Seasonal Storage of Solar Heat
The Anneberg Project
List of Contents
THE ANNEBERG SITE
HIGHLIGHTS
SYSTEM DESCRIPTION
Table
1. Annual system energy balance.
SYSTEM COSTS
Table.
2 Annual heating cost excluding heat distribution.
Photografies from
the construction cost
THE ANNEBERG SITE
A new residential area, Anneberg, is now being constructed in the city
of Danderyd, north of Stockholm, Sweden. In the city plan the utilisation
of renewable energy sources is encouraged. The project started with a pre-study
of a large-scale solar heating system with 60% solar fraction and seasonal
storage. This system was compared with more conventional heating systems.
In an open bid competition to exploit the area different plans were suggested
and in May 1999 the community selected a solar heated system for Anneberg.
The city of Danderyd first made a detailed development plan for Anneberg.
A land area of 130 x 400 m2 was reserved for the development
of single-family houses and a service/nursing home with a total floor area
of about 9000 m2. It was also decided that the area should use
a centralised heating system with water as heat carrier. These were the
presumptions for the open bid competition. There is presently no district
heating system in the city. The geological condition at the location of
the planned borehole heat store was considered to be favourable for the
construction and the operation of a ground storage system.
The total residential area corresponds to the equivalent of 90 single-family
houses with an annual heat demand of the buildings estimated to 120 kWh/m2
floor area. This results in a total heat demand of 1080 MWh with a peak
of 450 kW. The ventilation heat recovery system was assumed to reduce the
ventilation heat loss by 50%.
HIGHLIGHTS
-
Total heat demand of 1080 MWh with a peak of 450 kW.
-
The suggested system consists of 3000 m2 solar collectors; 400
kWh/m2
-
The rock store with 100 bore holes will have a storage volume of 60 000
m3.
-
Total annual cost 1000 SEK/MWh (100 USD/MWh)
Figure 1: Solar collectors and borehole heat store for residential
heating of Anneberg.
SYSTEM DESCRIPTION
The system is designed to operate at low temperatures in all essential
parts and no heat pump is required. The basic philosophy is to keep things
simple with the following main components: Low-temperature floor heating
and pre-heating of tap water, flat-plate solar collectors and electric
boosters, and seasonal storage in a borehole heat store in rock.
The suggested system consists of 3000 m2 solar collectors;
400 kWh/m2,year; 60 % solar fraction. The rock store with 100
boreholes (depth 65 m, diam. 0.115 m, borehole spacing 3.0 m) will have
a storage volume of 60 000 m3.
The space heating is performed with a low-temperature floor heating
system. The water supply temperature is designed to be 32oC.
To avoid bacterial growth the tap water must be heated to at least 55oC
before tapping. The tap water is preheated by the storage system to a temperature
of 32oC and raised to the desired temperature with individual auxiliary
electrical boosters.
Figure 2. Storage heat balance after 5 years of operation.
Relative losses in the store are estimated to 35-45 %, but would be
lower in a larger system. The heat loss to the surroundings will be large
initially, but will gradually approach a lower, steady state level. The
initial years will require a larger amount of auxiliary heating than later
when the store is in a steady-state operational mode. It is advantageous
to schedule the construction of the solar heating system so that the operation
begins with the charging of the store during the summer before the heat
demand sets in during the autumn.
| Table 1. Annual system
energy balance. |
|
| Annual energy balance |
(MWh) |
| Solar collectors |
1 075 |
| Suppl. Electricity |
120
|
| Storage losses |
-500 |
| Sub-units - |
-30 |
| Heat distribution |
- 100 |
| Heat demand |
- 420 |
| DHW demand |
- 145 |
The total annual heat demand, including heating, ventilation and DHW,
is estimated to about 11 000 kWh per residential unit of which almost 9
000 kWh is supplied by the solar system.
Figure 3: Energy supply to one residential unit.
SYSTEM COSTS
The solar collector cost (the additional cost compared with a conventional
roof design) is about 3 million SEK or 1 250 SEK/m² collector area.
The storage construction cost is about 2 million SEK.
The total investment cost for the building project amounts to 12-15
000 SEK/m² heated floor area of which the solar plant amounts to about
800 SEK/m², i.e. 5-8%.
| Table.
2 Annual heating cost excluding heat distribution. |
|
| Heating cost |
(SEK/a) |
| Solar heat |
8,000 |
| Suppl. Electricity |
1,200 |
| Total |
9,200 |
Table 2 shows the expected annual heating cost applying an annuity factor
of 0.08 on the solar system investment cost, an electricity price of 500
SEK/kWh (excl. fixed cost) and excluding heat distribution costs (assumed
to be the same for another heat supply) and initial heating.
Comparison with conventional heating systems
The annual heating cost was calculated based on the investment cost
of the solar heating system, including necessary installations in the buildings,
operation and maintenance. Similar estimations were made for two conventional
alternatives:
1. Solar heating system as described above – total annual cost 1000
SEK/MWh (100 USD/MWh)
2. A small-scale district heating system using biofuel (pellets) and
oil – total annual cost 1100 SEK/MWh (110 USD/MWh).
3. Individual ground-coupled heat pumps for each building - total annual
cost 920 SEK/MWh (92 USD/MWh).
The total annual cost of the solar heating system in Anneberg compares
favourably with conventional heating systems.
A solar heating system of this kind would be considerably more efficient,
if the heat loss from the store would be reduced. The storage heat loss
decrease with size. Consequently, the efficiency of the whole system increases
with size, since less solar collector area per unit delivered heat is required.
A three times larger system than Anneberg is estimated to show a 20% reduction
of the annual heating cost, which is lower than the best conventional alternative
evaluated in this preliminary study.
The Anneberg plant will become one of the ten largest in Europe and
the very first with a bore hole storage system in rock. The construction
started in 1999, the drilling of the heat store started in April 2000 and
the whole area will be completed by the spring of 2001.
ACKNOWLEDGEMENT
The Anneberg project is being realised by HSB Bostad AB with Askergrens
as architects, Peab AB as contractor and ÅF VVS Projekt AB as engineering
consultants. The solar heating system is partly financed with support from
EC within the THERMIE program (REB/0061/97).
For more information contact:
Sven-Erik Lundin
Tel: +46 18 30 02 11
Fax. +46 18 71 04 85
Email:
se.lundin@bjerking.se