5       Renewable heat and cold production

In cooperation with ITW

5.1       Passive solar thermal heating

The goal of passive solar heating design is to capture the heat of the sun within the building in order to reduce the heating needs. Optimized systems enable to store and release that heat during periods when the sun is not shining, so that space heating needs can be minimized.

The principle of passive solar thermal heating is easy: short-wave solar radiation penetrates the transparent elements of the building, is absorbed by the building components and then is transformed into long-wave solar radiation. The natural properties of glass let sunlight through but traps long-wave heat radiation, keeping the house warm (the greenhouse effect). The challenge often is to properly size the south-facing glass to balance heat gain and heat loss properties without overheating. The window area on north-, east-, and west-facing walls should be reduced, while still allowing for adequate daylight entrance. Effective south-facing windows require a high Solar Heat Gain Coefficient (SHGC) or g-value (approx. 0.60 or higher) to maximize heat gain and a low U-value (1.35 [W/(m² K)] or less) to reduce conductive heat transfer, and a high visible transmittance for good visible light transfer. In buildings with high internal gains, like offices, the g-value of the glazing can be lower in order to prevent overheating.

However, it has to be noted that in listed historical buildings, in order to preserve the original appearance of the building façade, often only the possibility to change the glazing or to build a double façade remains.

Passive solar thermal heating is getting more important if the heat energy demand and the heat losses of a building have been reduced with energy-efficient design strategies. By reducing heat losses and maximising gains, remaining energy loads can be effectively met with passive solar techniques. South-orientated rooms with big windows e.g. may remain unheated on cool but sunny days. At the same time, however, there exists a risk of overheating in summer. For this case preventive actions like shading should be foreseen. Properly sized window overhangs or awnings are an effective option to optimize southerly solar heat gain and shading. They shade windows from the summer sun and in the winter, when the sun is lower in the sky, they permit sunlight to pass through the window to warm the interior (See Part I – Chapter 4.5 - Integration of external shadowing and Part II – Chapter 4 - Interior shading devices).

Passive solar heating techniques generally fall into one of three categories: direct gain, indirect gain, and isolated gain. Direct gain is solar radiation that directly penetrates through windows and is stored in the living space. Indirect gain collects, stores, and distributes solar radiation using thermal storage devices by conduction, radiation, or convection and then transfers the energy indoors. Isolated gain systems (e.g. sunspace) collect solar radiation in an area that can be selectively closed off or opened to the rest of the house.

The use of passive solar thermal heating is particularly profitable, if the building is made of massive materials with a high thermal mass that can absorb, store and distribute the heat. Materials such as concrete, masonry or wallboard absorb heat during sunlit days and slowly release it as temperatures drop. This dampens the effects of outside air temperature changes and moderates indoor temperatures. Depending on the climate, mass-to-glass ratios should be optimized to prevent overheating and minimize energy consumption. Latest developments in this context are micro-encapsulated phase change materials that are integrated in the plaster and increase the thermal mass of the building. More information on thermal mass can be found in (See Part II – Chapter 2 - Thermal mass).

Points of attention

§  When assessing the potential solar gains, any shade and thermal radiation from adjacent buildings should be taken into account.

§  Wind speed and direction should be taken into account. Reduction of wind speed by deflection or creation of shelters on the site without reducing solar gains can reduce heat loss from buildings caused by winds [BIT08].

§  Windows on east and west façades are often the cause of overheating as they are difficult to shade without blocking out the sun, because of the low incidence angle of the solar radiation [BIT08].

Practical examples

§  Passive solar thermal heating is extensively used by the "Passive House Concept", which is developed in Germany by the Passive House Institute. For further information please refer to www.passiv.de. Experience indicates that there are no substantial extra costs to integrate passive solar heating into those houses.

§  Double skin façades have a second glazed skin installed at a distance from the main façade. In summer the double façade can reduce solar gains if the heat gains in the façade are evacuated by ventilation. In winter, the double façade acts as a buffer zone, reduces heat losses and improves U-values. In existing buildings double-skin façades can be used to renovate façades that have deteriorated.

§  Another innovative design with high attraction is the installation of a transparent insulation in front of an external wall. This concept is very suitable for façade renovation. The use of transparent insulation materials makes it possible to reduce transmission losses (like a conventional insulation does) and at the same time to capture solar heat in uninsulated massive walls. Solar rays cross the transparent insulation and hit the dark massive wall, where the solar radiation is converted into heat. The heat is stored in the wall and conducted into the interior with a time delay of several hours. It is most effective when used on massive uninsulated external walls with few openings. The transparent insulation modules can be installed either as a complete cladding of the façade or they can be distributed only on selected areas of the façade. Mostly they consist of an exterior glass pane with the transparent insulation layer behind. On the wall itself there is often a dark painting to increase absorption.

§  Trombe walls - A typical unvented Trombe wall consists of a 10 to 41 cm thick, southfacing masonry wall with a dark, heat-absorbing material on the exterior surface and faced with a single or double layer of glass. The glass is placed from 2 to 5 cm from the masonry wall to create a small airspace. Heat from sunlight passing through the glass is absorbed by the dark surface, stored in the wall, and conducted slowly inward through the masonry. For a 20 cm Trombe wall, heat will take about 8 to 10 hours to reach the interior of the building [TOR04]. This means that rooms receive slow and even heating for many hours after the sun sets, greatly reducing the need for conventional heating. Trombe walls can be used to maximise heat collection when views and glazing are oriented to the south or when site orientation is not ideal.

Figure 72: Schematic draw of a trombe wall

External links

§  Balcomb, J. Douglas., Jones, R. W. (ed.): "Passive Solar Design Handbook. Volume III: Passive Solar Design Analysis and Supplement. American Solar Energy Arbeitsgemeinschaft ERNEUERBARE ENERGIE – AEE 11/11 Society, Inc. Publications Office: 110 W. 34th St., New York 10001. ISBN 0-89553-124-0.

§  Passive House Institute, Darmstadt, Germany, http://www.passiv.de/

§  http://www.smarterhomes.org.nz/design/using-thermal-mass-for-heating-and-cooling/

§  Energy Efficiency and renewable energy, passive solar design for the home, NREL National Renewable Energy Laboratory, Colorado, USA http://www.nrel.gov/docs/fy01osti/27954.pdf

§  Association of Transparent Insulation (FVTWD e.V.), Gundelfingen, Germany, http://www.umwelt-wand.de/ti/index.html

 

5.2       Active solar thermal heating

Active solar thermal systems provide heat that can be used for different building needs: hot water, space heating and cooling (via a thermal driven cooling process).  This chapter treats general principles of solar hot water production and space heating, cooling is treated in (See Part II – Chapter 5.3 - Solar thermal cooling).  More information about the integration of the collector in historic buildings can be found in (See Part II – Chapter 5 - Integration of solar thermal collectors)

5.2.1    Solar water heaters

Today most solar thermal systems are used for domestic hot water preparation in single or small multi-family houses. These systems are designed to cover approximately 60 % of the annual hot water demand by means of solar thermal energy. Typical systems have a hot water storage volume of approximately 300 litres and a collector area between 3 to 6 m², depending on the type of collector (flat plate or evacuated tube). 

Figure 73 shows a typical design of a forced-circulating solar thermal system for domestic hot water preparation in a single family house. The collector on the roof heats up a transfer medium, which usually is a mixture of water and glycol (anti-freeze). For auxiliary heating, a second heat exchanger, which is connected to the oil- or gas boiler, is located in the upper part of the hot water store.  A variant of the back-up heating can be an electric resistance in this storage tank or a gas-fired direct heater at the outlet of the storage tank.

Figure 73: Schematic design of a forced circulating (pumped) solar thermal system for domestic hot water preparation

The collector loop consists of the collector, the solar loop pump, a non return valve, a heat exchanger and the safety group with membrane expansion vessel and safety valve. The heat exchanger of the collector loop is integrated near the bottom in the store. The heat exchanger is necessary to separate the heat transfer fluid of the collector loop from the domestic water in the buffer store. The electronic control unit monitors the temperature difference between the collector and the lower part of the hot water store. When the temperature difference between these two temperatures exceeds a certain limit (typically 6 to 10K) the solar loop pump is activated to circulate the heat transfer fluid through the collector loop. Via a heat exchanger, the solar heat is released into the water in the hot water store. An auxiliary heater keeps a part of the store volume at the top on a certain temperature level. This part of the store is named auxiliary volume. The heat is delivered to the store via a heat exchanger at the top of the store. The domestic hot water is withdrawn directly from the top of the store. As the temperature in the store can reach up to 95 °C in summer a cold water bypass and a mixing valve are necessary to mix the hot water down to the desired temperature (e.g. 45 °C).

Costs

The cost for a typical solar domestic hot water system for a single family house (approx. 300 litre store volume, 3,5 kW_th or 5 m² collector area respectively) is in the range of 4000 to 6000 EURO including installation and VAT.

Rules of thumb

There are some basic design figures for solar thermal systems for domestic hot water preparation in single or small multi-family houses in an average mid-European climate:

§  Collector area 1 to 1.5 m² per person (flat plate collectors) or 0.8 to 1.0 m² per person (evacuated tube collectors)

§  Buffer store volume about 50 l per m² collector area

With regard to large-scale systems, according to a VDI guideline [VDI04] the solar thermal system should be designed for the domestic hot water demand in the summer period when the normal demand is naturally lower and even lower as usually since some of the inhabitants are on holiday. To avoid stagnation of the collectors, which can damage collector loop components and reduce the economic efficiency, the system should be designed in such a way, that the solar gain from the collectors in the summer period can be either stored in the solar buffer store or is used by the consumers on the same day.

In order to achieve a reasonable solar fraction without too much stagnation during the summer the size of the collector field and the solar buffer store should be as follows:

§  Collector area 1 m² for every 65 l to 70 l of domestic hot water load (60 °C)

§  Solar buffer store volume approximately 50 l/m² collector area

Designing large scale solar domestic hot water systems, according to VDI [VDI04], leads typically to relative small solar fractions in the range of 30 % and there can be several good reasons to design a system in such a way that larger solar fractions will be achieved. Typically the most important aspects are that larger solar fractions correspond to a higher amount of financial savings with regard to purchasing conventional energy (oil or gas) as well as to higher CO₂ savings.  Another argument may be that systems with a 100 % solar fraction in the summer months can enable a complete shut down of the boiler in these months, thus reducing stand-by losses. 

5.2.2    Solar combisystems

In Central and Northern Europe it has become more and more common to install solar thermal systems that provide heat both for domestic hot water (DHW) and for space heating. These solar combisystems are similar to solar water heaters in the collection of solar energy and the transport of the produced heat to the storage device. The major difference is that the installed collector area of a combisystem, which provides two energy consumers, is larger than in the case of a solar water heater which supplies only one consumer. Solar combisystems consist typically of 5 - 50 m² collector area in combination with storage volumes of 0.5 to 3 m³. The size depends on the conditions in the country under consideration, the heat distribution system and the thermal insulation quality of the building [IEA00].

Combisystems are often more complex than solar thermal systems supplying DHW only. As a consequence, the system design must be adapted to the specific requirements of the building. Different practices are used in different countries. In Southern Europe, combisystems are still rarely used, but there is a big potential for so-called solar plus combisystems, systems generating space heating in winter, air-conditioning in summer and DHW throughout the year.

In northern latitudes the solar energy available in summer is more than twice as much as in winter. Virtually the opposite applies to the energy demand for space heating, as this heating load is dependent upon the outside air temperature. However, measurements of solar radiation and temperature in the transitional periods (September to October and March to May) clearly show that solar radiation availability is relatively high at the beginning and end of the space heating season, paving the way for solar space heating [IEA00].

The technology of combistores (storage tanks for combisystems) is more complex than of mere hot water stores. A lot of different systems and concepts are available on the market (two separate stores, tank-in-tank stores, use of stratifiers, geometry on fluid inlets, used flow rates, type of heat exchanger, etc.). There exist various possibilities to connect the heating circuit to the solar thermal system. One example is shown in Figure 74, which shows a combistore with two heat exchangers, one for the solar circuit and one for comestic hot water preparation. The auxiliary heat demand is covered with the help of an external boiler.

 

Figure 74: Scheme of a solar combisystem

To supply the two heat consumers (domestic hot water and space heating), water should be simultaneously available at two different temperature levels. This can be done, of course, by operating two different storage tanks with a clever control unit acting on valves and pumps. However, with regard to the reduction of heat losses and costs it is preferable to use a single storage tank. This requires that care is taken to avoid mixing water of water levels with different temperatures. As hot water has a lower density than cold water, hot water is always located in the upper part of the storage tank; conversely, cold water is found at its bottom. This feature is called thermal stratification of the storage tank.

Latest technologies that can be found are combistores with integrated burner, compact heating units that reduce considerable the installation effort, as all necessary equipment (pumps, controller, burner, etc.) is already installed in the unit. Other solutions are a clever control algorithm where mere water can be used as heat transfer fluid in the collector loop. Furthermore advanced storage technologies based on latent heat or sorption are under development.

Rules of thumb

§  Currently installed systems clearly show that solar space heating is possible even for mid- and northern European climates. How much heat is needed in a year for space heating in comparison to domestic hot water, depends on the building size, thermal insulation, ventilation, passive solar use, internal heat loads as well as on the number of its inhabitants and the location of the building. Usual figures indicate a DHW heat demand amounting to 10 to 40 % of the total heat demand for space heating and DHW [IEA00].

§  In a combisystem there are at least two energy sources used to supply heat to the two heat consumers: the solar collectors deliver heat as long as solar radiation is available, and the auxiliary energy source (oil, gas, wood, electricity, etc.) supplements the missing solar energy. As a general rule, collectors should be operated at the lowest possible temperature level in order to have a good efficiency; at higher temperature levels they have significant increased heat losses.

§  Whereas the typical store dimension for domestic hot water systems in single-family houses is around 300 litres, the required store dimension for solar combisystems is around 750-1500 litres.

§  Due to lower winter sun and the high heat demand in winter for space heating, solar thermal collectors should be installed with other inclination angels than for solar domestic hot water preparation only. As the winter sun is relatively low, façade integrated collectors are fitting well in combination with a solar combisystem. Another interesting solution in this case is the installation of solar thermal collectors in parapet of balconies.

§  Inappropriate inclination angles or orientations may be balanced with greater collector areas.

§  It should be analysed if it’s possible to install the equipment (store and burner) in the attic not far away from the collector field. This has the advantage of shorter piping, saves costs, reduces the heat losses of the piping and reduces installation effort.

Points of attention

§  Currently installed systems clearly show that solar space heating is possible even for mid- and northern European climatic conditions. However, before a solar-powered heating system is installed, due attention must be paid to solar energy availability, space heating energy demand, and required heat storage volume.

§  Particular attention should be paid to solar access, and the type and orientation of the building. Shading objects should be avoided. In spring and fall they may cause large reductions in solar heat production, especially at northern latitudes.

§  Another basic requirement for the efficient use of solar space heating systems is a high insulation standard of the building. In general, a badly insulated house should first have its insulation improved before a solar space heating system is considered.

§  A low-temperature heat emission system with low flow and return temperatures is an additional favourable prerequisite for solar space heating. A low return temperature at the outlet of the heat emission system is even more important in the case of solar space heating than a low flow temperature [IEA00].

Costs

The costs for solar combisystems differ considerably and depend on the system configuration, local market and system dimensions. A solar combisystem of 15 m² collector area and a 1000 litres store costs approx. 10000 Euro to 13000 Euro including installation.

Related chapters

There is another interaction between solar radiation and space heating demand: solar radiation on the building and through the windows produces heat. This is the so-called passive solar heating. This heat no longer needs to be emitted by the space heating system. More information on passive solar heating is given in Part II – Chapter 5.1 - Passive solar thermal heating.

Further information on solar thermal systems for domestic hot water preparation and information on façade integrated collect can be found in Part I – Chapter 5 - Integration of solar thermal collectors.

External links

§  www.estif.org European Solar Thermal Industry Federation

§  Weiss, W. (ed), Bales, Ch., Drück, H., et al.
Solar Heating Systems for Houses, A Design Handbook for Solar Combisystems, International Energy Agency (IEA), James&James (Science Publishes), 2003, London, ISBN: 1902916468

§  International Energy Agency (IEA) Solar Heating & Cooling Programme, IEA Task 26, Solar Combisystems, 2003. http://www.iea-shc.org/task26/index.html

§  A related project supported by the European Commission's ALTENER Programme, aimed at converting the findings of the IEA task 26 to information usable for the public. http://elle-kilde.dk/altener-combi/index.htm

§  Planning and Installing Solar Thermal Systems: A Guide for Installers, Architects and Engineers (Planning and Installing) by German Solar Energy Society (DGS), ISBN: 9781844071258, November 2004

§  IEA Task 32 - Advanced Storage Concepts for Solar and Low Energy Buildings, http://www.iea-shc.org/task32/index.html

§  IEA SHC Task 26 (2004): Solar Heating Systems for Houses - A Design Handbook for Solar Combisystems, W. Weiss and al., James & James, 2004, 313 pages

§  Hadorn J.-C. editor, (June 2005), Thermal energy storage for solar and low energy buildings - State of the Art, Printed by Servei de Publicacions Universidad Lleida, Spain, 170 pages ISBN 84-8409-877-X, available through Internet www.iea-shc.org Task32

 

5.3       Solar thermal cooling

Solar assistant cooling systems have the advantage that cooling requirements of a building largely coincide with high level of solar radiation. Typically relative harmless working fluids such as ammonia-water or solutions of certain salts are used. Solar thermal cooling cycles are based on a kind of sorption process: a liquid or gaseous substance is either adsorbed to a solid, porous substance or is absorbed in a liquid or solid substance. The thermal operating power can be delivered by solar thermal collectors.

In general a solar thermal cooling system consists of a typical solar thermal system (made up of solar thermal collectors, storage tank, control unit, pipes and pumps), a thermally driven cooling machine and components of a conventional air conditioning equipment (e.g. fan coils, chilled ceiling etc.). Solar cooling systems can be classified into two basic system categories: closed and open systems.                                                              

5.3.1    Closed systems

Closed systems are thermally driven chillers which provide chilled water, which is either used in central ventilation stations to supply conditioned air (cooled and/or dehumidified), or that is distributed via a chilled water network to the designated rooms to operate decentralized room installations, e.g. fan coils or chilled ceilings. The required chilled water temperature depends on the design of the cooling coils in the air handling unit or on the design of the installed cooling systems in the rooms. Closed cycle systems require a heat sink like a wet cooling tower, air-cooled heat exchanger (dry-cooler) or ground coupled systems to allow the heat rejection in the condenser. Market-available machines are absorption chillers (Figure 75) and adsorption chillers (Figure 76).

Absorption chillers are the most commonly used thermally driven chillers in solar assisted air conditioning systems today. Instead of using a mechanical compressor, absorption chillers achieve a thermal compression of the refrigerant by using a working pair (the refrigerant and a solvent e.g. ammonia / water or water / lithium bromide) and a heat source. The cooling effect is based on the evaporation of the refrigerant in the evaporator at very low pressure. After the absorption (thermal compression) the solution is continuously pumped into the generator, where the regeneration of the solution is achieved by applying driving heat (e.g. hot water from solar collectors). After leaving the generator the refrigerant condenses through the application of cooling water (e.g. wet cooling tower, air-cooled heat exchanger) in the condenser and circulates by means of an expansion valve back again into the evaporator.

Figure 75: Schematic drawing of an (solar driven) absorption chiller

In contrast to absorption chillers, adsorption chillers use solid sorption materials. Market available systems use water as refrigerant and silica gel as sorbent. Because of the fact that the solid sorbent cannot be circulated adsorption chillers consist of two separate chambers, which both contain the adsorbent.  Besides these two adsorbent chambers, there is one evaporator and one condenser (coupled to the heat sink).  Those 4 components are packed in a sealed vacuum.

The functional description below describes a system with only one adsorber (with 2 adsorbers the process is doubled) and in contra phase so as to enable continuous operation (Figure 76).

§  The adsorbent containing the water is heated (by the solar collectors) and the adsorbed water is expulsed as water vapor and condenses in the condenser

§  The condensed water is transferred to the evaporator

§  The adsorbent is cooled again, leading to a lower pressure in the sealed system. 

§  The water in the evaporator evaporates, taking up the heat from the chilled water circuit

§  The water vapor is adsorbed in the adsorbent (adsorption heat is evacuated)

Figure 76: Schematic drawing of an (solar driven) adsorption chiller.

5.3.2    Open systems

Open systems use a desiccant and evaporative cooling process (DEC systems) and allow complete air conditioning by supplying cooled and dehumidified air according to the comfort conditions. These systems can only be used in combination with an air handling unit, they are unable to produce chilled water. 

Most common open systems are desiccant cooling systems (DEC) using a rotating dehumidification wheel with solid sorbent (Figure 77). Warm and humid air (fresh air) enters the slowly rotating desiccant wheel and is dehumidified by adsorption of water. Because the air is heated up by the adsorption heat there is also installed a heat recovery wheel. The result is a significant pre-cooling of the supply air stream. After the pre-cooling the air is humidified and further cooled by a controlled humidifier according to the desired temperature and humidity of the supply air stream. The exhaust air stream (warm, humid) of the rooms is humidified close to the saturation point to exploit the full cooling potential in order to allow an effective heat recovery. To allow a continuous operation of the dehumidification process the sorption wheel has to be regenerated by applying heat in a comparatively low temperature range from 50 °C – 75 °C.

Figure 77: Schematic drawing of (solar) desiccant cooling system for direct treatment of fresh air in ventilation systems

Rules of thumb

For thermally driven cooling systems (absorption and adsorption cooling) it can be approximated that for 1 kW cooling capacity approx. 3 m² collector area is needed [HEB07, HEN04]. Regarding open cycles it can be assumed a value between 8 and 10 m² per 1000 m³/h installed air capacity [HEN04]. These values result as an average of all solar cooling systems realized in Europe. 

Costs

Solar assisted air conditioning systems require more technical equipment than conventional systems. Furthermore, they require the entire solar thermal system. This results in higher investment costs for solar cooling. It is best to use a solar thermal system that serves more than just the cooling needs of the building by incorporating other thermal consumers (heating, domestic hot water) to maximize the return on investment and not leave the system idle when cooling is not required.

Solar thermal cooling systems with absorption machines cost approximately up to 1.5 - 2 times more than a conventional one. A fully equipped solid solar thermal desiccant air handling unit costs approximately up to 1.3 to 2 times more than a standard unit [BIT08]. For DEC systems the solar collector field costs about 5000 € for every 1000 m³/h of handled air or 1500 € for every cooling kW [BIT08].

Related chapters

For more information on integration of solar thermal collectors, please refer to Part II – Chapter 5 - Integration of solar thermal collectors.

External links

§  Solar assisted cooling, state of the art. Key Issues for Renewable Heat in Europe (K4RES-H) Solar Assisted Cooling, WP3, Task 3.5 http://www.erec.org/fileadmin/erec_docs/Projcet_Documents/K4_RES-H/D23-solar-assisted-cooling.pdf

§  Hans-Martin Henning. Solar Cooling. ISES Solar World Congress 2007, Beijing International Convention Centre (BICC), Beijing China, 19 September 2007

§  Hans-Martin Henning. Solar-Assisted Air Conditioning in Buildings, A Handbook for Planners, ISBN 3-211-00647-8, Springer Wien / New York, published in the frame of Task 25 of the Solar Heating & Cooling Programme of the International Energy Agency (IEA), 2004

§  Brochure on Solar Cooling – General information and papers / summaries of expert workshop; Ed: Berliner Energie Agentur GmbH (BEA), this brochure was elaborated in the frame of the project ProEcoPolyNet, Contract No. TREN/05/FP6EN/SO7.554455/020114

§  V. Sabatelli, G. Fiorenza, D. Marano: Technical status report on solar desalination and solar cooling (WP5.D1). NEGST: New Generation of Solar Thermal Systems, November 2005

§  N.N.: Promoting Solar Air Conditioning. Technical overview of active techniques.  ALTENER Project Number 4.1030/Z/02-121/2002

§  M. Delorme, H.-M. Henning, et al.: Solar air conditioning. With the support of the European Commission and the Rhone-Alpes regional Council, 2004

§  IEA Task 25, Solar Assisted Air Conditioning in Buildings, http://www.iea-shc-task25.org/

§  IEA Task 38, Solar Air Conditioning and Refrigeration, http://www.iea-shc.org/task38/

 

5.4       Biomass

Biomass can be transformed via numerous processes into several useful forms of energy. Typical applications are the burning of solid biomass in furnaces, the use of pure plant oil in cogeneration plants with internal combustion engines, and fermenting solid and liquid bio-energy sources in biogas systems, followed by burning the biogas. For domestic applications of biomass the fuel usually takes the form of wood pellets, wood chips and wood logs. Log boilers must be loaded by hand and are therefore not suitable for every application.

Wood pellets are usually stored in closed rooms or silos and the delivery of the pellets is done by tank trucks. The pellets are generally blown into the storage room. From there they are transported automatically into the combustion chamber of the boiler.

In order to avoid partial load operation in summer when the pellet boiler would only be in operation to cover the hot water load, the pellet boiler should be combined with a solar thermal system. So the solar thermal system will be a pre-heating system in winter and the pellet boiler will act as the major energy source in winter. 

Biomass energy systems can be used in a wide range of sizes. If the heat load is large enough CHP (combined heat and power) can be considered [BIT08]. CHP systems are energy systems that can use biomass fuel to produce both heat and electricity. The overall efficiency is significantly higher than that of a separate heat and electricity production with conventional systems. The heat generated during electricity production is preferably used to meet the building’s heat demand.

Points of attention

For the integration of biomass as renewable energy heat source, it is important to ensure that there is enough storage space for the fuel, appropriate access to the boiler for loading and that there exists a local fuel supplier.

Wood chips contain more moisture and less energy than wood pellets. Therefore they require a larger store than wood pellets but are cheaper per unit of energy. However, wood chip feed mechanisms must be sturdier and stronger so that they are more costly. The equipment is different depending on whether pellets or chips are used. Wood pellets facilitate automatic feeding of the burner and they are less prone to clogging and gungus growth [BIT08]. Meanwhile there are also dual-fire boilers on the market which will fire both wood chips and pellets.

It should be kept in mind that the ash box of the boiler has to be emptied regularly. In order to keep this process rare there are systems available that are compressing the ash, so that the box has to be emptied only every 3 months, depending on the boiler output [REH08].

Rules of thumb

The size of a fuel store should be chosen in such a way that a reasonable volume of fuel can be delivered at one time. An under-sized store requires more frequent fuel deliveries and thus increases the overall fuel cost. As rough indication average energy densities of 13.6 GJ/ton for wood chips and 19 GJ/ton for wood pellets can be assumed [BIT08]. One kg of pellets corresponds approximately to the heating value of half a litre of oil (approx. 5 kWh/kg) [ALT08].

The pellet or chips store room should be located within splitting distance from the pellet boiler. The size of the storage room depends on the power of the boiler. With a building heat demand of 10 kW the size of the storage room is approx. 3-4 m² with a room height of 2.30 m [ALT08]. For a heating load of 1 kW it can be assumed a storage volume of 0.9 m³ for the storage of the entire heating demand of one year [ENE07].

Costs

The investment cost of a biomass heating system consisting of a 40 kW automatic wood chip boiler is around 35000 Euros. Pellet boilers are a bit cheaper. Wood chips cost between Euros 70-80 per ton [BIT08].

The investment cost of a pellet boiler for a typical detached house was in 2007 in Germany around 10000 to 15500 Euro (installation included), depending on the degree of automatisation of the transport system and without storage room. Wood pellets may cost between 180 up to 210 Euros per ton (average enduser price in summer 2007 in Germany) [BIN08].

Heat cost per kWh:
wood chips Euros 0.018-0.023
wood pellets Euros 0.021-0.035 [BIT08]
wood pellets Germany (end of 2006): 4.6 ct/kWh (incl. delivery and VAT) [ENE07]

Stand alone pellets room heaters generally cost around 3800 Euros, the cost for a typical 20 kW (average size required for a three-bedroom semi-detached house) pellet boiler would cost around 7000-15000 Euros [LOW08].

Practical examples

The refurbished Renewable Energy House in Brussels is equipped with a pellet heating system (2 boilers with 15 and 85 kW). With a storage capacity of 13 tons of pellets, pellets have to be delivered 2 - 3 times per year [REH08].

External links

§  European Biomass Industry Association http://www.eubia.org/

§  European Pellet Centre http://www.pelletcentre.info/cms/site.aspx?p=878

§  Biomass heating; BIT - The BRITA in PuBs Information Tool: BRITA = Bringing Retrofit Innovation to Application, demonstration of retrofit projects, computer tool for the first planning phase http://www.brita-in-pubs.eu/bit/uk/03viewer/retrofit_measures/renew_bioheat.html 

§  www.lowcarbonbuildings.org.uk

§  REH Brussels, http://www.erec.org/reh/

§  Renewables made in Germany: information about german renewable energy industries, companies and products http://www.renewables-made-in-germany.com/en/solid-biomass/

§  Fachagentur nachwachsender Rohstoffe e.V.; http://www.bio-energie.de/

§  Altbauten sanieren - Energie sparen - Solarpraxis - ISBN 978-3-934595-78-1

§  Energieeffizient sanieren - ISBN 978-3-934595-72-9

 

5.5       Heat pumps for heating

A heat pump is a device that transfers heat from a low temperature source, such as ambient air or soil, to a higher temperature sink by using additional energy. Typically heat pumps are used for space heating and domestic hot water preparation. In general between 60 to 70 % of the heat pumps heating energy is provided by the environment and only 30 to 40 % must be supplied in the form of electricity.

The working principle for electrical driven heat pumps is similar irrespective of the source of heat they are using. Figure 78 illustrates a basic heat pump cycle.

A refrigerant continuously flows in a closed loop between the heat source (evaporator) and the heat sink (condenser). In the evaporator the liquid refrigerant absorbs heat from the surroundings and vaporizes. It is then pressurized in a compressor and arrives in the condenser at a high temperature. It there condenses and gives off the useful heat. In the following expansion valve the fluid is expanded to the evaporating pressure and the cycle starts again.

 

Figure 78: Basic heat pump cycle [RBA08]

 

A heat pump system is suitable to be installed for heating purposes, when a refurbished building can be heated with low temperature heat e.g. with a floor heating system or oversized radiators. Existing heat emission systems can become overdimensioned as a result of energy efficiency measures like insulation and heat recovery thus reducing the heat loads (See Part II – Chapter 8 - Integrating heating and cooling emission systems).

For the operation a heat source is necessary, from which heat can be withdrawn. This heat will be lifted typically with electrical energy to a higher temperature level. As possible heat sources geothermal energy and ambient air are discussed in the following [ALT08].       

5.5.1    Geothermal

Geothermal heat pumps use the heat available in the earth as the heat source. The advantage of this system is the relatively constant temperature of the earth, from 7 °C in winter to approx. 12 °C in summer, over the year. This allows the system to reach fairly high efficiencies even during cold winter nights.

Ground source heat pumps can be categorized in two basic systems, the closed loop system and the open loop system. In a closed loop system the fluid circulates through the loop field’s pipe and there is no direct interaction between the fluid and the earth. Closed loop systems can be divided into ground source heat pump with horizontal collectors and ground source heat pump with vertical loops.

In an open system water is directly pumped from the source into the heat pump where the heat is extracted and then pumped back.

In addition to the usage of geothermal heat it is also possible to use the ground as a (seasonal) store for the storage of heat (e.g. generated by solar thermal collectors during the summer).

Rules of thumb

As far as closed systems are concerned ground source heat pumps with horizontal collectors are the most cost-effective solution, but a sufficient land size is necessary. The ducts are placed in trenches normally 1 to 2 meter deep. For a heating demand of 3.5 kW a piping length of 120 to 180 meter is needed, depending on the soil conditions.

Ground source heat pumps with vertical loops are mainly used in suburban area where lot space is restricted or when the soil is too shallow for trenching. Vertical holes with a diameter of about 150 mm are drilled about 18 to 100 meters deep. U-shaped loops of pipes are inserted into the holes and connected with horizontal pipes, placed in trenches and connected to the heat pump. About 80 to 110 m of piping is needed for 3.5 kW of heating capacity.

Costs

Specific initial investment costs for the usage of geothermal heat near the earth’s surface are given for Germany, depending for instance on the kind of installation. Typical values for geothermal collectors and boreholes are in the range of 240 to 600 €/kW and for an additional heat pump between 1500 and 2400 €/kW.

5.5.2    Air-based heat pumps

There are two types of air source heat pumps: the air-to-water heat pump and the air-to-air heat pump.

§  Air-to-water heat pump: The heat source of the air-to-water heat pump is the absorbed air (outside air or extraction air). The heat is transferred to water, the heat sink, which is used for space heating or hot water preparation.

§  Air-to-air heat pump: Similar to the air-to-water heat pump, the air-to-air heat pump uses outside air or extraction air as the heating source. But here, the heat is transferred to air as the space heating medium. For this type of heat pump a controlled ventilation system is necessary.

Air source heat pumps are relatively easy and inexpensive to install. But they suffer limitations due to their use of the outside air as a heat source. The disadvantage of using ambient air is that at the time when the most heat demand exists, the outside air has a low temperature level. The COP (coefficient of performance) decreases with the temperature of the ambient air. Typical values of the COP for medium temperatures (~10°C) lie in the range of 3.3 whereas the COP for a temperature of -8.3°C is around 2.3.

A specific application of air to water heat pumps are heat pump boilers. These systems use extraction air to produce sanitary hot water. These applications can be useful in situations where heat recovery is not possible in an air handling unit or when rooms with a lot of internal gains (e.g. server rooms) are situated in buildings with a certain hot water demand.

Costs

The costs for heat pumps are depending on the heating capacity. For an area of approx. 180 m² a heating capacity of 8-9 kW is necessary. In Germany the costs for domestic geothermal heat pumps lie approximately in the range from 8500 to 11500 Euro (incl. VAT, controller, pump and store, without delivery, installation and ground source system). The costs for air based heat pumps lie between 10.000 - 12.000 Euro incl. VAT. [ERD08] Installation costs for geothermal heat pumps are higher but they have a better COP than air-based heat pumps (COP between 3…5).

Related chapters

Part I– Chapter 2.2.4 - Heat pump applications describes the implication of geothermal systems on the outside of the building.

External links

§  http://oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/booklet.pdf, Heating and Cooling with a heat pump, Natural resources Canada’s office of energy efficiency

§  www.tea.ie/download.ashx?f=TEA+Geothermal+Energy+Resource+Report+Final901c7fa5.pdf

§  The European Heat Pump Network: http://www.ehpa.org

§  Heat Pump Centre of the IEA, http://www.heatpumpcentre.org

 

5.6       Heat pumps for cooling

Besides heat pumps for heating only, there are also reversible heat pumps (able of both heating and cooling) and heat pumps solemnly for cooling. Heat pumps for cooling are available as air-based heat pumps and geothermal heat pumps.

Air-based heat pumps are the reference technology for air-conditioning in buildings. This means that they can not be regarded as a renewable energy technology or an application of RUE. 

Geothermal heat pumps for cooling use the earth as heat sink for heat removed from the building in summer. In general, these are often reversible systems. These systems usually have higher efficiencies since the temperature of the heat sink is lower than in case of air based heat sinks. It is also possible to have geothermal heat pumps for heating where the geothermal installation is used for free cooling, without the use of the compressor of the heat pump. This is explained in Part II – Chapter 5.5 - Heat pumps for heating.

An amount of electricity input is required to run the compressor of the heat pump. If the electricity consumed by the compressor is generated from renewable sources, all the delivered energy is renewable. In order to maximize the delivery of renewable energy, it makes sense to couple renewable electricity, e.g. from a photovoltaic system or from a wind turbine, to heat pumps.

Related chapters

Details on heat pumps for heating will be found in Part II – Chapter 5.5 - Heat pumps for heating.

External links

§  Natural Resources Canadas Office of Energy Efficiency, Heating and cooling with a heat pump, ISBN 0-662-37827-X, Dec. 2004, http://oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/booklet.pdf

 

5.7       Passive cooling

Passive cooling stands for cooling technologies that do not use the compressor of air-conditioning equipment (also called mechanical cooling). Different possibilities exist, like night ventilation, geothermal free cooling or free chilling. Well applied passive cooling techniques in optimized buildings can seriously reduce or even eliminate the need for mechanical cooling, leading to strong energy and cost economies. 

5.7.1    Night ventilation

Apt use of outdoor air often can cool a building without need for mechanical cooling, especially when effective shading, insulation, window selection, and other means already reduce the cooling load. In many climates, opening windows at night to flush the house with cooler outdoor air and then closing windows and shades by day can greatly reduce the need for supplemental cooling.

Night ventilation uses design strategies that allow stored heat to be released to the outside. This strategy is particularly effective when the daytime-nighttime temperature differences are meaningful. Night flushing of buildings uses radiative cooling principles. The thermal mass of the building serves as a heat sink during the day, but releases the heat at night, while being cooled with night air.

However, for the average temperature of the building to be below the average ambient temperature, the building interior must be coupled to the outside selectively; that is, only when the ambient temperature is well below the temperature inside the building. This is done by providing high rates of ventilation at night (> 2 air changes per hour). Night ventilation can lower daytime temperatures in heavyweight buildings by about 3°C.

Free night ventilation is caused through pressure differences resulting from temperature differences between the outer shell and the internal space of the building that cause an air draught. Often natural ventilation systems use the passive stack effect and pressure differentials to bring fresh, cooling air through a building without mechanical systems. Buildings using this design will incorporate operable windows or other means of outdoor air intakes (e.g. fresh air inlets located near floor level) to enhance the stack effect. Exhausting naturally rising warmer air through upper-level openings (stack effect) encourages lower-level openings to admit cooler, refreshing, replacement air.

In mechanical night ventilation the ventilation group assures the necessary ventilation rates at night.  The heat recovery, if present, needs to be bypassed or stopped during the night ventilation. Caution has to be paid to the energy consumption of the fans.

Rules of thumb

§  Night ventilation is appropriate in hot weather with cool nights when the daytime temperature is expected to be above comfort temperature and in massive buildings with high internal gains. It is important that occupants are not isolated from the cooled mass. This may conflict with acoustic measures that include carpets and lightweight suspended ceilings.

§  It may be advantageous to direct the night ventilation airflow paths differently from daytime paths, to ensure maximum cooling of the mass. Some designers direct night ventilation through voids actually within the structure, such as hollow floors.

5.7.2    Geothermal free cooling

Ground coupled cooling is achieved by conductive heat transfer with the earth. The most common strategy is to cool air or water by channeling it through underground pipes. The idea is that as the fluid travels through underground plastic or metal pipes, it gives up some of its heat to the surrounding soil.  This will occur only if the earth is at least several degrees cooler than the incoming fluid. In the case of air, it can enter the building as cooler air.  In the case of water, this water can be used in radiant cooling systems like chilled floors or ceilings or concrete activation. 

Geothermal free cooling can either be realized as an open- or closed-loop configuration. In an open-loop system with air, outdoor air is drawn into the tubes and delivered directly to the inside of the building. In a closed-loop system interior air circulates through the earth cooling tubes. An alternative is to direct the cooled air from either type of system into a mechanical air conditioning system to reduce the air conditioner's cooling load.

Open-loop systems with water use directly the cooling energy of the ground water of a water-bearing layer. In most cases two wells are required, one for extracting the ground water and one for injecting it back into the water-bearing layer. The cool ground water is conducted through a radiant cooling system like chilled ceilings or concrete core activation where it cools the space below it by acting as a heat sink for the naturally rising warm air of the space. Once cooled, the air naturally drops back to the floor and the cycle starts again. Closed-loop systems with water use borehole heat exchangers or energy piles. Pure water as the cooling medium is circulated in a closed circuit. Since the tubes are earth-laid, antifreeze additives are mostly not necessary.

The higher the ambient temperature of the earth, the less effective the ground tubes are for cooling and dehumidification.

Rules of thumb for air systems

§  Tubes made of PVC or polypropylene are easier to install and more corrosion resistant as metal tubes.

§  The most appropriate tube diameters are between 15 and 45 cm (depends on tube length, flow velocity, flow volumes).

§  For determining the proper tube length local soil conditions, soil moisture and tube depth should be considered.

§  The inlets in open-loop systems and the cooling tubes should be placed in shady areas, as cooling tube performance vary significantly from sunny to shady locations.

§  The tubes should be buried at least 2 meters and not more than 4 meters below grade. When digging trenches at these depths, cave-ins are a hazard and precautions should be taken.

§  For best possible heat transfer, the tubes should be laid in solid ground (not in sand). Make sure the ground is well compressed around the tubes.

§  The tubes are to be laid at least 1 m from the building and from each other.

Rules of thumb for water systems

§  For borehole heat exchangers it has to be taken into account that the accomplishable cold water temperature is limited. The cooling capacity depends mainly on the thermal properties of the underground and can vary considerably from place to place.

§  For horizontal position of the heat exchangers, 35-60 m per kW of cooling capacity are required.

§  Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the earth deeper down in cooler in summer. A vertical borehole heat exchanger is drilled to a depth of 20-300 m with a diameter of 10-15 cm. Adjacent boreholes should have a separation distance of 5 m.

Points of attention

§  Geothermal free cooling is not appropriate in hot, humid areas because the ground does not remain sufficiently cool at a reasonable depth.

§  Mechanical dehumidifiers will be necessary as dehumidification is difficult to achieve with ground cooling.

§  Screens and gitters have to be installed at the tube inlets of open-loop systems to protect against insects and rodents.

Costs and economic benefits

§  The tube systems can be very expensive, it is unlikely that this technology is the most economic solution.

Practical examples

§  Air-ground heat exchanger AWADUKT, REHAU, http://export.rehau.com/construction/civil.engineering/ground.heat...geothermal.energy/awadukt.thermo.shtml

5.7.3    Free chilling

When the outside temperature is cooler than the internal target temperature, free chilling is an efficient method for cooling a building. It is a proven technology to achieve energy and money savings in both new built and refurbishment projects. The basic principle of free chilling is to dissipate heat to the ambient by using a cooling tower. The low temperature of the cooling tower water supply enables free cooling of laboratories, office buildings, and server rooms. It can be distinguished between dry cooling towers, wet cooling towers and hybrid systems. Wet cooling towers operate on the principle of evaporation. The advantage is that in comparison to dry cooling towers the heat exchanger area can be reduced and the wet bulb temperature can be considered instead of the ambient air temperature. Wet bulb temperature is lower than the ambient air temperature because the evaporation effect is considered. That means that recooling can be extended. Besides, the lower temperature has a positive effect on energy efficiency and maximum cooling capacity.

Common practice is to use cooling towers in combination with a heat exchanger to bypass the chiller. When outside wet-bulb temperature drops below the desired chilled water temperature, the chiller’s compressor can be shut down. With this energy-efficiency measure a lot of electric power can be saved.

Free chilling adapts a basic water chiller so it functions as a simple heat exchanger. Whenever condenser water is available at temperatures lower than the desired chilled water temperature, free chilling can provide up to 45 % of a chiller's nominal capacity without the operation of its compressor. This will result in substantial energy savings, because the majority of the cost associated with making chilled water is from operation of the compressor.

Rules of thumb

Free chilling is advantageous in the following condition:

§  Cooling loads are present when the ambient air temperature is relative low (server rooms, conference buildings, …)

External links

§  Santamouris, M.; Advances In Passive Cooling (Buildings, Energy and Solar Technology Series), ISBN-13: 978-1844072637, Earthscan Publications Ltd. (September 2007)

§  Brown G. Z., DeKay M.; Sun, Wind & Light: Architectural Design Strategies, ISBN-13: 978-0471348771, Wiley; 2nd edition (October 24, 2000) 
short summary: http://www.oikos.com/esb/51/passivecooling.html

§  Geros V., Santamouris M., Karatasou S., Tsangrasso; On the cooling potential of night ventilation techniques in the urban environment; Energy & Buildings, Elsevier (March 1, 2005)

§  Pfafferott, J., Herkel S., Wambsgansz M.; Design, monitoring and evaluation of a low energy office building with passive cooling by night ventilation; Energy & Buildings, Elsevier (May 1, 2004)

§  EREC Reference Briefs - Earth Cooling Tubes, Energy Efficiency and renewable Energy Network (EREN) U.S. Department of Energy, Dr. Dale Elifrits, University of Missouri-Rolla, 05.2000

§  http://www.builditsolar.com/Projects/Cooling/EarthtubeNotes.htm

§  BSRIA, UK, Free cooling systems, http://www.bsria.co.uk/press/?press=114

§  http://www.konvekta.ch/syskon/freecool.pdf

§  www.osti.gov/energycitations/product.biblio.jsp?osti_id=5043638

§  Florides, G., Kalogirou, S.; Ground heat exchangers - A review of systems, models and applications, Renewable Energy 32 (2007) 2461-2478

 

5.8       References

[BIT08]            www.brita-in-pubs.com (BIT = Brita in pubs information tool) BRITA = Bringing Retrofit Innovation to Application in Public Buildings, demonstration of retrofit projects, computer tool for the first planning phase

[TOR04]          P. Torcellini and S. Pless, Trombe Walls in Low-Energy Buildings: Practical Experiences, NREL/CP-550-36277, NREL National Renewable Energy Laboratory, Colorado, USA, World Renewable Energy Congress VIII and Expo Denver, Colorado August 29–September 3, 2004 http://www.nrel.gov/docs/fy04osti/36277.pdf

[IEA00]            IEA Task 26, coloured booklet, edited by Jean Marc Suter - Thomas Letz - Werner Weiss - Jürg Inäbnit. Solar Combisystems in Austria, Denmark, Finland, France, Germany, Sweden, Switzerland, the Netherlands and the USA – Overview 2000, http://www.iea-shc.org/task26/publications/index.html

[HEB07]          Bernd Hebenstreit, Solare Kühlung, Anwendungsbeispiele und Praxiserfahrung. VDI Tagung Heizen und Kühlen mit der Sonne, Stuttgart 10/07

[HEN04]          Dr. Hans-Martin Henning, ISE Freiburg; Realisierte Solare Kühlanlagen und Nutzungspotentiale; Tagung Solares Kühlen, AEE Intec, Österreich, 2004

[REH08]          Renewable Energy House Brussels, http://www.erec.org/fileadmin/erec_docs/Documents/
Publications/EREC-Brochure_House_2008_FINAL_VERSION.pdf

[ALT08]           Altbauten sanieren. Energie sparen, Solarpraxis, 2008, ISBN 978-3-934595-78-1

[ENE07]          Energieeffizient sanieren. 2007, ISBN 978-3-934595-72-9

[LOW08]         www.lowcarbonbuildings.org.uk

[ERD08]          Erdwaermepumpe.de, Das Verbraucherportal, August 6th, 2008 http://www.erdwaermepumpe.de/03kosten.php

[BIN08]            Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU), Germany; ERNEUERBARE ENERGIEN; Fragen und Antworten; http://www.erneuerbare-energien.de/files/pdfs/allgemein/application/pdf/broschuere_ee_faq.pdf

[VDI04]            VDI, Verein deutscher Ingenieure – Association of German engineers, guideline 6002-1

[RBA08]          www.rbair.com