120: Solar-heated Dwellings with Seasonal Storage

Technical Brochure No. 120
IEA
OECD
r e n e w a b l e e n e rg y
Solar-heated Dwellings with
Seasonal Storage
Summary
In 1984, 96 houses in the village of
Beijum in the Netherlands were
equipped with solar collectors, a
large underground heat storage unit
and low-temperature central
heating. The houses were
monitored from 1984 to 1996,
providing an excellent opportunity
to assess the long-term behaviour of
such systems. The monitoring
showed that the system functioned
consistently well for 12 years.
During this time, about 54% of the
total heat demand of the dwellings
was met by the solar energy system.
Highlights
▼
Solar energy collector area
of 2,358 m2
▼
23,000 m3 of underground
heat storage
▼
Effective, environmentallyfriendly heating systems
Heat pipe solar collectors are mounted on the roof of each participating house.
SOLAR – ACTIVE
Project Background
As concern grows regarding the
environmental effects of fossil-fuel
combustion, the Dutch government
is encouraging the use of solar
energy for heating purposes.
However, one problem to overcome
is the mismatch between peak
energy production in summer and
peak energy demand in winter.
Solving this requires long-term
storage of summer heat. There are
several ways to achieve this: one
method is to store heat in a sizeable
water vessel; another is to store
heat underground.
In the Netherlands, underground
storage projects have been carried
out since the mid-1980s. In 1984,
a large solar heating project was
established in Beijum in the most
northerly part of the Netherlands.
The project involves 96 houses
connected to a solar heating
system. The performance of the
system was monitored between
1984 and 1996, providing an
excellent opportunity to assess the
behaviour of such systems over an
extended period.
A diagram showing key elements of the scheme.
Roof collectors
Houses
Grid
To the
houses
Boilers
From the
houses
Insulation
Water tank
Seasonal
storage
The Project
The entire system is depicted in the
diagram below. It consists of four
subsystems:
roof-mounted collectors;
▼ a heat storage unit;
▼ a distribution grid;
▼ central heating systems in the
houses.
▼
The roof of each participating
house supports a solar collector of
the heat pipe type. The difference
between a water pipe collector and
a heat pipe collector is that, in the
heat pipe collector, a volatile
medium (neopentane) evaporates
inside the collector and condenses
in a heat exchanger, giving off
condensation heat to a water circuit.
Each collector has a surface of
24.56 m2, so the total collector
surface is 2,358 m2. The collectors
are oriented towards south-southeast at a 45° tilt angle.
The cooling water transports the heat
to another heat exchanger, which
charges an underground heat store.
This consists of an underground
aquifer (a water-bearing sand layer)
with a total depth of 20 m and a
diameter of 38 m (hence a volume of
23,000 m3). The aquifer is charged
with heat through a network of 360
polybutane flexible tubes that are
dug into it. The tubes, which have a
total length of over 15 km, have
been connected to form 60 groups of
six loops in a circular array. The
heat store is charged from the centre.
The heat store provides space
heating to the houses on the estate.
Heat is supplied first to a small
100 m3 buffer and is withdrawn by a
heat exchanger from there to the
heat distribution network. This is to
prevent overheating of the system
on sunny days when the heat store
cannot absorb heat fast enough to
store all the solar energy.
Two gas-fired boilers can provide
additional heating to meet the
demand when the supply in the heat
store is too low. Each house is
equipped with a low-temperature
heating system, operating at a
supply temperature of 42.5°C and a
return temperature of 32.3°C.
The heat store also provides
preheating for tap water, which is
heated to 60°C in a 95 litre boiler in
the dwellings. (The additional
heating to 60°C is necessary to kill
the Legionella pneumophilia
bacteria.)
Performance
The system has been fully
operational since 1984. During the
first five years (1984-1989)
monitoring showed that the average
heat demand was 1,001 MWh/year.
The average solar irradiation
amounted to 2,411 MWh/year,
however, up to 462 MWh/year were
lost through system components and
the average collector losses were
1,407 MWh/year. The net heat
production of the system was
therefore 542 MWh/year, yielding
an average efficiency of 22%. To
cover the total heat demand, about
459 MWh/year had to be supplied
by natural gas.
Between 1984 and 1996, the system
functioned without serious
problems. Some difficulties
occurred in the operation and
maintenance of the collector circuit,
Artist’s impression of the underground heat store.
mainly because of vapour losses.
The low-temperature heating
systems caused some trouble
because the households had to learn
how to operate the systems correctly
(since they operate at lower
temperatures, the systems respond
more slowly than conventional
heaters and are more sensitive to
blockages in radiators).
The main benefit of the project is the
considerable experience it has
provided concerning the design and
operation of large-scale solar heating
systems combined with heat storage.
Economics
At 1995 prices, the 1984 investment
in the collectors amounted to
NLG 3,887,500 (where NLG is the
Dutch guilder). The heat store cost
NLG 1,675,000, while the
distribution system and the lowtemperature heaters required an
investment of NLG 456,000. The
total investment was therefore
NLG 6,018,500. At a gas price of
NLG 0.5/m3 and a boiler efficiency
of 90%, the 542 MWh/year
produced by the unit represents a
value of NLG 34,250/year.
The project has no realistic payback
period. However, the main aim was
to demonstrate the technical
feasibility of a combined
collector/storage system. In that
respect, the project has certainly
been a success.
Environment
The solar heating system produces
542 MWh/year for district heating
which would otherwise have been
supplied from fossil fuel sources. It
therefore avoids the emission to the
atmosphere of around 110 tonnes/
year of carbon dioxide (CO2).
Technical Brochure No. 120
Host Organisation
De Huismeester Building Society
Friesestraatweg 18, PO Box 546
NL-9700 AM Groningen
The Netherlands
Contact: P K Hillenga
Tel: +31 50 365 7171
Fax: +31 50 318 3124
Information Organisations
DWA Installatie- en energieadvies
Van Tolstraat 4c, PO Box 274
NL-2410 AG Bodegraven
The Netherlands
Contact: J J Buitenhuis
Tel: +31 172 650 260
Fax: +31 172 651 499
Novem bv
Swentiboldstraat 21, PO Box 17
NL-6130 AA Sittard
The Netherlands
Contact: W van Zanten
Tel: +31 464 202 329
Fax: +31 464 528 260
Please write to the address below if you require more information.
IEA
OECD
r e n e w a b l e e n e rg y
CADDET Centre for Renewable Energy
ETSU, 168 Harwell, Didcot
Oxfordshire OX11 0RA
United Kingdom
Tel: +44 1235 432719
Fax: +44 1235 433595
E-mail: [email protected]
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First printed: March 2000