2.6 HEAT RECOVERY, ENERGY STORAGE AND
ACTIVE SOLAR SYSTEMS
Integration of Heat Recovery, Energy Storage and Active
Solar Systems for Improvement of Indoor Air Quality,
Energy Efficiency and Optimisation of Operation Costs
A general problem of museums are the contrasting conservation
requirements of exhibition objects and the comfort requirements
of the museum visitors. This diverging requirements usually lead
to a complication of the definition and the improvement of the
indoor air quality and thermal comfort. Further on historical
constructions often make it difficult to integrate building services
like air channel systems or active solar. The demands on the
preservation or protection of these monuments restrict the variety
of solutions.
For this the integral planing and the inclusion of experts as early
as possible is an important aspect for the design or retrofitting
concept of energy efficient museum buildings.
Heat recovery, energy storage, and active solar systems are
suitable means to achieve energy efficient buildings and to reduce
their operating costs. In detail it means to reduce the annual
energy demand for heating and cooling, the electricity
consumption and, in consequence, the CO2-emission of buildings.
The integration of heat recovery systems or components leads to a
reduction of thermal ventilation losses of the building and to a
reduction of the required energy support. Energy storage can be
inserted to transfer excessive gained or recovered energy from a
period of time with surplus energy gain to a period of time when
this surplus energy can be used as supply again. The use of
internal energy storage resources in walls and ceilings can enable
an improvement of the indoor air climate and the thermal comfort
with relative low additional costs. The use of renewable energy
like active solar systems leads to a conservation of natural
resources in addition. Systems like solar air and water collectors
usually have to be combined with energy storage systems to shift
the energy to a later period of time when it is needed. Photovoltaic
elements also lead to a reduction of primary energy consumption
and CO2-emission.
The integration of existing structural elements like walls and
ceilings inside the building and its foundation as energy storage
components and the implementation of heat recovery systems and
active solar components within an energy design can lead to
economically and ecologically optimised solutions also for
museum buildings. The dimension of the different components
have to be co-ordinated to reach advanced solutions. Due to the
complex requirements on indoor climate and monument
conservation the concepts have to be “custom-made”.
State of the Art Heat Recovery, Energy Storage and Active
Solar Systems Design
Precondition to reach energy efficient buildings is the reduction of
thermal losses. The improvement of the thermal insulation and the
airtightness of the building in combination with a controlled air
change decrease the required supply energy. This offers the use of
energy optimised low temperature heating and high temperature
cooling systems.
Central air to air system
Heat recovery, energy storage and the integration of active solar
systems can be inserted as basic components of advanced energy
concepts. In practice different system of heat recovery, energy
storage and active solar are possible.
In case of already existing ventilation systems the integration of
heat recovery is a sensible measure for saving energy and
operation costs. „Air to Air“, „Air to Water“ and central or
decentral systems are different possibilities to realise these
systems. Central air to air systems are usable in new and existing
museums buildings with already integrated air channel systems.
The exhaust and supply air have to be placed central in close
connection. Standard heat recovery components are recuperators
or regenerators.
Decentral air to air systems are particularly suitable for retrofitting
existing museum buildings. It is possible to add fresh air to
circulating air. In this way the air change can be minimised to the
smallest amount required by hygienically aspects. The air
conditioning can be realised as a four-wire-system with cooling
and heating coils. Generally air to water systems are less efficient
than air to air systems, but central air to water systems are
favourable in case of distant adjustment of supply and exhaust air
channels or in case of simple exhaust air systems. In the second
case the recovered heat can support low temperature systems like
floor and wall heating.
The thermal use of construction units or natural storage capacities
in the environment offers another way to increase the energy
efficiency of a new or an existing building. Most simple is the
activation of thermal masses like ceilings, floors and walls by
ventilation. In consideration of security aspects a nocturnal
ventilation exhausts the heat stored over the day. In the morning
pre-cooled construction units in combination with a reduced air
change stabilise the indoor climate and avoid an overheating of
the rooms. The system efficiency depends on the capacity of the
construction material. Especially old museum buildings with big
construction masses are suitable. The advantages of this solution
are the simple system configuration and in consequence low
capital costs and low energy consumption. In new buildings
concrete core activation is a possibility for using construction
units for energy storage and active cooling or heating. Therefore
water filled tubes are integrated for example in the ceiling
construction.
Decentral air to air system
Central air to water system
Activation of thermal masses by
ventilation
Concrete core activation and
heat exchanger piles
Energy piles and borehole heat exchangers use the surrounding
ground for heat exchange and thermal energy storage. Heat
exchanger piles are additionally used to support the building
foundation. Both systems work with plastic tubes, in which a fluid
circulates to exchange heat with the environment. In most cases
the heat carrier fluid is water or a water-glycol mixture. An
advantage is the parallel usage of foundation piles as channel and
foundation system. The extra costs for construction are reduced to
the integration of the tubes in the reinforcement basket and the
connection of the different vertical “U-tubes” arranged in the
piles.
Generating energy with active solar systems is a possibility of
using renewable energy. Solar assisted space heating, solar tap
water heating, solar air heating and solar cooling are systems
using thermal solar energy. The efficiency of a solar assisted
heating system depends on the thermal insulation of the building.
Suitable are buildings of low-energy-standard (heat demand up to
70 kWh/(m²a)) in connection with low temperature heat
distribution systems like floor or wall heating. Due to the missing
heat demand in summer, solar assisted space heating normally is
not economic for museums. In case of further heat demand or the
addition of further energy consumption units within an district
heating system the integration of a solar system has to be
considered. In this case the roof of the museum can be used for the
arrangement of the collector area (if a greater coherent roof area is
available). Economic solar tap water systems start by a daily hot
water demand of 500 l. In case of low hot water demand and noncentral tapping points as in most museum buildings the integration
of a plant for the thermal use of solar energy is not
recommendable. For this decentral electric flow heaters should be
installed to reduce investment costs and heat losses (standby and
distribution losses).
Solar air collectors transfer energy directly to the air flow inside
the collector. In an open loop system the pre-heated air can be
used as supply air for building ventilation. Another possibility is
the integration of the solar air collector in a closed loop. In this
case the circulating air is used as heat carrier for energy transfer to
the building structure, like a hypocaust system.
Solar cooling systems use the direct coupling of the cooling
demand and the available solar energy – both rise simultaneously.
Heat delivered by a solar collector generates warm air to
dehumidify and to cool down a water absorbing material
(evaporative cooling). This cooling energy is transferred to the air
of the ventilation system. Opposed to solar heating absolutely no
storage is needed.
In addition to the thermal use of solar energy, photovoltaic
systems transform solar energy into electrical energy. The
electrical energy can be used inside the building for every
purposes or it can be fed into the grid. The possibility of the
fabrication of PV-elements in different forms, colours and sizes
offers a wide range of possibilities for their architectural
integration. A double function as shading device and energy
collector leads also to a reduction of the additional investment
costs for the integration of PV-elements.
Solar tap water system
Solar air collector, open loop
Solar air collector, closed loop
Solar cooling system
Photovoltaic integrated to a
shading system
Heat Recovery, Energy Storage and Active Solar Systems
Design Techniques to Optimise Performance
In case of the improvement of the indoor air quality and thermal
comfort different conditions have to be complied. Climate
demands of people and exhibits have to be defined and
differentiate by agreeing and dissenting requirements. In the
second case sealed spaces and/or showcases for the exhibition
parts have to be implemented.
Demands on the preservation or protection of the outward
appearance of the building, regarding historical monuments, have
a great influence on selection and design of components of energy
concepts for museums and restrict the variety of solutions.
Therefore a sensible combination of system components is
necessary. Hybrid systems should be inserted in an integral
concept. For this the integral planing and the inclusion of experts
as early as possible is an important aspect for the design or
retrofitting concept of energy efficient museum buildings.
In the following some expedient applications and combinations of
components are composed.
The integration of heat recovery and therefore the reduction of
ventilation losses leads to a decrease of the heat demand of the
museum building and offers the possibility of the integration of
low temperature heating and high temperature cooling devices. A
controlled air change and the airtightness of the building envelope
are preconditions of the installation of heat recovery units.
Standard air to air heat recovery components are recuperators and
regenerators. The regenerator can be used for heat exchange as
well as for humidification and dehumidification of the supply air.
The heat exchange between the two air flows in regenerators is
realised by alternative passing of the heat exchanger by the two
flows. Driven by the partial pressure gradient between in- and
outgoing air also a moisture exchange between the two flows is
possible. In the recuperator the two air flows are totally separated.
The exchange of moisture is not possible. To avoid condensation
and freezing in the recuperator (exhaust air) the outside air has to
be pre-heated during frost periods in winter.
An advantage of central heat recovery systems is the simple
integration of components like other heat sources and heat pumps.
Also an earth heat exchanger for pre-heating or pre-cooling could
be provided. In summer the earth heat exchanger achieves 10 15 K lower temperatures of the supply air than outside air for a
period of time. In winter the earth heat exchanger avoids
condensation water and freezing in the recuperator. The
disadvantage of the combination with an earth heat exchanger is
the less effectiveness of the recuperator because of the small
temperature gradient between the two air flows.
Recuperator
Regenerator
The thermal activation of ceilings by embedded coils for example
is an useful combination of an active component for heating /
cooling with an additional energy storage function. For concrete
core activation the medium temperature is approximately 20 °C
(corresponding to the required indoor temperature). In a self
regulating effect which depends on the temperature gradient
between water and indoor air the ceiling is used for cooling or
heating. As heat source for concrete core activation often the
environment in connection with energy piles is used. In summer
the cold of the ground is used for direct cooling. Coupled with a
heat pump, which generates a higher temperature range, the
system can be used for heating in winter. Generally, the heating
power of concrete core activation does not cover the heat demand
of a building. Therefore the system is normally supported by a
conventional space heating system.
Energy storage components outside the building like energy piles
are possible systems to shift energy amounts over a defined time
period or to use earth heat.
Ceiling with a system of plastic tubes for
concrete core activation
The implementation of active solar systems is independent of the
age of the building. For historic buildings demands on the
preservation or protection of historic monuments have to be
regarded. Integral planning of new buildings can lead to solutions
which are able to fit multiple functions. A photovoltaic facade for
example generates power and protects the building against
weather conditions (rain, wind, solar irradiation).
Therefore, the first step in designing active solar systems is to
check possible solar receiver surfaces (roof and facade) and the
demand of solar energy (solar heating / cooling or electric power).
Suitable are collector arrangements with tilt angles of 15 - 50° and
vertical elements with orientations from south-west to south-east.
The use of active solar components for the energy supply in
museums usually is restricted to the integration of photovoltaic
elements, solar air collectors and solar cooling. In special cases
the thermal use of solar energy can be utilised for heating and tap
water preparation.
The integration of solar water collectors depends on the heat and
first of all on the hot water demand and is not common for
museums. However, in some cases simple systems with absorbers
are suitable. These systems are generally used for swimming
pools, but they are also suitable in connection with low
temperature systems like wall tempering. For this the wall is used
as storage.
Flat plate and vacuum collectors are used for higher temperature
ranges like tap water or space heating. Vacuum collectors are
more energy efficient than flat plat collectors and can be easily
oriented into the required slope, but they are also more expensive.
By integration of the corresponding control and hydraulic parts the
collector of a solar cooling system can be used for cooling in
summer as well as for heating in winter.
Reinforcement and U-tubes of energy piles
Other sensible applications for the thermal use of active solar
components within energy concepts for museums are solar air
collectors, which pre-heat the air before entering the building. The
absorbed radiation energy is transferred directly to the air flowing
through. In an open loop system the air can be used as supply air
for building ventilation. These systems are an optimal addition to
low temperature systems and systems with a high thermal inertia.
In case of solar air collectors heat recovery is not necessary.
Another possibility is the integration of the solar air collector in a
closed loop. Here the circulating air is used as heat carrier for
energy transfer to the building structure, like a hypocaust system.
The structure warms the enclosed spaces by radiant heat. In this
case a powerful and speedy heating systems is needed in addition.
The low density and heat capacity of air compared to water as heat
carrier cause large channel dimensions and volumes, which have
to be considered in the building design.
Solar air collectors integrated to the facade or the roof of a
building in addition act as building insulation. The simple,
lightweight and cost-efficient construction of the solar air
collector are other advantages.
As already described, different solutions of integrating
photovoltaic elements are possible. Hybrid systems like
photovoltaic facades, which generate power and protect buildings
against weather conditions, roof sealing elements with
implemented photovoltaic cells or photovoltaic cells integrated in
a shading system are available on market.
Nowadays Photovoltaic elements occur in different kinds of
colours and geometry. Also partly translucent elements are
possible. Therefore the integration of photovoltaic could be an
element of architectural and lightning design.
Because of extensive coherence of building and system
engineering and numerous dynamic boundary conditions like
temperature and moisture, the design and optimisation of energy
systems has to carry out with computer simulations. Different
tools allow the investigation of system behaviour at different load
cases and offers the opportunity to optimise the system
configuration. The coupled dynamic building and system
simulation enables the accommodation of different components on
each other and on the building demand. The simulation results
give design certainty and are an important tool for the
development of efficient and adapted concepts for the complex
demands on indoor air quality and thermal comfort in museums.
Hybrid system – shading system with
photovoltaic
Building surface of translucent photovoltaic
modules
Photovoltaic elements of different colours
and geometry
Heat Recovery, Energy Storage and Active Solar Systems
Design Performance Criteria
In energy optimised buildings the ventilation losses can amount to
over 50% of the thermal losses of the building. To reduce these
losses and to reach a low energy standard for the building (net
heat demand lower than 60 to 50 kWh/(m²a)) the integration of a
heat recovery unit is required. Central air to air heat recovery
devices in any size and construction (recuperators, regenerators,
heat pumps etc.) can be integrated into a ventilation system for
supply and exhaust air. Normally a sensible energy recovery rate
of 70 to 90 % can be expected by using recuperators (channel
counter flow units). Regenerators used as “enthalpy wheels” can
also recover humidity and reach humidity recovery levels of 50 to
60 %. In any case the COP of heat pumps should be greater than 3
to save primary energy.
Energy piles and borehole heat exchangers exploit the surrounding
earth as storage medium for heat and cold. In case of energy piles
the number and depth of the piles usually depend on building
constructional necessities. In case of separate piles only for energy
transfer purposes important influences of the distance between the
piles (greater than 5 to 6 m), their depth (usual range 30 to 150 m)
and of course the relevant parameters of the ground (thermal
conduction, ground water flow, humidity) have to be regarded
within their dimensioning. The diameter of the tubes used for the
circulation of the fluid is usually about 26 to 46 mm. The
dimensioning of the tubes depends on the depth, the set up of the
water ~20°C
19-21°C
indoor temperature
18°C
heating
22°C
cooling
Effects of self regulating for thermal
activated constructions
tube distance 15 cm
heating
power
cooling
power
25 W/m²
13 W/m²
10 W/m²
15 W/m²
28 cm
According to the aim of cooling or heating the water temperature
range for concrete core activation is about 18 to 28°C. As already
described water temperatures corresponding to the required indoor
temperature achieve a self regulating effect. The activated
building unit cools or heats depending on the temperature gradient
between water and indoor air. In order to adjust water and indoor
temperature the temperature spread between outflow and inflow
has to be 2 to 5 K.
Apart from the temperature gradient and the water flow the
cooling or heating power depends on the position and distance of
the tubes in the building unit. In case of an activated ceiling the
tubes could be placed at the top, at the bottom or in the core. The
distance of the tubes amounts generally 15 to 30 cm. In case of the
mentioned boundary conditions the cooling or heating power
achieves a value of 5 to 40 W/m².
Influence of material and material thickness
on the storage efficiency
tube distance 30 cm
heating
power
cooling
power
12 W/m²
6 W/m²
15 W/m²
22 W/m²
28 cm
The efficiency of the thermal use of construction units depends on
the capacity and on the thickness of the construction material (see
picture). Low capacity and/or thin constructions like light weight
constructions can be covered with a plaster including a phase
change material. The plaster stores energy at a specific
temperature or temperature range without temperature rise until all
the phase change has happened. This effect is adjustable by the
choice of material in a range between 23 to 26 °C. The latent heat
is circa 110 J/kg. Therefore phase change materials can make light
weight constructions behave thermally like heavy weight
constructions.
Cooling and heating power depending on the
tube position and distance (steady-state
condition ∆logT = 5 K), IndustrieBAU 2/99
piles, the corresponding heat transfer rate and the resulting
pressure drop. Depending on the ground parameters a medium
heating or cooling power in the range of 50 to 70 W/m can be
expected. To be sure of the long time development of the
temperature level within the ground simulation studies have to be
carried out.
preheating of air in winter:
temperature [°C]
10
5
ϑ1
-5
Earth heat exchangers can be used for the pre-heating or -cooling
of fresh air before entering the ventilation unit or the building.
The main influences on the reachable heating or cooling power of
an earth heat exchanger are parameters of the earth (thermal
capacity, humidity...), its disposition (under or beside the building,
depth) and the length of the channels. To reach a good efficiency
the depth of the earth heat exchanger should be chosen to 1,8 to
2,4 m (6 to 8 ft). The channels have to be positioned in the
beginning frost free zone of the ground therefore the depends on
climate conditions. To reach a good heat or cold transfer within
the earth heat exchangers and to avoid unwanted high pressure
losses the diameter of the channels should be adapted to the
mainstream. The corresponding flow velocities should be about
1,5 to 3 m/s (300 to 600 fpm). With a length of about 35 to 45 m a
temperature level of 0 °C can be reached at the end of the earth
heat exchanger at ambient temperatures of about -15 °C. In
summer the air can be cooled down to a temperature level of
below 20 °C at ambient temperature of 25 to 30 °C.
ϑ2
-10
-15
-20
0
8
16
24
32
40
length of pipe [m]
ϑE .. earth temperature
ϑ1 .. temperature of air in the pipe at ϑA = -15°C
ϑ2 .. temperature of air in the pipe at ϑA = -20°C
cooling of air in summer:
temperature [°C]
35
30
25
ϑ1
20
ϑ2
15
10
5
ϑE
0
8
16
32
40
24
length of pipe [m]
ϑE .. earth temperature
ϑ1 .. temperature of air in the pipe at ϑA = 30°C
ϑ2 .. temperature of air in the pipe at ϑA = 25°C
parameters:
Vair=155m³/h; depth of laying 1,8m; length of pipe 40m;
Reachable air outlet temperature in earth
heat exchangers
tilt angle
east,
west
tilt angle
For active solar systems collector arrangements with tilt angles of
15 - 50° and vertical elements with orientations from south-west
to south-east are suitable. In addition to the tilt angle and the
orientation of the collector the yearly energy gain depends on the
yearly global irradiation and on the size of the collector area.
Further on shading of close-by buildings, trees as well as other
collectors have to be considered when planning solar receiver
surfaces.
Solar tap water systems can work economically only from a daily
hot water demand of about 500 l. Therefor in most cases only
museums with cafeterias or restaurants are recommendable. To
achieve a yearly solar fraction of about 50 % 1 m² collector area
per 50 l daily hot water demand (water temperature 45 °C) have to
be installed. The store volume should be 60 l per square meter.
The heat gain of solar air collector systems depend on numerous
factors like the yearly global irradiation, the dimension of the
collector area and on the type, the utilisation and the air change of
the building. The efficiency of solar air collectors increases with
low temperature gradients between outflow and ambient air and
with a rising airflow. In general solar air collector systems are able
to generate 100 to 350 kWh heat per year and square meter
collector area for heating and ventilation.
The efficiency of photovoltaic depends on the kind of installed
solar cells (amorphous silicon, polycrystalline silicon or
monocrystalline silicon cells). The ratio between generated power
and solar radiation lies in a range of 5 to 18 %. Therefore the
efficiency of the solar cells the orientation, the tilt angle and the
yearly global irradiation are the main influences on the electrical
energy gain.
ϑE
0
south
Influence of the tilt angle of the solar
collector to the received solar irradiation
Results and Lessons learned with respect to the Heat
Recovery, Energy Storage and Active Solar Systems Design
Energy storage, heat recovery and the use of active solar systems
can be integrated in energy design concepts for museum buildings
and contribute to lower the energy consumption and to reach a low
energy standard.
The knowledge of the operation characteristic of the different
measures is important for reasonable system integration and the
development of efficient concepts. The integration of energy
storage devices like energy piles or even construction units within
the thermal activation of ceilings improve energy consumption
and thermal comfort. The use of active solar components for the
energy support of museums usually is reduced to solar air
collectors or photovoltaic. Heat recovery units can be inserted as
decentral or central systems. Different already known solutions
can be applied and contribute to reduce the energy consumption.
The special requirements of the exhibition rooms and the problem
of the divergence of the indoor air requirements for exhibits and
visitors within the museum has to be regarded in any case.
Coupled tools for system and building simulation should be used
to predict the parameters of thermal comfort and indoor air quality
and to adjust the different components of efficient energy
concepts.
Within the museum project several advanced energy designs using
active solar components, heat storage and ventilation systems
were realised and monitored.
A very good energy concept including solar air collectors energy
storage and heat recovery has been implemented at the “Museum
of Modern Art” at Kristinehamn, Sweden.
The museum is hosted in a former hospital which was build
during the 1960s and was rebuild to an art gallery in 1977. In the
meantime the gallery needed more space. Therefore a nearby,
former boiler house was rebuild and incorporated.
Because of the insufficient thermal insulation new economic and
innovative methods for heating were necessary in order to use the
building in wintertime.
The solution is a south orientated air collector with an area of
48 m² which has been mounted at the former charcoal tower for
pre-heating ventilation air in winter. Outside air flows through the
collector. The solar heated air is driven to the bottom of the tower
and through the basement by fan. Solar energy is stored by the
thermal mass of the building structure. Afterwards the tempered
air is used for ventilation of the exhibition halls. The exhaust air is
evacuated through the tall charcoal tower (See principle scheme).
When the museum is closed the exhibition halls are ventilated by
recirculating air. In summer seawater is used for cooling the
supply air.
Secondary rooms like offices, library, cafeteria and shop are
ventilated conventionally by a ventilation system with a central air
to air heat recovery.
The main energy supply of the floor heating in the exhibition halls
and the heating in the secondary rooms as well as the post-heating
of the ventilation air is provided by district heating.
Museum of Modern Art with the charcoal
tower at Kristinehamn
Solar air collector installed to the charcoal
tower
Principle scheme of the solar air heating at the “Museum of Modern Art” at Kristinehamn
The “C/PLEX Art Centre” at West Bromwich in Great Britain
shows, that planning energy efficient buildings already starts with
the building design. In this example a special “heat recovery”
mechanism is integrated to the building design. Therefor a concept
with different climatic zones is realised.
At the moment the C/PLEX Art Centre is under construction. It
will be a modern, flagship building containing gallery-, work- and
creative spaces, retail opportunities, restaurant facilities, public
areas, conference rooms and other multi-function spaces.
Due to the diverse nature of the spaces , a number of distinct
comfort requirements exist. These can be broadly distinguished
into three groups:
-
-
Areas requiring close control like exhibition type spaces.
Areas with more relaxed comfort requirements, where natural
ventilation and heating is proposed like office and circulating
spaces.
The bioclimatic area like general public areas and some
exhibition spaces, closely following outside conditions,
where minimal heating is proposed.
The bioclimatic areas are located within the „bioclimatic
enclosure“ which takes the form of an intelligent building skin.
The close control areas lie entirely within the bioclimatic
enclosure and therefore do not experience the outside climate but
the mitigated conditions created by the bioclimatic enclosure. A
„heat recovery“ mechanism is specifically based upon the „box
within a box“ strategy. If appropriate exhaust air from the closed
conditioned spaces, is dumped into the bioclimatic zone.
For pre-heating and pre-cooling the close controlled spaces and
some of the naturally ventilated spaces borehole heat exchangers
will be installed at the C/PLEX Art Centre.
“Box within a box” strategy of the different
climate zones