperature of the stored water is above the required process temperature, this is mixed with mains water to obtain the required temperature. If no water of adequate temperature is available in the storage tank its temperature is topped-up with an auxiliary heater before use. The system considered provides 1000 kg/hr of hot water at a temperature of 80°C (load). This is an average consumption of hot water for medium size food industries. The load is required for the first three quarters of each hour. The specifications of the system are shown in Table 3.


TRNSYS can give results in an annual, monthly, daily or hourly basis. Here mainly annual results are presented together with some typical monthly ones.

Concept Development

However, plastics are widely used in comparable applications (e. g. in buildings as doors and windows) and even in the highly sophisticated automobile industry under increased requirements concerning structural stability and quality of surface.

Despite the lack of flexibility, a trough was favoured to a frame design for cost reasons. In any case, it is first of all necessary to find the appropriate way of production: injection moulding or deep-drawing. Apparently, injection moulding offers much more freedom of design, however, the economically favourable solution counts. A commercially interesting alternative to the existing aluminium troughs means production costs of less than 25 € per trough (2 m2 collector, ready for assembly).

For that reason, a detailed cost simulation was carried out regarding several approaches of injection moulding. As was expected, caused by the necessary high investments, injection moulding is interesting only above 60.000 pieces p. a., yet provides opportunities to reduce the cost to about 20 € per trough. At the time of the project, such a high number of collectors of one single type was not produced by any of the big producers of solar — thermal collectors. Hence, for cost reasons, a deep-drawn trough design was selected, that leads to only moderate investments of tooling. The cost limit can be met using this production technology.

The following design work, as shown in figure 7 and figure 8, was accompanied by detailed material selection. Typical solar-thermal collector operation conditions have to be considered, such as

► life-time of at least 20 years,

► all-year unprotected weather exposure,

► temperature changes from -20 °C up to +80 °C,

► UV-exposure,

► salt water atmosphere, etc.

Thermoplastics such as ASA, especially when combined with polycarbonate and glass fibres, appear well suited and are already used in relevant applications, however, they are relatively expensive (figure 6).

Luran® S, produced by BASF AG, Ludwigshafen/D, for example is a thermoplastic based on ASA and is widely used in applications comparable to solar-thermal collectors:

► automotive construction (figure 6; also commercial and agricultural vehicles),

► electrical engineering (TV antennae parts, cable connection housings, weatherproof protective housings),

► sports and other outdoor uses (sailboats, surfboards, snowboards).

PFLEIDERER DACHZIEGEL GmbH, Winnenden/D, is even using this material for roof — integrated PV-systems as shown in figure 9.